Remove PYTHON-5668.patch

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

.evergreen/resync-specs.sh

@@ -94,6 +94,9 @@ do
     change-streams|change_streams)
       cpjson change-streams/tests/ change_streams/
       ;;
+    client-backpressure|client_backpressure)
+      cpjson client-backpressure/tests client-backpressure
+      ;;
     client-side-encryption|csfle|fle)
       cpjson client-side-encryption/tests/ client-side-encryption/spec
       cpjson client-side-encryption/corpus/ client-side-encryption/corpus

doc/changelog.rst

@@ -14,6 +14,9 @@ PyMongo 4.17 brings a number of changes including:
 - Added the :meth:`~pymongo.asynchronous.client_session.AsyncClientSession.bind` and :meth:`~pymongo.client_session.ClientSession.bind` methods
   that allow users to bind a session to all database operations within the scope of a context manager instead of having to explicitly pass the session to each individual operation.
   See <PLACEHOLDER> for examples and more information.
+- Added support for MongoDB's Intelligent Workload Management (IWM) and ingress connection rate limiting features.
+  The driver now gracefully handles write-blocking scenarios and optimizes connection establishment during high-load conditions to maintain application availability.
+  See <DOCSP-55426> and <DOCSP-57078> for more information.
 
 Changes in Version 4.16.0 (2026/01/07)
 --------------------------------------

pymongo/asynchronous/client_bulk.py

@@ -59,6 +59,7 @@ from pymongo.errors import (
     InvalidOperation,
     NotPrimaryError,
     OperationFailure,
+    PyMongoError,
     WaitQueueTimeoutError,
 )
 from pymongo.helpers_shared import _RETRYABLE_ERROR_CODES
@@ -563,9 +564,17 @@ class _AsyncClientBulk:
                         error, ConnectionFailure
                     ) and not isinstance(error, (NotPrimaryError, WaitQueueTimeoutError))
 
+                    retryable_label_error = isinstance(
+                        error, PyMongoError
+                    ) and error.has_error_label("RetryableError")
+
                     # Synthesize the full bulk result without modifying the
                     # current one because this write operation may be retried.
-                    if retryable and (retryable_top_level_error or retryable_network_error):
+                    if retryable and (
+                        retryable_top_level_error
+                        or retryable_network_error
+                        or retryable_label_error
+                    ):
                         full = copy.deepcopy(full_result)
                         _merge_command(self.ops, self.idx_offset, full, result)
                         _throw_client_bulk_write_exception(full, self.verbose_results)

pymongo/asynchronous/client_session.py

@@ -135,7 +135,9 @@ Classes
 
 from __future__ import annotations
 
+import asyncio
 import collections
+import random
 import time
 import uuid
 from collections.abc import Mapping as _Mapping
@@ -162,7 +164,9 @@ from pymongo.asynchronous.cursor_base import _ConnectionManager
 from pymongo.errors import (
     ConfigurationError,
     ConnectionFailure,
+    ExecutionTimeout,
     InvalidOperation,
+    NetworkTimeout,
     OperationFailure,
     PyMongoError,
     WTimeoutError,
@@ -427,6 +431,7 @@ class _Transaction:
         self.recovery_token = None
         self.attempt = 0
         self.client = client
+        self.has_completed_command = False
 
     def active(self) -> bool:
         return self.state in (_TxnState.STARTING, _TxnState.IN_PROGRESS)
@@ -434,6 +439,9 @@ class _Transaction:
     def starting(self) -> bool:
         return self.state == _TxnState.STARTING
 
+    def set_starting(self) -> None:
+        self.state = _TxnState.STARTING
+
     @property
     def pinned_conn(self) -> Optional[AsyncConnection]:
         if self.active() and self.conn_mgr:
@@ -459,6 +467,7 @@ class _Transaction:
         self.sharded = False
         self.recovery_token = None
         self.attempt = 0
+        self.has_completed_command = False
 
     def __del__(self) -> None:
         if self.conn_mgr:
@@ -493,11 +502,29 @@ _UNKNOWN_COMMIT_ERROR_CODES: frozenset = _RETRYABLE_ERROR_CODES | frozenset(  #
 # This limit is non-configurable and was chosen to be twice the 60 second
 # default value of MongoDB's `transactionLifetimeLimitSeconds` parameter.
 _WITH_TRANSACTION_RETRY_TIME_LIMIT = 120
+_BACKOFF_MAX = 0.500  # 500ms max backoff
+_BACKOFF_INITIAL = 0.005  # 5ms initial backoff
 
 
-def _within_time_limit(start_time: float) -> bool:
+def _within_time_limit(start_time: float, backoff: float = 0) -> bool:
     """Are we within the with_transaction retry limit?"""
-    return time.monotonic() - start_time < _WITH_TRANSACTION_RETRY_TIME_LIMIT
+    remaining = _csot.remaining()
+    if remaining is not None and remaining <= 0:
+        return False
+    return time.monotonic() + backoff - start_time < _WITH_TRANSACTION_RETRY_TIME_LIMIT
+
+
+def _make_timeout_error(error: BaseException) -> PyMongoError:
+    """Convert error to a NetworkTimeout or ExecutionTimeout as appropriate."""
+    if _csot.remaining() is not None:
+        timeout_error: PyMongoError = ExecutionTimeout(
+            str(error), 50, {"ok": 0, "errmsg": str(error), "code": 50}
+        )
+    else:
+        timeout_error = NetworkTimeout(str(error))
+    if isinstance(error, PyMongoError):
+        timeout_error._error_labels = error._error_labels.copy()
+    return timeout_error
 
 
 _T = TypeVar("_T")
@@ -744,7 +771,17 @@ class AsyncClientSession:
             https://github.com/mongodb/specifications/blob/master/source/transactions-convenient-api/transactions-convenient-api.md#handling-errors-inside-the-callback
         """
         start_time = time.monotonic()
+        retry = 0
+        last_error: Optional[BaseException] = None
         while True:
+            if retry:  # Implement exponential backoff on retry.
+                jitter = random.random()  # noqa: S311
+                backoff = jitter * min(_BACKOFF_INITIAL * (1.5**retry), _BACKOFF_MAX)
+                if not _within_time_limit(start_time, backoff):
+                    assert last_error is not None
+                    raise _make_timeout_error(last_error) from last_error
+                await asyncio.sleep(backoff)
+            retry += 1
             await self.start_transaction(
                 read_concern, write_concern, read_preference, max_commit_time_ms
             )
@@ -752,15 +789,16 @@ class AsyncClientSession:
                 ret = await callback(self)
             # Catch KeyboardInterrupt, CancelledError, etc. and cleanup.
             except BaseException as exc:
+                last_error = exc
                 if self.in_transaction:
                     await self.abort_transaction()
-                if (
-                    isinstance(exc, PyMongoError)
-                    and exc.has_error_label("TransientTransactionError")
-                    and _within_time_limit(start_time)
+                if isinstance(exc, PyMongoError) and exc.has_error_label(
+                    "TransientTransactionError"
                 ):
+                    if _within_time_limit(start_time):
                         # Retry the entire transaction.
                         continue
+                    raise _make_timeout_error(last_error) from exc
                 raise
 
             if not self.in_transaction:
@@ -771,17 +809,18 @@ class AsyncClientSession:
                 try:
                     await self.commit_transaction()
                 except PyMongoError as exc:
-                    if (
-                        exc.has_error_label("UnknownTransactionCommitResult")
-                        and _within_time_limit(start_time)
-                        and not _max_time_expired_error(exc)
-                    ):
+                    last_error = exc
+                    if exc.has_error_label(
+                        "UnknownTransactionCommitResult"
+                    ) and not _max_time_expired_error(exc):
+                        if not _within_time_limit(start_time):
+                            raise _make_timeout_error(last_error) from exc
                         # Retry the commit.
                         continue
 
-                    if exc.has_error_label("TransientTransactionError") and _within_time_limit(
-                        start_time
-                    ):
+                    if exc.has_error_label("TransientTransactionError"):
+                        if not _within_time_limit(start_time):
+                            raise _make_timeout_error(last_error) from exc
                         # Retry the entire transaction.
                         break
                     raise

pymongo/asynchronous/collection.py

@@ -20,7 +20,6 @@ from collections import abc
 from typing import (
     TYPE_CHECKING,
     Any,
-    AsyncContextManager,
     Callable,
     Coroutine,
     Generic,
@@ -571,11 +570,6 @@ class AsyncCollection(common.BaseObject, Generic[_DocumentType]):
         await change_stream._initialize_cursor()
         return change_stream
 
-    async def _conn_for_writes(
-        self, session: Optional[AsyncClientSession], operation: str
-    ) -> AsyncContextManager[AsyncConnection]:
-        return await self._database.client._conn_for_writes(session, operation)
-
     async def _command(
         self,
         conn: AsyncConnection,
@@ -652,7 +646,10 @@ class AsyncCollection(common.BaseObject, Generic[_DocumentType]):
             if "size" in options:
                 options["size"] = float(options["size"])
             cmd.update(options)
-        async with await self._conn_for_writes(session, operation=_Op.CREATE) as conn:
+
+        async def inner(
+            session: Optional[AsyncClientSession], conn: AsyncConnection, _retryable_write: bool
+        ) -> None:
             if qev2_required and conn.max_wire_version < 21:
                 raise ConfigurationError(
                     "Driver support of Queryable Encryption is incompatible with server. "
@@ -669,6 +666,8 @@ class AsyncCollection(common.BaseObject, Generic[_DocumentType]):
                 session=session,
             )
 
+        await self.database.client._retryable_write(False, inner, session, _Op.CREATE)
+
     async def _create(
         self,
         options: MutableMapping[str, Any],
@@ -2240,7 +2239,10 @@ class AsyncCollection(common.BaseObject, Generic[_DocumentType]):
             command (like maxTimeMS) can be passed as keyword arguments.
         """
         names = []
-        async with await self._conn_for_writes(session, operation=_Op.CREATE_INDEXES) as conn:
+
+        async def inner(
+            session: Optional[AsyncClientSession], conn: AsyncConnection, _retryable_write: bool
+        ) -> list[str]:
             supports_quorum = conn.max_wire_version >= 9
 
             def gen_indexes() -> Iterator[Mapping[str, Any]]:
@@ -2271,6 +2273,10 @@ class AsyncCollection(common.BaseObject, Generic[_DocumentType]):
             )
             return names
 
+        return await self.database.client._retryable_write(
+            False, inner, session, _Op.CREATE_INDEXES
+        )
+
     async def create_index(
         self,
         keys: _IndexKeyHint,
@@ -2422,7 +2428,6 @@ class AsyncCollection(common.BaseObject, Generic[_DocumentType]):
             kwargs["comment"] = comment
         await self._drop_index("*", session=session, **kwargs)
 
-    @_csot.apply
     async def drop_index(
         self,
         index_or_name: _IndexKeyHint,
@@ -2490,7 +2495,10 @@ class AsyncCollection(common.BaseObject, Generic[_DocumentType]):
         cmd.update(kwargs)
         if comment is not None:
             cmd["comment"] = comment
-        async with await self._conn_for_writes(session, operation=_Op.DROP_INDEXES) as conn:
+
+        async def inner(
+            session: Optional[AsyncClientSession], conn: AsyncConnection, _retryable_write: bool
+        ) -> None:
             await self._command(
                 conn,
                 cmd,
@@ -2500,6 +2508,8 @@ class AsyncCollection(common.BaseObject, Generic[_DocumentType]):
                 session=session,
             )
 
+        await self.database.client._retryable_write(False, inner, session, _Op.DROP_INDEXES)
+
     async def list_indexes(
         self,
         session: Optional[AsyncClientSession] = None,
@@ -2763,17 +2773,22 @@ class AsyncCollection(common.BaseObject, Generic[_DocumentType]):
         cmd = {"createSearchIndexes": self.name, "indexes": list(gen_indexes())}
         cmd.update(kwargs)
 
-        async with await self._conn_for_writes(
-            session, operation=_Op.CREATE_SEARCH_INDEXES
-        ) as conn:
+        async def inner(
+            session: Optional[AsyncClientSession], conn: AsyncConnection, _retryable_write: bool
+        ) -> list[str]:
             resp = await self._command(
                 conn,
                 cmd,
                 read_preference=ReadPreference.PRIMARY,
                 codec_options=_UNICODE_REPLACE_CODEC_OPTIONS,
+                session=session,
             )
             return [index["name"] for index in resp["indexesCreated"]]
 
+        return await self.database.client._retryable_write(
+            False, inner, session, _Op.CREATE_SEARCH_INDEXES
+        )
+
     async def drop_search_index(
         self,
         name: str,
@@ -2799,15 +2814,21 @@ class AsyncCollection(common.BaseObject, Generic[_DocumentType]):
         cmd.update(kwargs)
         if comment is not None:
             cmd["comment"] = comment
-        async with await self._conn_for_writes(session, operation=_Op.DROP_SEARCH_INDEXES) as conn:
+
+        async def inner(
+            session: Optional[AsyncClientSession], conn: AsyncConnection, _retryable_write: bool
+        ) -> None:
             await self._command(
                 conn,
                 cmd,
                 read_preference=ReadPreference.PRIMARY,
                 allowable_errors=["ns not found", 26],
                 codec_options=_UNICODE_REPLACE_CODEC_OPTIONS,
+                session=session,
             )
 
+        await self.database.client._retryable_write(False, inner, session, _Op.DROP_SEARCH_INDEXES)
+
     async def update_search_index(
         self,
         name: str,
@@ -2835,15 +2856,21 @@ class AsyncCollection(common.BaseObject, Generic[_DocumentType]):
         cmd.update(kwargs)
         if comment is not None:
             cmd["comment"] = comment
-        async with await self._conn_for_writes(session, operation=_Op.UPDATE_SEARCH_INDEX) as conn:
+
+        async def inner(
+            session: Optional[AsyncClientSession], conn: AsyncConnection, _retryable_write: bool
+        ) -> None:
             await self._command(
                 conn,
                 cmd,
                 read_preference=ReadPreference.PRIMARY,
                 allowable_errors=["ns not found", 26],
                 codec_options=_UNICODE_REPLACE_CODEC_OPTIONS,
+                session=session,
             )
 
+        await self.database.client._retryable_write(False, inner, session, _Op.UPDATE_SEARCH_INDEX)
+
     async def options(
         self,
         session: Optional[AsyncClientSession] = None,
@@ -2918,6 +2945,7 @@ class AsyncCollection(common.BaseObject, Generic[_DocumentType]):
             session,
             retryable=not cmd._performs_write,
             operation=_Op.AGGREGATE,
+            is_aggregate_write=cmd._performs_write,
         )
 
     async def aggregate(
@@ -3123,18 +3151,22 @@ class AsyncCollection(common.BaseObject, Generic[_DocumentType]):
         if comment is not None:
             cmd["comment"] = comment
         write_concern = self._write_concern_for_cmd(cmd, session)
+        client = self._database.client
 
-        async with await self._conn_for_writes(session, operation=_Op.RENAME) as conn:
-            async with self._database.client._tmp_session(session) as s:
+        async def inner(
+            session: Optional[AsyncClientSession], conn: AsyncConnection, _retryable_write: bool
+        ) -> MutableMapping[str, Any]:
             return await conn.command(
                 "admin",
                 cmd,
                 write_concern=write_concern,
                 parse_write_concern_error=True,
-                    session=s,
-                    client=self._database.client,
+                session=session,
+                client=client,
             )
 
+        return await client._retryable_write(False, inner, session, _Op.RENAME)
+
     async def distinct(
         self,
         key: str,

pymongo/asynchronous/database.py

@@ -931,14 +931,15 @@ class AsyncDatabase(common.BaseObject, Generic[_DocumentType]):
 
         if read_preference is None:
             read_preference = (session and session._txn_read_preference()) or ReadPreference.PRIMARY
-        async with await self._client._conn_for_reads(
-            read_preference, session, operation=command_name
-        ) as (
-            connection,
-            read_preference,
-        ):
+
+        async def inner(
+            session: Optional[AsyncClientSession],
+            _server: Server,
+            conn: AsyncConnection,
+            read_preference: _ServerMode,
+        ) -> Union[dict[str, Any], _CodecDocumentType]:
             return await self._command(
-                connection,
+                conn,
                 command,
                 value,
                 check,
@@ -949,6 +950,10 @@ class AsyncDatabase(common.BaseObject, Generic[_DocumentType]):
                 **kwargs,
             )
 
+        return await self._client._retryable_read(
+            inner, read_preference, session, command_name, None, False, is_run_command=True
+        )
+
     @_csot.apply
     async def cursor_command(
         self,
@@ -1016,17 +1021,17 @@ class AsyncDatabase(common.BaseObject, Generic[_DocumentType]):
 
         async with self._client._tmp_session(session) as tmp_session:
             opts = codec_options or DEFAULT_CODEC_OPTIONS
-
             if read_preference is None:
                 read_preference = (
                     tmp_session and tmp_session._txn_read_preference()
                 ) or ReadPreference.PRIMARY
-            async with await self._client._conn_for_reads(
-                read_preference, tmp_session, command_name
-            ) as (
-                conn,
-                read_preference,
-            ):
+
+            async def inner(
+                session: Optional[AsyncClientSession],
+                _server: Server,
+                conn: AsyncConnection,
+                read_preference: _ServerMode,
+            ) -> AsyncCommandCursor[_DocumentType]:
                 response = await self._command(
                     conn,
                     command,
@@ -1035,7 +1040,7 @@ class AsyncDatabase(common.BaseObject, Generic[_DocumentType]):
                     None,
                     read_preference,
                     opts,
-                    session=tmp_session,
+                    session=session,
                     **kwargs,
                 )
                 coll = self.get_collection("$cmd", read_preference=read_preference)
@@ -1045,7 +1050,7 @@ class AsyncDatabase(common.BaseObject, Generic[_DocumentType]):
                         response["cursor"],
                         conn.address,
                         max_await_time_ms=max_await_time_ms,
-                        session=tmp_session,
+                        session=session,
                         comment=comment,
                     )
                     await cmd_cursor._maybe_pin_connection(conn)
@@ -1053,6 +1058,10 @@ class AsyncDatabase(common.BaseObject, Generic[_DocumentType]):
                 else:
                     raise InvalidOperation("Command does not return a cursor.")
 
+            return await self.client._retryable_read(
+                inner, read_preference, tmp_session, command_name, None, False
+            )
+
     async def _retryable_read_command(
         self,
         command: Union[str, MutableMapping[str, Any]],
@@ -1254,9 +1263,11 @@ class AsyncDatabase(common.BaseObject, Generic[_DocumentType]):
         if comment is not None:
             command["comment"] = comment
 
-        async with await self._client._conn_for_writes(session, operation=_Op.DROP) as connection:
+        async def inner(
+            session: Optional[AsyncClientSession], conn: AsyncConnection, _retryable_write: bool
+        ) -> dict[str, Any]:
             return await self._command(
-                connection,
+                conn,
                 command,
                 allowable_errors=["ns not found", 26],
                 write_concern=self._write_concern_for(session),
@@ -1264,6 +1275,8 @@ class AsyncDatabase(common.BaseObject, Generic[_DocumentType]):
                 session=session,
             )
 
+        return await self.client._retryable_write(False, inner, session, _Op.DROP)
+
     @_csot.apply
     async def drop_collection(
         self,

pymongo/asynchronous/helpers.py

@@ -17,8 +17,11 @@ from __future__ import annotations
 
 import asyncio
 import builtins
+import functools
+import random
 import socket
 import sys
+import time as time  # noqa: PLC0414 # needed in sync version
 from typing import (
     Any,
     Callable,
@@ -26,6 +29,8 @@ from typing import (
     cast,
 )
 
+from pymongo import _csot
+from pymongo.common import MAX_ADAPTIVE_RETRIES
 from pymongo.errors import (
     OperationFailure,
 )
@@ -38,6 +43,7 @@ F = TypeVar("F", bound=Callable[..., Any])
 
 
 def _handle_reauth(func: F) -> F:
+    @functools.wraps(func)
     async def inner(*args: Any, **kwargs: Any) -> Any:
         no_reauth = kwargs.pop("no_reauth", False)
         from pymongo.asynchronous.pool import AsyncConnection
@@ -70,6 +76,46 @@ def _handle_reauth(func: F) -> F:
     return cast(F, inner)
 
 
+_BACKOFF_INITIAL = 0.1
+_BACKOFF_MAX = 10
+
+
+def _backoff(
+    attempt: int, initial_delay: float = _BACKOFF_INITIAL, max_delay: float = _BACKOFF_MAX
+) -> float:
+    jitter = random.random()  # noqa: S311
+    return jitter * min(initial_delay * (2**attempt), max_delay)
+
+
+class _RetryPolicy:
+    """A retry limiter that performs exponential backoff with jitter."""
+
+    def __init__(
+        self,
+        attempts: int = MAX_ADAPTIVE_RETRIES,
+        backoff_initial: float = _BACKOFF_INITIAL,
+        backoff_max: float = _BACKOFF_MAX,
+    ):
+        self.attempts = attempts
+        self.backoff_initial = backoff_initial
+        self.backoff_max = backoff_max
+
+    def backoff(self, attempt: int) -> float:
+        """Return the backoff duration for the given attempt."""
+        return _backoff(max(0, attempt - 1), self.backoff_initial, self.backoff_max)
+
+    async def should_retry(self, attempt: int, delay: float) -> bool:
+        """Return if we have retry attempts remaining and the next backoff would not exceed a timeout."""
+        if attempt > self.attempts:
+            return False
+
+        if _csot.get_timeout():
+            if time.monotonic() + delay > _csot.get_deadline():
+                return False
+
+        return True
+
+
 async def _getaddrinfo(
     host: Any, port: Any, **kwargs: Any
 ) -> list[

pymongo/asynchronous/mongo_client.py

@@ -35,6 +35,7 @@ from __future__ import annotations
 import asyncio
 import contextlib
 import os
+import time as time  # noqa: PLC0414 # needed in sync version
 import warnings
 import weakref
 from collections import defaultdict
@@ -67,6 +68,9 @@ from pymongo.asynchronous.change_stream import AsyncChangeStream, AsyncClusterCh
 from pymongo.asynchronous.client_bulk import _AsyncClientBulk
 from pymongo.asynchronous.client_session import _SESSION, _EmptyServerSession
 from pymongo.asynchronous.command_cursor import AsyncCommandCursor
+from pymongo.asynchronous.helpers import (
+    _RetryPolicy,
+)
 from pymongo.asynchronous.settings import TopologySettings
 from pymongo.asynchronous.topology import Topology, _ErrorContext
 from pymongo.client_options import ClientOptions
@@ -610,8 +614,18 @@ class AsyncMongoClient(common.BaseObject, Generic[_DocumentType]):
             client to use Stable API. See `versioned API <https://www.mongodb.com/docs/manual/reference/stable-api/#what-is-the-stable-api--and-should-you-use-it->`_ for
             details.
 
+          | **Overload retry options:**
+
+          - `max_adaptive_retries`: (int) How many retries to allow for overload errors. Defaults to ``2``.
+          - `enable_overload_retargeting`: (boolean) Whether overload retargeting is enabled for this client.
+            If enabled, server overload errors will cause retry attempts to select a server that has not yet returned an overload error, if possible.
+            Defaults to ``False``.
+
         .. seealso:: The MongoDB documentation on `connections <https://dochub.mongodb.org/core/connections>`_.
 
+        .. versionchanged:: 4.17
+           Added the ``max_adaptive_retries`` and ``enable_overload_retargeting`` URI and keyword arguments.
+
         .. versionchanged:: 4.5
            Added the ``serverMonitoringMode`` keyword argument.
 
@@ -879,11 +893,14 @@ class AsyncMongoClient(common.BaseObject, Generic[_DocumentType]):
             self._options.read_concern,
         )
 
+        self._retry_policy = _RetryPolicy(attempts=self._options.max_adaptive_retries)
+
         self._init_based_on_options(self._seeds, srv_max_hosts, srv_service_name)
 
         self._opened = False
         self._closed = False
         self._loop: Optional[asyncio.AbstractEventLoop] = None
+
         if not is_srv:
             self._init_background()
 
@@ -1991,6 +2008,8 @@ class AsyncMongoClient(common.BaseObject, Generic[_DocumentType]):
         read_pref: Optional[_ServerMode] = None,
         retryable: bool = False,
         operation_id: Optional[int] = None,
+        is_run_command: bool = False,
+        is_aggregate_write: bool = False,
     ) -> T:
         """Internal retryable helper for all client transactions.
 
@@ -2002,6 +2021,8 @@ class AsyncMongoClient(common.BaseObject, Generic[_DocumentType]):
         :param address: Server Address, defaults to None
         :param read_pref: Topology of read operation, defaults to None
         :param retryable: If the operation should be retried once, defaults to None
+        :param is_run_command: If this is a runCommand operation, defaults to False
+        :param is_aggregate_write: If this is a aggregate operation with a write, defaults to False.
 
         :return: Output of the calling func()
         """
@@ -2016,6 +2037,8 @@ class AsyncMongoClient(common.BaseObject, Generic[_DocumentType]):
             address=address,
             retryable=retryable,
             operation_id=operation_id,
+            is_run_command=is_run_command,
+            is_aggregate_write=is_aggregate_write,
         ).run()
 
     async def _retryable_read(
@@ -2027,6 +2050,8 @@ class AsyncMongoClient(common.BaseObject, Generic[_DocumentType]):
         address: Optional[_Address] = None,
         retryable: bool = True,
         operation_id: Optional[int] = None,
+        is_run_command: bool = False,
+        is_aggregate_write: bool = False,
     ) -> T:
         """Execute an operation with consecutive retries if possible
 
@@ -2042,6 +2067,8 @@ class AsyncMongoClient(common.BaseObject, Generic[_DocumentType]):
         :param address: Optional address when sending a message, defaults to None
         :param retryable: if we should attempt retries
             (may not always be supported even if supplied), defaults to False
+        :param is_run_command: If this is a runCommand operation, defaults to False.
+        :param is_aggregate_write: If this is a aggregate operation with a write, defaults to False.
         """
 
         # Ensure that the client supports retrying on reads and there is no session in
@@ -2060,6 +2087,8 @@ class AsyncMongoClient(common.BaseObject, Generic[_DocumentType]):
                 read_pref=read_pref,
                 retryable=retryable,
                 operation_id=operation_id,
+                is_run_command=is_run_command,
+                is_aggregate_write=is_aggregate_write,
             )
 
     async def _retryable_write(
@@ -2454,9 +2483,7 @@ class AsyncMongoClient(common.BaseObject, Generic[_DocumentType]):
                 f"name_or_database must be an instance of str or a AsyncDatabase, not {type(name)}"
             )
 
-        async with await self._conn_for_writes(session, operation=_Op.DROP_DATABASE) as conn:
-            await self[name]._command(
-                conn,
+        await self[name].command(
             {"dropDatabase": 1, "comment": comment},
             read_preference=ReadPreference.PRIMARY,
             write_concern=self._write_concern_for(session),
@@ -2746,12 +2773,15 @@ class _ClientConnectionRetryable(Generic[T]):
         address: Optional[_Address] = None,
         retryable: bool = False,
         operation_id: Optional[int] = None,
+        is_run_command: bool = False,
+        is_aggregate_write: bool = False,
     ):
         self._last_error: Optional[Exception] = None
         self._retrying = False
+        self._always_retryable = False
         self._multiple_retries = _csot.get_timeout() is not None
         self._client = mongo_client
-
+        self._retry_policy = mongo_client._retry_policy
         self._func = func
         self._bulk = bulk
         self._session = session
@@ -2767,6 +2797,8 @@ class _ClientConnectionRetryable(Generic[T]):
         self._operation = operation
         self._operation_id = operation_id
         self._attempt_number = 0
+        self._is_run_command = is_run_command
+        self._is_aggregate_write = is_aggregate_write
 
     async def run(self) -> T:
         """Runs the supplied func() and attempts a retry
@@ -2786,7 +2818,13 @@ class _ClientConnectionRetryable(Generic[T]):
         while True:
             self._check_last_error(check_csot=True)
             try:
-                return await self._read() if self._is_read else await self._write()
+                res = await self._read() if self._is_read else await self._write()
+                # Track whether the transaction has completed a command.
+                # If we need to apply backpressure to the first command,
+                # we will need to revert back to starting state.
+                if self._session is not None and self._session.in_transaction:
+                    self._session._transaction.has_completed_command = True
+                return res
             except ServerSelectionTimeoutError:
                 # The application may think the write was never attempted
                 # if we raise ServerSelectionTimeoutError on the retry
@@ -2797,37 +2835,76 @@ class _ClientConnectionRetryable(Generic[T]):
                 # most likely be a waste of time.
                 raise
             except PyMongoError as exc:
+                always_retryable = False
+                overloaded = False
+                exc_to_check = exc
+
+                if self._is_run_command and not (
+                    self._client.options.retry_reads and self._client.options.retry_writes
+                ):
+                    raise
+                if self._is_aggregate_write and not self._client.options.retry_writes:
+                    raise
+
                 # Execute specialized catch on read
                 if self._is_read:
                     if isinstance(exc, (ConnectionFailure, OperationFailure)):
                         # ConnectionFailures do not supply a code property
                         exc_code = getattr(exc, "code", None)
-                        if self._is_not_eligible_for_retry() or (
+                        overloaded = exc.has_error_label("SystemOverloadedError")
+                        always_retryable = exc.has_error_label("RetryableError") and overloaded
+                        if not self._client.options.retry_reads or (
+                            not always_retryable
+                            and (
+                                self._is_not_eligible_for_retry()
+                                or (
                                     isinstance(exc, OperationFailure)
                                     and exc_code not in helpers_shared._RETRYABLE_ERROR_CODES
+                                )
+                            )
                         ):
                             raise
                         self._retrying = True
                         self._last_error = exc
                         self._attempt_number += 1
+
+                        # Revert back to starting state if we're in a transaction but haven't completed the first
+                        # command.
+                        if (
+                            overloaded
+                            and self._session is not None
+                            and self._session.in_transaction
+                        ):
+                            transaction = self._session._transaction
+                            if not transaction.has_completed_command:
+                                transaction.set_starting()
+                            transaction.attempt = 0
                     else:
                         raise
 
                 # Specialized catch on write operation
                 if not self._is_read:
-                    if not self._retryable:
-                        raise
-                    if isinstance(exc, ClientBulkWriteException) and exc.error:
-                        retryable_write_error_exc = isinstance(
+                    if isinstance(exc, ClientBulkWriteException) and isinstance(
                         exc.error, PyMongoError
-                        ) and exc.error.has_error_label("RetryableWriteError")
-                    else:
-                        retryable_write_error_exc = exc.has_error_label("RetryableWriteError")
-                    if retryable_write_error_exc:
+                    ):
+                        exc_to_check = exc.error
+                    retryable_write_label = exc_to_check.has_error_label("RetryableWriteError")
+                    overloaded = exc_to_check.has_error_label("SystemOverloadedError")
+                    always_retryable = exc_to_check.has_error_label("RetryableError") and overloaded
+
+                    # Always retry abortTransaction and commitTransaction up to once
+                    if self._operation not in ["abortTransaction", "commitTransaction"] and (
+                        not self._client.options.retry_writes
+                        or not (self._retryable or always_retryable)
+                    ):
+                        raise
+                    if retryable_write_label or always_retryable:
                         assert self._session
                         await self._session._unpin()
-                    if not retryable_write_error_exc or self._is_not_eligible_for_retry():
-                        if exc.has_error_label("NoWritesPerformed") and self._last_error:
+                    if not always_retryable and (
+                        not retryable_write_label or self._is_not_eligible_for_retry()
+                    ):
+                        if exc_to_check.has_error_label("NoWritesPerformed") and self._last_error:
                             raise self._last_error from exc
                         else:
                             raise
@@ -2836,18 +2913,34 @@ class _ClientConnectionRetryable(Generic[T]):
                         self._bulk.retrying = True
                     else:
                         self._retrying = True
-                    if not exc.has_error_label("NoWritesPerformed"):
+                    if not exc_to_check.has_error_label("NoWritesPerformed"):
                         self._last_error = exc
                     if self._last_error is None:
                         self._last_error = exc
+                    # Revert back to starting state if we're in a transaction but haven't completed the first
+                    # command.
+                    if overloaded and self._session is not None and self._session.in_transaction:
+                        transaction = self._session._transaction
+                        if not transaction.has_completed_command:
+                            transaction.set_starting()
+                        transaction.attempt = 0
 
-                if (
-                    self._server is not None
-                    and self._client.topology_description.topology_type_name == "Sharded"
-                    or exc.has_error_label("SystemOverloadedError")
+                if self._server is not None and (
+                    self._client.topology_description.topology_type_name == "Sharded"
+                    or (overloaded and self._client.options.enable_overload_retargeting)
                 ):
                     self._deprioritized_servers.append(self._server)
 
+                self._always_retryable = always_retryable
+                if overloaded:
+                    delay = self._retry_policy.backoff(self._attempt_number)
+                    if not await self._retry_policy.should_retry(self._attempt_number, delay):
+                        if exc_to_check.has_error_label("NoWritesPerformed") and self._last_error:
+                            raise self._last_error from exc
+                        else:
+                            raise
+                    await asyncio.sleep(delay)
+
     def _is_not_eligible_for_retry(self) -> bool:
         """Checks if the exchange is not eligible for retry"""
         return not self._retryable or (self._is_retrying() and not self._multiple_retries)
@@ -2909,7 +3002,7 @@ class _ClientConnectionRetryable(Generic[T]):
                     and conn.supports_sessions
                 )
                 is_mongos = conn.is_mongos
-                if not sessions_supported:
+                if not self._always_retryable and not sessions_supported:
                     # A retry is not possible because this server does
                     # not support sessions raise the last error.
                     self._check_last_error()
@@ -2941,7 +3034,7 @@ class _ClientConnectionRetryable(Generic[T]):
             conn,
             read_pref,
         ):
-            if self._retrying and not self._retryable:
+            if self._retrying and not self._retryable and not self._always_retryable:
                 self._check_last_error()
             if self._retrying:
                 _debug_log(

pymongo/asynchronous/pool.py

@@ -19,6 +19,8 @@ import collections
 import contextlib
 import logging
 import os
+import socket
+import ssl
 import sys
 import time
 import weakref
@@ -52,10 +54,12 @@ from pymongo.errors import (  # type:ignore[attr-defined]
     DocumentTooLarge,
     ExecutionTimeout,
     InvalidOperation,
+    NetworkTimeout,
     NotPrimaryError,
     OperationFailure,
     PyMongoError,
     WaitQueueTimeoutError,
+    _CertificateError,
 )
 from pymongo.hello import Hello, HelloCompat
 from pymongo.helpers_shared import _get_timeout_details, format_timeout_details
@@ -250,6 +254,7 @@ class AsyncConnection:
         cmd = self.hello_cmd()
         performing_handshake = not self.performed_handshake
         awaitable = False
+        cmd["backpressure"] = True
         if performing_handshake:
             self.performed_handshake = True
             cmd["client"] = self.opts.metadata
@@ -752,8 +757,8 @@ class Pool:
         # Enforces: maxConnecting
         # Also used for: clearing the wait queue
         self._max_connecting_cond = _async_create_condition(self.lock)
-        self._max_connecting = self.opts.max_connecting
         self._pending = 0
+        self._max_connecting = self.opts.max_connecting
         self._client_id = client_id
         if self.enabled_for_cmap:
             assert self.opts._event_listeners is not None
@@ -986,6 +991,21 @@ class Pool:
                     self.requests -= 1
                     self.size_cond.notify()
 
+    def _handle_connection_error(self, error: BaseException) -> None:
+        # Handle system overload condition for non-sdam pools.
+        # Look for errors of type AutoReconnect and add error labels if appropriate.
+        if self.is_sdam or type(error) not in (AutoReconnect, NetworkTimeout):
+            return
+        assert isinstance(error, AutoReconnect)  # Appease type checker.
+        # If the original error was a DNS, certificate, or SSL error, ignore it.
+        if isinstance(error.__cause__, (_CertificateError, SSLErrors, socket.gaierror)):
+            # End of file errors are excluded, because the server may have disconnected
+            # during the handshake.
+            if not isinstance(error.__cause__, (ssl.SSLEOFError, ssl.SSLZeroReturnError)):
+                return
+        error._add_error_label("SystemOverloadedError")
+        error._add_error_label("RetryableError")
+
     async def connect(self, handler: Optional[_MongoClientErrorHandler] = None) -> AsyncConnection:
         """Connect to Mongo and return a new AsyncConnection.
 
@@ -1037,10 +1057,10 @@ class Pool:
                     reason=_verbose_connection_error_reason(ConnectionClosedReason.ERROR),
                     error=ConnectionClosedReason.ERROR,
                 )
+            self._handle_connection_error(error)
             if isinstance(error, (IOError, OSError, *SSLErrors)):
                 details = _get_timeout_details(self.opts)
                 _raise_connection_failure(self.address, error, timeout_details=details)
-
             raise
 
         conn = AsyncConnection(networking_interface, self, self.address, conn_id, self.is_sdam)  # type: ignore[arg-type]
@@ -1049,18 +1069,22 @@ class Pool:
             self.active_contexts.discard(tmp_context)
         if tmp_context.cancelled:
             conn.cancel_context.cancel()
+        completed_hello = False
         try:
             if not self.is_sdam:
                 await conn.hello()
+                completed_hello = True
                 self.is_writable = conn.is_writable
             if handler:
                 handler.contribute_socket(conn, completed_handshake=False)
 
             await conn.authenticate()
         # Catch KeyboardInterrupt, CancelledError, etc. and cleanup.
-        except BaseException:
+        except BaseException as e:
             async with self.lock:
                 self.active_contexts.discard(conn.cancel_context)
+            if not completed_hello:
+                self._handle_connection_error(e)
             await conn.close_conn(ConnectionClosedReason.ERROR)
             raise
 
@@ -1389,8 +1413,8 @@ class Pool:
         :class:`~pymongo.errors.AutoReconnect` exceptions on server
         hiccups, etc. We only check if the socket was closed by an external
         error if it has been > 1 second since the socket was checked into the
-        pool, to keep performance reasonable - we can't avoid AutoReconnects
-        completely anyway.
+        pool to keep performance reasonable -
+        we can't avoid AutoReconnects completely anyway.
         """
         idle_time_seconds = conn.idle_time_seconds()
         # If socket is idle, open a new one.
@@ -1401,8 +1425,9 @@ class Pool:
             await conn.close_conn(ConnectionClosedReason.IDLE)
             return True
 
-        if self._check_interval_seconds is not None and (
-            self._check_interval_seconds == 0 or idle_time_seconds > self._check_interval_seconds
+        check_interval_seconds = self._check_interval_seconds
+        if check_interval_seconds is not None and (
+            check_interval_seconds == 0 or idle_time_seconds > check_interval_seconds
         ):
             if conn.conn_closed():
                 await conn.close_conn(ConnectionClosedReason.ERROR)

pymongo/asynchronous/topology.py

@@ -913,7 +913,9 @@ class Topology:
                 # Clear the pool.
                 await server.reset(service_id)
         elif isinstance(error, ConnectionFailure):
-            if isinstance(error, WaitQueueTimeoutError):
+            if isinstance(error, WaitQueueTimeoutError) or (
+                error.has_error_label("SystemOverloadedError")
+            ):
                 return
             # "Client MUST replace the server's description with type Unknown
             # ... MUST NOT request an immediate check of the server."

pymongo/client_options.py

@@ -235,6 +235,16 @@ class ClientOptions:
         self.__server_monitoring_mode = options.get(
             "servermonitoringmode", common.SERVER_MONITORING_MODE
         )
+        self.__max_adaptive_retries = (
+            options.get("max_adaptive_retries", common.MAX_ADAPTIVE_RETRIES)
+            if "max_adaptive_retries" in options
+            else options.get("maxadaptiveretries", common.MAX_ADAPTIVE_RETRIES)
+        )
+        self.__enable_overload_retargeting = (
+            options.get("enable_overload_retargeting", common.ENABLE_OVERLOAD_RETARGETING)
+            if "enable_overload_retargeting" in options
+            else options.get("enableoverloadretargeting", common.ENABLE_OVERLOAD_RETARGETING)
+        )
 
     @property
     def _options(self) -> Mapping[str, Any]:
@@ -346,3 +356,19 @@ class ClientOptions:
         .. versionadded:: 4.5
         """
         return self.__server_monitoring_mode
+
+    @property
+    def max_adaptive_retries(self) -> int:
+        """The configured maxAdaptiveRetries option.
+
+        .. versionadded:: 4.17
+        """
+        return self.__max_adaptive_retries
+
+    @property
+    def enable_overload_retargeting(self) -> bool:
+        """The configured enableOverloadRetargeting option.
+
+        .. versionadded:: 4.17
+        """
+        return self.__enable_overload_retargeting

pymongo/common.py

@@ -140,6 +140,12 @@ SRV_SERVICE_NAME = "mongodb"
 # Default value for serverMonitoringMode
 SERVER_MONITORING_MODE = "auto"  # poll/stream/auto
 
+# Default value for max adaptive retries
+MAX_ADAPTIVE_RETRIES = 2
+
+# Default value for enableOverloadRetargeting
+ENABLE_OVERLOAD_RETARGETING = False
+
 # Auth mechanism properties that must raise an error instead of warning if they invalidate.
 _MECH_PROP_MUST_RAISE = ["CANONICALIZE_HOST_NAME"]
 
@@ -717,6 +723,8 @@ URI_OPTIONS_VALIDATOR_MAP: dict[str, Callable[[Any, Any], Any]] = {
     "srvmaxhosts": validate_non_negative_integer,
     "timeoutms": validate_timeoutms,
     "servermonitoringmode": validate_server_monitoring_mode,
+    "maxadaptiveretries": validate_non_negative_integer,
+    "enableoverloadretargeting": validate_boolean_or_string,
 }
 
 # Dictionary where keys are the names of URI options specific to pymongo,
@@ -750,6 +758,8 @@ KW_VALIDATORS: dict[str, Callable[[Any, Any], Any]] = {
     "server_selector": validate_is_callable_or_none,
     "auto_encryption_opts": validate_auto_encryption_opts_or_none,
     "authoidcallowedhosts": validate_list,
+    "max_adaptive_retries": validate_non_negative_integer,
+    "enable_overload_retargeting": validate_boolean_or_string,
 }
 
 # Dictionary where keys are any URI option name, and values are the

pymongo/synchronous/client_bulk.py

@@ -59,6 +59,7 @@ from pymongo.errors import (
     InvalidOperation,
     NotPrimaryError,
     OperationFailure,
+    PyMongoError,
     WaitQueueTimeoutError,
 )
 from pymongo.helpers_shared import _RETRYABLE_ERROR_CODES
@@ -561,9 +562,17 @@ class _ClientBulk:
                         error, ConnectionFailure
                     ) and not isinstance(error, (NotPrimaryError, WaitQueueTimeoutError))
 
+                    retryable_label_error = isinstance(
+                        error, PyMongoError
+                    ) and error.has_error_label("RetryableError")
+
                     # Synthesize the full bulk result without modifying the
                     # current one because this write operation may be retried.
-                    if retryable and (retryable_top_level_error or retryable_network_error):
+                    if retryable and (
+                        retryable_top_level_error
+                        or retryable_network_error
+                        or retryable_label_error
+                    ):
                         full = copy.deepcopy(full_result)
                         _merge_command(self.ops, self.idx_offset, full, result)
                         _throw_client_bulk_write_exception(full, self.verbose_results)

pymongo/synchronous/client_session.py

@@ -136,6 +136,7 @@ Classes
 from __future__ import annotations
 
 import collections
+import random
 import time
 import uuid
 from collections.abc import Mapping as _Mapping
@@ -160,7 +161,9 @@ from pymongo import _csot
 from pymongo.errors import (
     ConfigurationError,
     ConnectionFailure,
+    ExecutionTimeout,
     InvalidOperation,
+    NetworkTimeout,
     OperationFailure,
     PyMongoError,
     WTimeoutError,
@@ -426,6 +429,7 @@ class _Transaction:
         self.recovery_token = None
         self.attempt = 0
         self.client = client
+        self.has_completed_command = False
 
     def active(self) -> bool:
         return self.state in (_TxnState.STARTING, _TxnState.IN_PROGRESS)
@@ -433,6 +437,9 @@ class _Transaction:
     def starting(self) -> bool:
         return self.state == _TxnState.STARTING
 
+    def set_starting(self) -> None:
+        self.state = _TxnState.STARTING
+
     @property
     def pinned_conn(self) -> Optional[Connection]:
         if self.active() and self.conn_mgr:
@@ -458,6 +465,7 @@ class _Transaction:
         self.sharded = False
         self.recovery_token = None
         self.attempt = 0
+        self.has_completed_command = False
 
     def __del__(self) -> None:
         if self.conn_mgr:
@@ -492,11 +500,29 @@ _UNKNOWN_COMMIT_ERROR_CODES: frozenset = _RETRYABLE_ERROR_CODES | frozenset(  #
 # This limit is non-configurable and was chosen to be twice the 60 second
 # default value of MongoDB's `transactionLifetimeLimitSeconds` parameter.
 _WITH_TRANSACTION_RETRY_TIME_LIMIT = 120
+_BACKOFF_MAX = 0.500  # 500ms max backoff
+_BACKOFF_INITIAL = 0.005  # 5ms initial backoff
 
 
-def _within_time_limit(start_time: float) -> bool:
+def _within_time_limit(start_time: float, backoff: float = 0) -> bool:
     """Are we within the with_transaction retry limit?"""
-    return time.monotonic() - start_time < _WITH_TRANSACTION_RETRY_TIME_LIMIT
+    remaining = _csot.remaining()
+    if remaining is not None and remaining <= 0:
+        return False
+    return time.monotonic() + backoff - start_time < _WITH_TRANSACTION_RETRY_TIME_LIMIT
+
+
+def _make_timeout_error(error: BaseException) -> PyMongoError:
+    """Convert error to a NetworkTimeout or ExecutionTimeout as appropriate."""
+    if _csot.remaining() is not None:
+        timeout_error: PyMongoError = ExecutionTimeout(
+            str(error), 50, {"ok": 0, "errmsg": str(error), "code": 50}
+        )
+    else:
+        timeout_error = NetworkTimeout(str(error))
+    if isinstance(error, PyMongoError):
+        timeout_error._error_labels = error._error_labels.copy()
+    return timeout_error
 
 
 _T = TypeVar("_T")
@@ -743,21 +769,32 @@ class ClientSession:
             https://github.com/mongodb/specifications/blob/master/source/transactions-convenient-api/transactions-convenient-api.md#handling-errors-inside-the-callback
         """
         start_time = time.monotonic()
+        retry = 0
+        last_error: Optional[BaseException] = None
         while True:
+            if retry:  # Implement exponential backoff on retry.
+                jitter = random.random()  # noqa: S311
+                backoff = jitter * min(_BACKOFF_INITIAL * (1.5**retry), _BACKOFF_MAX)
+                if not _within_time_limit(start_time, backoff):
+                    assert last_error is not None
+                    raise _make_timeout_error(last_error) from last_error
+                time.sleep(backoff)
+            retry += 1
             self.start_transaction(read_concern, write_concern, read_preference, max_commit_time_ms)
             try:
                 ret = callback(self)
             # Catch KeyboardInterrupt, CancelledError, etc. and cleanup.
             except BaseException as exc:
+                last_error = exc
                 if self.in_transaction:
                     self.abort_transaction()
-                if (
-                    isinstance(exc, PyMongoError)
-                    and exc.has_error_label("TransientTransactionError")
-                    and _within_time_limit(start_time)
+                if isinstance(exc, PyMongoError) and exc.has_error_label(
+                    "TransientTransactionError"
                 ):
+                    if _within_time_limit(start_time):
                         # Retry the entire transaction.
                         continue
+                    raise _make_timeout_error(last_error) from exc
                 raise
 
             if not self.in_transaction:
@@ -768,17 +805,18 @@ class ClientSession:
                 try:
                     self.commit_transaction()
                 except PyMongoError as exc:
-                    if (
-                        exc.has_error_label("UnknownTransactionCommitResult")
-                        and _within_time_limit(start_time)
-                        and not _max_time_expired_error(exc)
-                    ):
+                    last_error = exc
+                    if exc.has_error_label(
+                        "UnknownTransactionCommitResult"
+                    ) and not _max_time_expired_error(exc):
+                        if not _within_time_limit(start_time):
+                            raise _make_timeout_error(last_error) from exc
                         # Retry the commit.
                         continue
 
-                    if exc.has_error_label("TransientTransactionError") and _within_time_limit(
-                        start_time
-                    ):
+                    if exc.has_error_label("TransientTransactionError"):
+                        if not _within_time_limit(start_time):
+                            raise _make_timeout_error(last_error) from exc
                         # Retry the entire transaction.
                         break
                     raise

pymongo/synchronous/collection.py

@@ -21,7 +21,6 @@ from typing import (
     TYPE_CHECKING,
     Any,
     Callable,
-    ContextManager,
     Generic,
     Iterable,
     Iterator,
@@ -572,11 +571,6 @@ class Collection(common.BaseObject, Generic[_DocumentType]):
         change_stream._initialize_cursor()
         return change_stream
 
-    def _conn_for_writes(
-        self, session: Optional[ClientSession], operation: str
-    ) -> ContextManager[Connection]:
-        return self._database.client._conn_for_writes(session, operation)
-
     def _command(
         self,
         conn: Connection,
@@ -653,7 +647,10 @@ class Collection(common.BaseObject, Generic[_DocumentType]):
             if "size" in options:
                 options["size"] = float(options["size"])
             cmd.update(options)
-        with self._conn_for_writes(session, operation=_Op.CREATE) as conn:
+
+        def inner(
+            session: Optional[ClientSession], conn: Connection, _retryable_write: bool
+        ) -> None:
             if qev2_required and conn.max_wire_version < 21:
                 raise ConfigurationError(
                     "Driver support of Queryable Encryption is incompatible with server. "
@@ -670,6 +667,8 @@ class Collection(common.BaseObject, Generic[_DocumentType]):
                 session=session,
             )
 
+        self.database.client._retryable_write(False, inner, session, _Op.CREATE)
+
     def _create(
         self,
         options: MutableMapping[str, Any],
@@ -2237,7 +2236,10 @@ class Collection(common.BaseObject, Generic[_DocumentType]):
             command (like maxTimeMS) can be passed as keyword arguments.
         """
         names = []
-        with self._conn_for_writes(session, operation=_Op.CREATE_INDEXES) as conn:
+
+        def inner(
+            session: Optional[ClientSession], conn: Connection, _retryable_write: bool
+        ) -> list[str]:
             supports_quorum = conn.max_wire_version >= 9
 
             def gen_indexes() -> Iterator[Mapping[str, Any]]:
@@ -2268,6 +2270,8 @@ class Collection(common.BaseObject, Generic[_DocumentType]):
             )
             return names
 
+        return self.database.client._retryable_write(False, inner, session, _Op.CREATE_INDEXES)
+
     def create_index(
         self,
         keys: _IndexKeyHint,
@@ -2419,7 +2423,6 @@ class Collection(common.BaseObject, Generic[_DocumentType]):
             kwargs["comment"] = comment
         self._drop_index("*", session=session, **kwargs)
 
-    @_csot.apply
     def drop_index(
         self,
         index_or_name: _IndexKeyHint,
@@ -2487,7 +2490,10 @@ class Collection(common.BaseObject, Generic[_DocumentType]):
         cmd.update(kwargs)
         if comment is not None:
             cmd["comment"] = comment
-        with self._conn_for_writes(session, operation=_Op.DROP_INDEXES) as conn:
+
+        def inner(
+            session: Optional[ClientSession], conn: Connection, _retryable_write: bool
+        ) -> None:
             self._command(
                 conn,
                 cmd,
@@ -2497,6 +2503,8 @@ class Collection(common.BaseObject, Generic[_DocumentType]):
                 session=session,
             )
 
+        self.database.client._retryable_write(False, inner, session, _Op.DROP_INDEXES)
+
     def list_indexes(
         self,
         session: Optional[ClientSession] = None,
@@ -2760,15 +2768,22 @@ class Collection(common.BaseObject, Generic[_DocumentType]):
         cmd = {"createSearchIndexes": self.name, "indexes": list(gen_indexes())}
         cmd.update(kwargs)
 
-        with self._conn_for_writes(session, operation=_Op.CREATE_SEARCH_INDEXES) as conn:
+        def inner(
+            session: Optional[ClientSession], conn: Connection, _retryable_write: bool
+        ) -> list[str]:
             resp = self._command(
                 conn,
                 cmd,
                 read_preference=ReadPreference.PRIMARY,
                 codec_options=_UNICODE_REPLACE_CODEC_OPTIONS,
+                session=session,
             )
             return [index["name"] for index in resp["indexesCreated"]]
 
+        return self.database.client._retryable_write(
+            False, inner, session, _Op.CREATE_SEARCH_INDEXES
+        )
+
     def drop_search_index(
         self,
         name: str,
@@ -2794,15 +2809,21 @@ class Collection(common.BaseObject, Generic[_DocumentType]):
         cmd.update(kwargs)
         if comment is not None:
             cmd["comment"] = comment
-        with self._conn_for_writes(session, operation=_Op.DROP_SEARCH_INDEXES) as conn:
+
+        def inner(
+            session: Optional[ClientSession], conn: Connection, _retryable_write: bool
+        ) -> None:
             self._command(
                 conn,
                 cmd,
                 read_preference=ReadPreference.PRIMARY,
                 allowable_errors=["ns not found", 26],
                 codec_options=_UNICODE_REPLACE_CODEC_OPTIONS,
+                session=session,
             )
 
+        self.database.client._retryable_write(False, inner, session, _Op.DROP_SEARCH_INDEXES)
+
     def update_search_index(
         self,
         name: str,
@@ -2830,15 +2851,21 @@ class Collection(common.BaseObject, Generic[_DocumentType]):
         cmd.update(kwargs)
         if comment is not None:
             cmd["comment"] = comment
-        with self._conn_for_writes(session, operation=_Op.UPDATE_SEARCH_INDEX) as conn:
+
+        def inner(
+            session: Optional[ClientSession], conn: Connection, _retryable_write: bool
+        ) -> None:
             self._command(
                 conn,
                 cmd,
                 read_preference=ReadPreference.PRIMARY,
                 allowable_errors=["ns not found", 26],
                 codec_options=_UNICODE_REPLACE_CODEC_OPTIONS,
+                session=session,
             )
 
+        self.database.client._retryable_write(False, inner, session, _Op.UPDATE_SEARCH_INDEX)
+
     def options(
         self,
         session: Optional[ClientSession] = None,
@@ -2911,6 +2938,7 @@ class Collection(common.BaseObject, Generic[_DocumentType]):
             session,
             retryable=not cmd._performs_write,
             operation=_Op.AGGREGATE,
+            is_aggregate_write=cmd._performs_write,
         )
 
     def aggregate(
@@ -3116,18 +3144,22 @@ class Collection(common.BaseObject, Generic[_DocumentType]):
         if comment is not None:
             cmd["comment"] = comment
         write_concern = self._write_concern_for_cmd(cmd, session)
+        client = self._database.client
 
-        with self._conn_for_writes(session, operation=_Op.RENAME) as conn:
-            with self._database.client._tmp_session(session) as s:
+        def inner(
+            session: Optional[ClientSession], conn: Connection, _retryable_write: bool
+        ) -> MutableMapping[str, Any]:
             return conn.command(
                 "admin",
                 cmd,
                 write_concern=write_concern,
                 parse_write_concern_error=True,
-                    session=s,
-                    client=self._database.client,
+                session=session,
+                client=client,
             )
 
+        return client._retryable_write(False, inner, session, _Op.RENAME)
+
     def distinct(
         self,
         key: str,

pymongo/synchronous/database.py

@@ -931,12 +931,15 @@ class Database(common.BaseObject, Generic[_DocumentType]):
 
         if read_preference is None:
             read_preference = (session and session._txn_read_preference()) or ReadPreference.PRIMARY
-        with self._client._conn_for_reads(read_preference, session, operation=command_name) as (
-            connection,
-            read_preference,
-        ):
+
+        def inner(
+            session: Optional[ClientSession],
+            _server: Server,
+            conn: Connection,
+            read_preference: _ServerMode,
+        ) -> Union[dict[str, Any], _CodecDocumentType]:
             return self._command(
-                connection,
+                conn,
                 command,
                 value,
                 check,
@@ -947,6 +950,10 @@ class Database(common.BaseObject, Generic[_DocumentType]):
                 **kwargs,
             )
 
+        return self._client._retryable_read(
+            inner, read_preference, session, command_name, None, False, is_run_command=True
+        )
+
     @_csot.apply
     def cursor_command(
         self,
@@ -1014,15 +1021,17 @@ class Database(common.BaseObject, Generic[_DocumentType]):
 
         with self._client._tmp_session(session) as tmp_session:
             opts = codec_options or DEFAULT_CODEC_OPTIONS
-
             if read_preference is None:
                 read_preference = (
                     tmp_session and tmp_session._txn_read_preference()
                 ) or ReadPreference.PRIMARY
-            with self._client._conn_for_reads(read_preference, tmp_session, command_name) as (
-                conn,
-                read_preference,
-            ):
+
+            def inner(
+                session: Optional[ClientSession],
+                _server: Server,
+                conn: Connection,
+                read_preference: _ServerMode,
+            ) -> CommandCursor[_DocumentType]:
                 response = self._command(
                     conn,
                     command,
@@ -1031,7 +1040,7 @@ class Database(common.BaseObject, Generic[_DocumentType]):
                     None,
                     read_preference,
                     opts,
-                    session=tmp_session,
+                    session=session,
                     **kwargs,
                 )
                 coll = self.get_collection("$cmd", read_preference=read_preference)
@@ -1041,7 +1050,7 @@ class Database(common.BaseObject, Generic[_DocumentType]):
                         response["cursor"],
                         conn.address,
                         max_await_time_ms=max_await_time_ms,
-                        session=tmp_session,
+                        session=session,
                         comment=comment,
                     )
                     cmd_cursor._maybe_pin_connection(conn)
@@ -1049,6 +1058,10 @@ class Database(common.BaseObject, Generic[_DocumentType]):
                 else:
                     raise InvalidOperation("Command does not return a cursor.")
 
+            return self.client._retryable_read(
+                inner, read_preference, tmp_session, command_name, None, False
+            )
+
     def _retryable_read_command(
         self,
         command: Union[str, MutableMapping[str, Any]],
@@ -1247,9 +1260,11 @@ class Database(common.BaseObject, Generic[_DocumentType]):
         if comment is not None:
             command["comment"] = comment
 
-        with self._client._conn_for_writes(session, operation=_Op.DROP) as connection:
+        def inner(
+            session: Optional[ClientSession], conn: Connection, _retryable_write: bool
+        ) -> dict[str, Any]:
             return self._command(
-                connection,
+                conn,
                 command,
                 allowable_errors=["ns not found", 26],
                 write_concern=self._write_concern_for(session),
@@ -1257,6 +1272,8 @@ class Database(common.BaseObject, Generic[_DocumentType]):
                 session=session,
             )
 
+        return self.client._retryable_write(False, inner, session, _Op.DROP)
+
     @_csot.apply
     def drop_collection(
         self,

pymongo/synchronous/helpers.py

@@ -17,8 +17,11 @@ from __future__ import annotations
 
 import asyncio
 import builtins
+import functools
+import random
 import socket
 import sys
+import time as time  # noqa: PLC0414 # needed in sync version
 from typing import (
     Any,
     Callable,
@@ -26,6 +29,8 @@ from typing import (
     cast,
 )
 
+from pymongo import _csot
+from pymongo.common import MAX_ADAPTIVE_RETRIES
 from pymongo.errors import (
     OperationFailure,
 )
@@ -38,6 +43,7 @@ F = TypeVar("F", bound=Callable[..., Any])
 
 
 def _handle_reauth(func: F) -> F:
+    @functools.wraps(func)
     def inner(*args: Any, **kwargs: Any) -> Any:
         no_reauth = kwargs.pop("no_reauth", False)
         from pymongo.message import _BulkWriteContext
@@ -70,6 +76,46 @@ def _handle_reauth(func: F) -> F:
     return cast(F, inner)
 
 
+_BACKOFF_INITIAL = 0.1
+_BACKOFF_MAX = 10
+
+
+def _backoff(
+    attempt: int, initial_delay: float = _BACKOFF_INITIAL, max_delay: float = _BACKOFF_MAX
+) -> float:
+    jitter = random.random()  # noqa: S311
+    return jitter * min(initial_delay * (2**attempt), max_delay)
+
+
+class _RetryPolicy:
+    """A retry limiter that performs exponential backoff with jitter."""
+
+    def __init__(
+        self,
+        attempts: int = MAX_ADAPTIVE_RETRIES,
+        backoff_initial: float = _BACKOFF_INITIAL,
+        backoff_max: float = _BACKOFF_MAX,
+    ):
+        self.attempts = attempts
+        self.backoff_initial = backoff_initial
+        self.backoff_max = backoff_max
+
+    def backoff(self, attempt: int) -> float:
+        """Return the backoff duration for the given attempt."""
+        return _backoff(max(0, attempt - 1), self.backoff_initial, self.backoff_max)
+
+    def should_retry(self, attempt: int, delay: float) -> bool:
+        """Return if we have retry attempts remaining and the next backoff would not exceed a timeout."""
+        if attempt > self.attempts:
+            return False
+
+        if _csot.get_timeout():
+            if time.monotonic() + delay > _csot.get_deadline():
+                return False
+
+        return True
+
+
 def _getaddrinfo(
     host: Any, port: Any, **kwargs: Any
 ) -> list[

pymongo/synchronous/mongo_client.py

@@ -35,6 +35,7 @@ from __future__ import annotations
 import asyncio
 import contextlib
 import os
+import time as time  # noqa: PLC0414 # needed in sync version
 import warnings
 import weakref
 from collections import defaultdict
@@ -110,6 +111,9 @@ from pymongo.synchronous.change_stream import ChangeStream, ClusterChangeStream
 from pymongo.synchronous.client_bulk import _ClientBulk
 from pymongo.synchronous.client_session import _SESSION, _EmptyServerSession
 from pymongo.synchronous.command_cursor import CommandCursor
+from pymongo.synchronous.helpers import (
+    _RetryPolicy,
+)
 from pymongo.synchronous.settings import TopologySettings
 from pymongo.synchronous.topology import Topology, _ErrorContext
 from pymongo.topology_description import TOPOLOGY_TYPE, TopologyDescription
@@ -610,8 +614,18 @@ class MongoClient(common.BaseObject, Generic[_DocumentType]):
             client to use Stable API. See `versioned API <https://www.mongodb.com/docs/manual/reference/stable-api/#what-is-the-stable-api--and-should-you-use-it->`_ for
             details.
 
+          | **Overload retry options:**
+
+          - `max_adaptive_retries`: (int) How many retries to allow for overload errors. Defaults to ``2``.
+          - `enable_overload_retargeting`: (boolean) Whether overload retargeting is enabled for this client.
+            If enabled, server overload errors will cause retry attempts to select a server that has not yet returned an overload error, if possible.
+            Defaults to ``False``.
+
         .. seealso:: The MongoDB documentation on `connections <https://dochub.mongodb.org/core/connections>`_.
 
+        .. versionchanged:: 4.17
+           Added the ``max_adaptive_retries`` and ``enable_overload_retargeting`` URI and keyword arguments.
+
         .. versionchanged:: 4.5
            Added the ``serverMonitoringMode`` keyword argument.
 
@@ -879,11 +893,14 @@ class MongoClient(common.BaseObject, Generic[_DocumentType]):
             self._options.read_concern,
         )
 
+        self._retry_policy = _RetryPolicy(attempts=self._options.max_adaptive_retries)
+
         self._init_based_on_options(self._seeds, srv_max_hosts, srv_service_name)
 
         self._opened = False
         self._closed = False
         self._loop: Optional[asyncio.AbstractEventLoop] = None
+
         if not is_srv:
             self._init_background()
 
@@ -1987,6 +2004,8 @@ class MongoClient(common.BaseObject, Generic[_DocumentType]):
         read_pref: Optional[_ServerMode] = None,
         retryable: bool = False,
         operation_id: Optional[int] = None,
+        is_run_command: bool = False,
+        is_aggregate_write: bool = False,
     ) -> T:
         """Internal retryable helper for all client transactions.
 
@@ -1998,6 +2017,8 @@ class MongoClient(common.BaseObject, Generic[_DocumentType]):
         :param address: Server Address, defaults to None
         :param read_pref: Topology of read operation, defaults to None
         :param retryable: If the operation should be retried once, defaults to None
+        :param is_run_command: If this is a runCommand operation, defaults to False
+        :param is_aggregate_write: If this is a aggregate operation with a write, defaults to False.
 
         :return: Output of the calling func()
         """
@@ -2012,6 +2033,8 @@ class MongoClient(common.BaseObject, Generic[_DocumentType]):
             address=address,
             retryable=retryable,
             operation_id=operation_id,
+            is_run_command=is_run_command,
+            is_aggregate_write=is_aggregate_write,
         ).run()
 
     def _retryable_read(
@@ -2023,6 +2046,8 @@ class MongoClient(common.BaseObject, Generic[_DocumentType]):
         address: Optional[_Address] = None,
         retryable: bool = True,
         operation_id: Optional[int] = None,
+        is_run_command: bool = False,
+        is_aggregate_write: bool = False,
     ) -> T:
         """Execute an operation with consecutive retries if possible
 
@@ -2038,6 +2063,8 @@ class MongoClient(common.BaseObject, Generic[_DocumentType]):
         :param address: Optional address when sending a message, defaults to None
         :param retryable: if we should attempt retries
             (may not always be supported even if supplied), defaults to False
+        :param is_run_command: If this is a runCommand operation, defaults to False.
+        :param is_aggregate_write: If this is a aggregate operation with a write, defaults to False.
         """
 
         # Ensure that the client supports retrying on reads and there is no session in
@@ -2056,6 +2083,8 @@ class MongoClient(common.BaseObject, Generic[_DocumentType]):
                 read_pref=read_pref,
                 retryable=retryable,
                 operation_id=operation_id,
+                is_run_command=is_run_command,
+                is_aggregate_write=is_aggregate_write,
             )
 
     def _retryable_write(
@@ -2444,9 +2473,7 @@ class MongoClient(common.BaseObject, Generic[_DocumentType]):
                 f"name_or_database must be an instance of str or a Database, not {type(name)}"
             )
 
-        with self._conn_for_writes(session, operation=_Op.DROP_DATABASE) as conn:
-            self[name]._command(
-                conn,
+        self[name].command(
             {"dropDatabase": 1, "comment": comment},
             read_preference=ReadPreference.PRIMARY,
             write_concern=self._write_concern_for(session),
@@ -2736,12 +2763,15 @@ class _ClientConnectionRetryable(Generic[T]):
         address: Optional[_Address] = None,
         retryable: bool = False,
         operation_id: Optional[int] = None,
+        is_run_command: bool = False,
+        is_aggregate_write: bool = False,
     ):
         self._last_error: Optional[Exception] = None
         self._retrying = False
+        self._always_retryable = False
         self._multiple_retries = _csot.get_timeout() is not None
         self._client = mongo_client
-
+        self._retry_policy = mongo_client._retry_policy
         self._func = func
         self._bulk = bulk
         self._session = session
@@ -2757,6 +2787,8 @@ class _ClientConnectionRetryable(Generic[T]):
         self._operation = operation
         self._operation_id = operation_id
         self._attempt_number = 0
+        self._is_run_command = is_run_command
+        self._is_aggregate_write = is_aggregate_write
 
     def run(self) -> T:
         """Runs the supplied func() and attempts a retry
@@ -2776,7 +2808,13 @@ class _ClientConnectionRetryable(Generic[T]):
         while True:
             self._check_last_error(check_csot=True)
             try:
-                return self._read() if self._is_read else self._write()
+                res = self._read() if self._is_read else self._write()
+                # Track whether the transaction has completed a command.
+                # If we need to apply backpressure to the first command,
+                # we will need to revert back to starting state.
+                if self._session is not None and self._session.in_transaction:
+                    self._session._transaction.has_completed_command = True
+                return res
             except ServerSelectionTimeoutError:
                 # The application may think the write was never attempted
                 # if we raise ServerSelectionTimeoutError on the retry
@@ -2787,37 +2825,76 @@ class _ClientConnectionRetryable(Generic[T]):
                 # most likely be a waste of time.
                 raise
             except PyMongoError as exc:
+                always_retryable = False
+                overloaded = False
+                exc_to_check = exc
+
+                if self._is_run_command and not (
+                    self._client.options.retry_reads and self._client.options.retry_writes
+                ):
+                    raise
+                if self._is_aggregate_write and not self._client.options.retry_writes:
+                    raise
+
                 # Execute specialized catch on read
                 if self._is_read:
                     if isinstance(exc, (ConnectionFailure, OperationFailure)):
                         # ConnectionFailures do not supply a code property
                         exc_code = getattr(exc, "code", None)
-                        if self._is_not_eligible_for_retry() or (
+                        overloaded = exc.has_error_label("SystemOverloadedError")
+                        always_retryable = exc.has_error_label("RetryableError") and overloaded
+                        if not self._client.options.retry_reads or (
+                            not always_retryable
+                            and (
+                                self._is_not_eligible_for_retry()
+                                or (
                                     isinstance(exc, OperationFailure)
                                     and exc_code not in helpers_shared._RETRYABLE_ERROR_CODES
+                                )
+                            )
                         ):
                             raise
                         self._retrying = True
                         self._last_error = exc
                         self._attempt_number += 1
+
+                        # Revert back to starting state if we're in a transaction but haven't completed the first
+                        # command.
+                        if (
+                            overloaded
+                            and self._session is not None
+                            and self._session.in_transaction
+                        ):
+                            transaction = self._session._transaction
+                            if not transaction.has_completed_command:
+                                transaction.set_starting()
+                            transaction.attempt = 0
                     else:
                         raise
 
                 # Specialized catch on write operation
                 if not self._is_read:
-                    if not self._retryable:
-                        raise
-                    if isinstance(exc, ClientBulkWriteException) and exc.error:
-                        retryable_write_error_exc = isinstance(
+                    if isinstance(exc, ClientBulkWriteException) and isinstance(
                         exc.error, PyMongoError
-                        ) and exc.error.has_error_label("RetryableWriteError")
-                    else:
-                        retryable_write_error_exc = exc.has_error_label("RetryableWriteError")
-                    if retryable_write_error_exc:
+                    ):
+                        exc_to_check = exc.error
+                    retryable_write_label = exc_to_check.has_error_label("RetryableWriteError")
+                    overloaded = exc_to_check.has_error_label("SystemOverloadedError")
+                    always_retryable = exc_to_check.has_error_label("RetryableError") and overloaded
+
+                    # Always retry abortTransaction and commitTransaction up to once
+                    if self._operation not in ["abortTransaction", "commitTransaction"] and (
+                        not self._client.options.retry_writes
+                        or not (self._retryable or always_retryable)
+                    ):
+                        raise
+                    if retryable_write_label or always_retryable:
                         assert self._session
                         self._session._unpin()
-                    if not retryable_write_error_exc or self._is_not_eligible_for_retry():
-                        if exc.has_error_label("NoWritesPerformed") and self._last_error:
+                    if not always_retryable and (
+                        not retryable_write_label or self._is_not_eligible_for_retry()
+                    ):
+                        if exc_to_check.has_error_label("NoWritesPerformed") and self._last_error:
                             raise self._last_error from exc
                         else:
                             raise
@@ -2826,18 +2903,34 @@ class _ClientConnectionRetryable(Generic[T]):
                         self._bulk.retrying = True
                     else:
                         self._retrying = True
-                    if not exc.has_error_label("NoWritesPerformed"):
+                    if not exc_to_check.has_error_label("NoWritesPerformed"):
                         self._last_error = exc
                     if self._last_error is None:
                         self._last_error = exc
+                    # Revert back to starting state if we're in a transaction but haven't completed the first
+                    # command.
+                    if overloaded and self._session is not None and self._session.in_transaction:
+                        transaction = self._session._transaction
+                        if not transaction.has_completed_command:
+                            transaction.set_starting()
+                        transaction.attempt = 0
 
-                if (
-                    self._server is not None
-                    and self._client.topology_description.topology_type_name == "Sharded"
-                    or exc.has_error_label("SystemOverloadedError")
+                if self._server is not None and (
+                    self._client.topology_description.topology_type_name == "Sharded"
+                    or (overloaded and self._client.options.enable_overload_retargeting)
                 ):
                     self._deprioritized_servers.append(self._server)
 
+                self._always_retryable = always_retryable
+                if overloaded:
+                    delay = self._retry_policy.backoff(self._attempt_number)
+                    if not self._retry_policy.should_retry(self._attempt_number, delay):
+                        if exc_to_check.has_error_label("NoWritesPerformed") and self._last_error:
+                            raise self._last_error from exc
+                        else:
+                            raise
+                    time.sleep(delay)
+
     def _is_not_eligible_for_retry(self) -> bool:
         """Checks if the exchange is not eligible for retry"""
         return not self._retryable or (self._is_retrying() and not self._multiple_retries)
@@ -2899,7 +2992,7 @@ class _ClientConnectionRetryable(Generic[T]):
                     and conn.supports_sessions
                 )
                 is_mongos = conn.is_mongos
-                if not sessions_supported:
+                if not self._always_retryable and not sessions_supported:
                     # A retry is not possible because this server does
                     # not support sessions raise the last error.
                     self._check_last_error()
@@ -2931,7 +3024,7 @@ class _ClientConnectionRetryable(Generic[T]):
             conn,
             read_pref,
         ):
-            if self._retrying and not self._retryable:
+            if self._retrying and not self._retryable and not self._always_retryable:
                 self._check_last_error()
             if self._retrying:
                 _debug_log(

pymongo/synchronous/pool.py

@@ -19,6 +19,8 @@ import collections
 import contextlib
 import logging
 import os
+import socket
+import ssl
 import sys
 import time
 import weakref
@@ -49,10 +51,12 @@ from pymongo.errors import (  # type:ignore[attr-defined]
     DocumentTooLarge,
     ExecutionTimeout,
     InvalidOperation,
+    NetworkTimeout,
     NotPrimaryError,
     OperationFailure,
     PyMongoError,
     WaitQueueTimeoutError,
+    _CertificateError,
 )
 from pymongo.hello import Hello, HelloCompat
 from pymongo.helpers_shared import _get_timeout_details, format_timeout_details
@@ -250,6 +254,7 @@ class Connection:
         cmd = self.hello_cmd()
         performing_handshake = not self.performed_handshake
         awaitable = False
+        cmd["backpressure"] = True
         if performing_handshake:
             self.performed_handshake = True
             cmd["client"] = self.opts.metadata
@@ -750,8 +755,8 @@ class Pool:
         # Enforces: maxConnecting
         # Also used for: clearing the wait queue
         self._max_connecting_cond = _create_condition(self.lock)
-        self._max_connecting = self.opts.max_connecting
         self._pending = 0
+        self._max_connecting = self.opts.max_connecting
         self._client_id = client_id
         if self.enabled_for_cmap:
             assert self.opts._event_listeners is not None
@@ -982,6 +987,21 @@ class Pool:
                     self.requests -= 1
                     self.size_cond.notify()
 
+    def _handle_connection_error(self, error: BaseException) -> None:
+        # Handle system overload condition for non-sdam pools.
+        # Look for errors of type AutoReconnect and add error labels if appropriate.
+        if self.is_sdam or type(error) not in (AutoReconnect, NetworkTimeout):
+            return
+        assert isinstance(error, AutoReconnect)  # Appease type checker.
+        # If the original error was a DNS, certificate, or SSL error, ignore it.
+        if isinstance(error.__cause__, (_CertificateError, SSLErrors, socket.gaierror)):
+            # End of file errors are excluded, because the server may have disconnected
+            # during the handshake.
+            if not isinstance(error.__cause__, (ssl.SSLEOFError, ssl.SSLZeroReturnError)):
+                return
+        error._add_error_label("SystemOverloadedError")
+        error._add_error_label("RetryableError")
+
     def connect(self, handler: Optional[_MongoClientErrorHandler] = None) -> Connection:
         """Connect to Mongo and return a new Connection.
 
@@ -1033,10 +1053,10 @@ class Pool:
                     reason=_verbose_connection_error_reason(ConnectionClosedReason.ERROR),
                     error=ConnectionClosedReason.ERROR,
                 )
+            self._handle_connection_error(error)
             if isinstance(error, (IOError, OSError, *SSLErrors)):
                 details = _get_timeout_details(self.opts)
                 _raise_connection_failure(self.address, error, timeout_details=details)
-
             raise
 
         conn = Connection(networking_interface, self, self.address, conn_id, self.is_sdam)  # type: ignore[arg-type]
@@ -1045,18 +1065,22 @@ class Pool:
             self.active_contexts.discard(tmp_context)
         if tmp_context.cancelled:
             conn.cancel_context.cancel()
+        completed_hello = False
         try:
             if not self.is_sdam:
                 conn.hello()
+                completed_hello = True
                 self.is_writable = conn.is_writable
             if handler:
                 handler.contribute_socket(conn, completed_handshake=False)
 
             conn.authenticate()
         # Catch KeyboardInterrupt, CancelledError, etc. and cleanup.
-        except BaseException:
+        except BaseException as e:
             with self.lock:
                 self.active_contexts.discard(conn.cancel_context)
+            if not completed_hello:
+                self._handle_connection_error(e)
             conn.close_conn(ConnectionClosedReason.ERROR)
             raise
 
@@ -1385,8 +1409,8 @@ class Pool:
         :class:`~pymongo.errors.AutoReconnect` exceptions on server
         hiccups, etc. We only check if the socket was closed by an external
         error if it has been > 1 second since the socket was checked into the
-        pool, to keep performance reasonable - we can't avoid AutoReconnects
-        completely anyway.
+        pool to keep performance reasonable -
+        we can't avoid AutoReconnects completely anyway.
         """
         idle_time_seconds = conn.idle_time_seconds()
         # If socket is idle, open a new one.
@@ -1397,8 +1421,9 @@ class Pool:
             conn.close_conn(ConnectionClosedReason.IDLE)
             return True
 
-        if self._check_interval_seconds is not None and (
-            self._check_interval_seconds == 0 or idle_time_seconds > self._check_interval_seconds
+        check_interval_seconds = self._check_interval_seconds
+        if check_interval_seconds is not None and (
+            check_interval_seconds == 0 or idle_time_seconds > check_interval_seconds
         ):
             if conn.conn_closed():
                 conn.close_conn(ConnectionClosedReason.ERROR)

pymongo/synchronous/topology.py

@@ -911,7 +911,9 @@ class Topology:
                 # Clear the pool.
                 server.reset(service_id)
         elif isinstance(error, ConnectionFailure):
-            if isinstance(error, WaitQueueTimeoutError):
+            if isinstance(error, WaitQueueTimeoutError) or (
+                error.has_error_label("SystemOverloadedError")
+            ):
                 return
             # "Client MUST replace the server's description with type Unknown
             # ... MUST NOT request an immediate check of the server."

test/asynchronous/test_client.py

@@ -652,6 +652,38 @@ class AsyncClientUnitTest(AsyncUnitTest):
             with self.assertWarns(UserWarning):
                 self.simple_client(multi_host)
 
+    async def test_max_adaptive_retries(self):
+        # Assert that max adaptive retries defaults to 2.
+        c = self.simple_client(connect=False)
+        self.assertEqual(c.options.max_adaptive_retries, 2)
+
+        # Assert that max adaptive retries can be configured through connection or client options.
+        c = self.simple_client(connect=False, max_adaptive_retries=10)
+        self.assertEqual(c.options.max_adaptive_retries, 10)
+
+        c = self.simple_client(connect=False, maxAdaptiveRetries=10)
+        self.assertEqual(c.options.max_adaptive_retries, 10)
+
+        c = self.simple_client(host="mongodb://localhost/?maxAdaptiveRetries=10", connect=False)
+        self.assertEqual(c.options.max_adaptive_retries, 10)
+
+    async def test_enable_overload_retargeting(self):
+        # Assert that overload retargeting defaults to false.
+        c = self.simple_client(connect=False)
+        self.assertFalse(c.options.enable_overload_retargeting)
+
+        # Assert that overload retargeting can be enabled through connection or client options.
+        c = self.simple_client(connect=False, enable_overload_retargeting=True)
+        self.assertTrue(c.options.enable_overload_retargeting)
+
+        c = self.simple_client(connect=False, enableOverloadRetargeting=True)
+        self.assertTrue(c.options.enable_overload_retargeting)
+
+        c = self.simple_client(
+            host="mongodb://localhost/?enableOverloadRetargeting=true", connect=False
+        )
+        self.assertTrue(c.options.enable_overload_retargeting)
+
 
 class TestClient(AsyncIntegrationTest):
     def test_multiple_uris(self):

test/asynchronous/test_client_backpressure.py

@@ -0,0 +1,312 @@
+# Copyright 2025-present MongoDB, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Test Client Backpressure spec."""
+from __future__ import annotations
+
+import os
+import pathlib
+import sys
+from time import perf_counter
+from unittest.mock import patch
+
+from pymongo.common import MAX_ADAPTIVE_RETRIES
+
+sys.path[0:0] = [""]
+
+from test.asynchronous import (
+    AsyncIntegrationTest,
+    async_client_context,
+    unittest,
+)
+from test.asynchronous.unified_format import generate_test_classes
+from test.utils_shared import EventListener, OvertCommandListener
+
+from pymongo.errors import OperationFailure, PyMongoError
+
+_IS_SYNC = False
+
+# Mock a system overload error.
+mock_overload_error = {
+    "configureFailPoint": "failCommand",
+    "mode": {"times": 1},
+    "data": {
+        "failCommands": ["find", "insert", "update"],
+        "errorCode": 462,  # IngressRequestRateLimitExceeded
+        "errorLabels": ["RetryableError", "SystemOverloadedError"],
+    },
+}
+
+
+def get_mock_overload_error(times: int):
+    error = mock_overload_error.copy()
+    error["mode"] = {"times": times}
+    return error
+
+
+class TestBackpressure(AsyncIntegrationTest):
+    RUN_ON_LOAD_BALANCER = True
+
+    @async_client_context.require_failCommand_appName
+    async def test_retry_overload_error_command(self):
+        await self.db.t.insert_one({"x": 1})
+
+        # Ensure command is retried on overload error.
+        fail_many = get_mock_overload_error(MAX_ADAPTIVE_RETRIES)
+        async with self.fail_point(fail_many):
+            await self.db.command("find", "t")
+
+        # Ensure command stops retrying after MAX_ADAPTIVE_RETRIES.
+        fail_too_many = get_mock_overload_error(MAX_ADAPTIVE_RETRIES + 1)
+        async with self.fail_point(fail_too_many):
+            with self.assertRaises(PyMongoError) as error:
+                await self.db.command("find", "t")
+
+        self.assertIn("RetryableError", str(error.exception))
+        self.assertIn("SystemOverloadedError", str(error.exception))
+
+    @async_client_context.require_failCommand_appName
+    async def test_retry_overload_error_find(self):
+        await self.db.t.insert_one({"x": 1})
+
+        # Ensure command is retried on overload error.
+        fail_many = get_mock_overload_error(MAX_ADAPTIVE_RETRIES)
+        async with self.fail_point(fail_many):
+            await self.db.t.find_one()
+
+        # Ensure command stops retrying after MAX_ADAPTIVE_RETRIES.
+        fail_too_many = get_mock_overload_error(MAX_ADAPTIVE_RETRIES + 1)
+        async with self.fail_point(fail_too_many):
+            with self.assertRaises(PyMongoError) as error:
+                await self.db.t.find_one()
+
+        self.assertIn("RetryableError", str(error.exception))
+        self.assertIn("SystemOverloadedError", str(error.exception))
+
+    @async_client_context.require_failCommand_appName
+    async def test_retry_overload_error_insert_one(self):
+        # Ensure command is retried on overload error.
+        fail_many = get_mock_overload_error(MAX_ADAPTIVE_RETRIES)
+        async with self.fail_point(fail_many):
+            await self.db.t.insert_one({"x": 1})
+
+        # Ensure command stops retrying after MAX_ADAPTIVE_RETRIES.
+        fail_too_many = get_mock_overload_error(MAX_ADAPTIVE_RETRIES + 1)
+        async with self.fail_point(fail_too_many):
+            with self.assertRaises(PyMongoError) as error:
+                await self.db.t.insert_one({"x": 1})
+
+        self.assertIn("RetryableError", str(error.exception))
+        self.assertIn("SystemOverloadedError", str(error.exception))
+
+    @async_client_context.require_failCommand_appName
+    async def test_retry_overload_error_update_many(self):
+        # Even though update_many is not a retryable write operation, it will
+        # still be retried via the "RetryableError" error label.
+        await self.db.t.insert_one({"x": 1})
+
+        # Ensure command is retried on overload error.
+        fail_many = get_mock_overload_error(MAX_ADAPTIVE_RETRIES)
+        async with self.fail_point(fail_many):
+            await self.db.t.update_many({}, {"$set": {"x": 2}})
+
+        # Ensure command stops retrying after MAX_ADAPTIVE_RETRIES.
+        fail_too_many = get_mock_overload_error(MAX_ADAPTIVE_RETRIES + 1)
+        async with self.fail_point(fail_too_many):
+            with self.assertRaises(PyMongoError) as error:
+                await self.db.t.update_many({}, {"$set": {"x": 2}})
+
+        self.assertIn("RetryableError", str(error.exception))
+        self.assertIn("SystemOverloadedError", str(error.exception))
+
+    @async_client_context.require_failCommand_appName
+    async def test_retry_overload_error_getMore(self):
+        coll = self.db.t
+        await coll.insert_many([{"x": 1} for _ in range(10)])
+
+        # Ensure command is retried on overload error.
+        fail_many = {
+            "configureFailPoint": "failCommand",
+            "mode": {"times": MAX_ADAPTIVE_RETRIES},
+            "data": {
+                "failCommands": ["getMore"],
+                "errorCode": 462,  # IngressRequestRateLimitExceeded
+                "errorLabels": ["RetryableError", "SystemOverloadedError"],
+            },
+        }
+        cursor = coll.find(batch_size=2)
+        await cursor.next()
+        async with self.fail_point(fail_many):
+            await cursor.to_list()
+
+        # Ensure command stops retrying after MAX_ADAPTIVE_RETRIES.
+        fail_too_many = fail_many.copy()
+        fail_too_many["mode"] = {"times": MAX_ADAPTIVE_RETRIES + 1}
+        cursor = coll.find(batch_size=2)
+        await cursor.next()
+        async with self.fail_point(fail_too_many):
+            with self.assertRaises(PyMongoError) as error:
+                await cursor.to_list()
+
+        self.assertIn("RetryableError", str(error.exception))
+        self.assertIn("SystemOverloadedError", str(error.exception))
+
+
+# Prose tests.
+class AsyncTestClientBackpressure(AsyncIntegrationTest):
+    listener: EventListener
+
+    @classmethod
+    def setUpClass(cls) -> None:
+        cls.listener = OvertCommandListener()
+
+    @async_client_context.require_connection
+    async def asyncSetUp(self) -> None:
+        await super().asyncSetUp()
+        self.listener.reset()
+        self.app_name = self.__class__.__name__.lower()
+        self.client = await self.async_rs_or_single_client(
+            event_listeners=[self.listener], appName=self.app_name
+        )
+
+    @patch("random.random")
+    @async_client_context.require_failCommand_appName
+    async def test_01_operation_retry_uses_exponential_backoff(self, random_func):
+        # Drivers should test that retries do not occur immediately when a SystemOverloadedError is encountered.
+
+        # 1. let `client` be a `MongoClient`
+        client = self.client
+
+        # 2. let `collection` be a collection
+        collection = client.test.test
+
+        # 3. Now, run transactions without backoff:
+
+        # a. Configure the random number generator used for jitter to always return `0` -- this effectively disables backoff.
+        random_func.return_value = 0
+
+        # b. Configure the following failPoint:
+        fail_point = dict(
+            mode="alwaysOn",
+            data=dict(
+                failCommands=["insert"],
+                errorCode=2,
+                errorLabels=["SystemOverloadedError", "RetryableError"],
+                appName=self.app_name,
+            ),
+        )
+        async with self.fail_point(fail_point):
+            # c. Execute the following command. Expect that the command errors. Measure the duration of the command execution.
+            start0 = perf_counter()
+            with self.assertRaises(OperationFailure):
+                await collection.insert_one({"a": 1})
+            end0 = perf_counter()
+
+            # d. Configure the random number generator used for jitter to always return `1`.
+            random_func.return_value = 1
+
+            # e. Execute step c again.
+            start1 = perf_counter()
+            with self.assertRaises(OperationFailure):
+                await collection.insert_one({"a": 1})
+            end1 = perf_counter()
+
+            # f. Compare the times between the two runs.
+            # The sum of 2 backoffs is 0.3 seconds. There is a 0.3-second window to account for potential variance between the two
+            # runs.
+            self.assertTrue(abs((end1 - start1) - (end0 - start0 + 0.3)) < 0.3)
+
+    @async_client_context.require_failCommand_appName
+    async def test_03_overload_retries_limited(self):
+        # Drivers should test that overload errors are retried a maximum of two times.
+
+        # 1. Let `client` be a `MongoClient`.
+        client = self.client
+        # 2. Let `coll` be a collection.
+        coll = client.pymongo_test.coll
+
+        # 3. Configure the following failpoint:
+        failpoint = {
+            "configureFailPoint": "failCommand",
+            "mode": "alwaysOn",
+            "data": {
+                "failCommands": ["find"],
+                "errorCode": 462,  # IngressRequestRateLimitExceeded
+                "errorLabels": ["RetryableError", "SystemOverloadedError"],
+            },
+        }
+
+        # 4. Perform a find operation with `coll` that fails.
+        async with self.fail_point(failpoint):
+            with self.assertRaises(PyMongoError) as error:
+                await coll.find_one({})
+
+        # 5. Assert that the raised error contains both the `RetryableError` and `SystemOverloadedError` error labels.
+        self.assertIn("RetryableError", str(error.exception))
+        self.assertIn("SystemOverloadedError", str(error.exception))
+
+        # 6. Assert that the total number of started commands is MAX_ADAPTIVE_RETRIES + 1.
+        self.assertEqual(len(self.listener.started_events), MAX_ADAPTIVE_RETRIES + 1)
+
+    @async_client_context.require_failCommand_appName
+    async def test_04_overload_retries_limited_configured(self):
+        # Drivers should test that overload errors are retried a maximum of maxAdaptiveRetries times.
+        max_retries = 1
+
+        # 1. Let `client` be a `MongoClient` with `maxAdaptiveRetries=1` and command event monitoring enabled.
+        client = await self.async_single_client(
+            maxAdaptiveRetries=max_retries, event_listeners=[self.listener]
+        )
+        # 2. Let `coll` be a collection.
+        coll = client.pymongo_test.coll
+
+        # 3. Configure the following failpoint:
+        failpoint = {
+            "configureFailPoint": "failCommand",
+            "mode": "alwaysOn",
+            "data": {
+                "failCommands": ["find"],
+                "errorCode": 462,  # IngressRequestRateLimitExceeded
+                "errorLabels": ["RetryableError", "SystemOverloadedError"],
+            },
+        }
+
+        # 4. Perform a find operation with `coll` that fails.
+        async with self.fail_point(failpoint):
+            with self.assertRaises(PyMongoError) as error:
+                await coll.find_one({})
+
+        # 5. Assert that the raised error contains both the `RetryableError` and `SystemOverloadedError` error labels.
+        self.assertIn("RetryableError", str(error.exception))
+        self.assertIn("SystemOverloadedError", str(error.exception))
+
+        # 6. Assert that the total number of started commands is max_retries + 1.
+        self.assertEqual(len(self.listener.started_events), max_retries + 1)
+
+
+# Location of JSON test specifications.
+if _IS_SYNC:
+    _TEST_PATH = os.path.join(pathlib.Path(__file__).resolve().parent, "client-backpressure")
+else:
+    _TEST_PATH = os.path.join(pathlib.Path(__file__).resolve().parent.parent, "client-backpressure")
+
+globals().update(
+    generate_test_classes(
+        _TEST_PATH,
+        module=__name__,
+    )
+)
+
+if __name__ == "__main__":
+    unittest.main()

test/asynchronous/test_client_metadata.py

@@ -219,6 +219,19 @@ class TestClientMetadataProse(AsyncIntegrationTest):
         # add same metadata again
         await self.check_metadata_added(client, "Framework", None, None)
 
+    async def test_handshake_documents_include_backpressure(self):
+        # Create a `MongoClient` that is configured to record all handshake documents sent to the server as a part of
+        # connection establishment.
+        client = await self.async_rs_or_single_client("mongodb://" + self.server.address_string)
+
+        # Send a `ping` command to the server and verify that the command succeeds. This ensure that a connection is
+        # established on all topologies.  Note: MockupDB only supports standalone servers.
+        await client.admin.command("ping")
+
+        # Assert that for every handshake document intercepted:
+        # the document has a field `backpressure` whose value is `true`.
+        self.assertEqual(self.handshake_req["backpressure"], True)
+
 
 if __name__ == "__main__":
     unittest.main()

test/asynchronous/test_discovery_and_monitoring.py

@@ -25,8 +25,10 @@ from asyncio import StreamReader, StreamWriter
 from pathlib import Path
 from test.asynchronous.helpers import ConcurrentRunner
 from test.asynchronous.utils import flaky
+from test.utils_shared import delay
 
 from pymongo.asynchronous.pool import AsyncConnection
+from pymongo.errors import ConnectionFailure
 from pymongo.operations import _Op
 from pymongo.server_selectors import writable_server_selector
 
@@ -70,7 +72,12 @@ from pymongo.errors import (
 )
 from pymongo.hello import Hello, HelloCompat
 from pymongo.helpers_shared import _check_command_response, _check_write_command_response
-from pymongo.monitoring import ServerHeartbeatFailedEvent, ServerHeartbeatStartedEvent
+from pymongo.monitoring import (
+    ConnectionCheckOutFailedEvent,
+    PoolClearedEvent,
+    ServerHeartbeatFailedEvent,
+    ServerHeartbeatStartedEvent,
+)
 from pymongo.server_description import SERVER_TYPE, ServerDescription
 from pymongo.topology_description import TOPOLOGY_TYPE
 
@@ -131,6 +138,9 @@ async def got_app_error(topology, app_error):
         raise AssertionError
     except (AutoReconnect, NotPrimaryError, OperationFailure) as e:
         if when == "beforeHandshakeCompletes":
+            # The pool would have added the SystemOverloadedError in this case.
+            if isinstance(e, AutoReconnect):
+                e._add_error_label("SystemOverloadedError")
             completed_handshake = False
         elif when == "afterHandshakeCompletes":
             completed_handshake = True
@@ -439,6 +449,59 @@ class TestPoolManagement(AsyncIntegrationTest):
             AsyncConnection.close_conn = original_close
 
 
+class TestPoolBackpressure(AsyncIntegrationTest):
+    @async_client_context.require_version_min(7, 0, 0)
+    async def test_connection_pool_is_not_cleared(self):
+        listener = CMAPListener()
+
+        # Create a client that listens to CMAP events, with maxConnecting=100.
+        client = await self.async_rs_or_single_client(maxConnecting=100, event_listeners=[listener])
+
+        # Enable the ingress rate limiter.
+        await client.admin.command(
+            "setParameter", 1, ingressConnectionEstablishmentRateLimiterEnabled=True
+        )
+        await client.admin.command("setParameter", 1, ingressConnectionEstablishmentRatePerSec=20)
+        await client.admin.command(
+            "setParameter", 1, ingressConnectionEstablishmentBurstCapacitySecs=1
+        )
+        await client.admin.command("setParameter", 1, ingressConnectionEstablishmentMaxQueueDepth=1)
+
+        # Disable the ingress rate limiter on teardown.
+        # Sleep for 1 second before disabling to avoid the rate limiter.
+        async def teardown():
+            await asyncio.sleep(1)
+            await client.admin.command(
+                "setParameter", 1, ingressConnectionEstablishmentRateLimiterEnabled=False
+            )
+
+        self.addAsyncCleanup(teardown)
+
+        # Make sure the collection has at least one document.
+        await client.test.test.delete_many({})
+        await client.test.test.insert_one({})
+
+        # Run a slow operation to tie up the connection.
+        async def target():
+            try:
+                await client.test.test.find_one({"$where": delay(0.1)})
+            except ConnectionFailure:
+                pass
+
+        # Run 100 parallel operations that contend for connections.
+        tasks = []
+        for _ in range(100):
+            tasks.append(ConcurrentRunner(target=target))
+        for t in tasks:
+            await t.start()
+        for t in tasks:
+            await t.join()
+
+        # Verify there were at least 10 connection checkout failed event but no pool cleared events.
+        self.assertGreater(len(listener.events_by_type(ConnectionCheckOutFailedEvent)), 10)
+        self.assertEqual(len(listener.events_by_type(PoolClearedEvent)), 0)
+
+
 class TestServerMonitoringMode(AsyncIntegrationTest):
     @async_client_context.require_no_load_balancer
     async def asyncSetUp(self):

test/asynchronous/test_pooling.py

@@ -513,6 +513,39 @@ class TestPooling(_TestPoolingBase):
             str(error.exception),
         )
 
+    @async_client_context.require_failCommand_appName
+    async def test_pool_backpressure_preserves_existing_connections(self):
+        client = await self.async_rs_or_single_client()
+        coll = client.pymongo_test.t
+        pool = await async_get_pool(client)
+        await coll.insert_many([{"x": 1} for _ in range(10)])
+        t = SocketGetter(self.c, pool)
+        await t.start()
+        while t.state != "connection":
+            await asyncio.sleep(0.1)
+
+        assert not t.sock.conn_closed()
+
+        # Mock a session establishment overload.
+        mock_connection_fail = {
+            "configureFailPoint": "failCommand",
+            "mode": {"times": 1},
+            "data": {
+                "closeConnection": True,
+            },
+        }
+
+        async with self.fail_point(mock_connection_fail):
+            await coll.find_one({})
+
+        # Make sure the existing socket was not affected.
+        assert not t.sock.conn_closed()
+
+        # Cleanup
+        await t.release_conn()
+        await t.join()
+        await pool.close()
+
 
 class TestPoolMaxSize(_TestPoolingBase):
     async def test_max_pool_size(self):

test/asynchronous/test_retryable_reads.py

@@ -265,14 +265,17 @@ class TestRetryableReads(AsyncIntegrationTest):
     @async_client_context.require_secondaries_count(1)
     @async_client_context.require_failCommand_fail_point
     @async_client_context.require_version_min(4, 4, 0)
-    async def test_03_01_retryable_reads_caused_by_overload_errors_are_retried_on_a_different_replicaset_server_when_one_is_available(
+    async def test_03_01_retryable_reads_caused_by_overload_errors_are_retried_on_a_different_replicaset_server_when_one_is_available_and_overload_retargeting_is_enabled(
         self
     ):
         listener = OvertCommandListener()
 
-        # 1. Create a client `client` with `retryReads=true`, `readPreference=primaryPreferred`, and command event monitoring enabled.
+        # 1. Create a client `client` with `retryReads=true`, `readPreference=primaryPreferred`, `enableOverloadRetargeting=True`, and command event monitoring enabled.
         client = await self.async_rs_or_single_client(
-            event_listeners=[listener], retryReads=True, readPreference="primaryPreferred"
+            event_listeners=[listener],
+            retryReads=True,
+            readPreference="primaryPreferred",
+            enableOverloadRetargeting=True,
         )
 
         # 2. Configure a fail point with the RetryableError and SystemOverloadedError error labels.
@@ -339,6 +342,47 @@ class TestRetryableReads(AsyncIntegrationTest):
         # 6. Assert that both events occurred the same server.
         assert listener.failed_events[0].connection_id == listener.succeeded_events[0].connection_id
 
+    @async_client_context.require_replica_set
+    @async_client_context.require_secondaries_count(1)
+    @async_client_context.require_failCommand_fail_point
+    @async_client_context.require_version_min(4, 4, 0)
+    async def test_03_03_retryable_reads_caused_by_overload_errors_are_retried_on_the_same_replicaset_server_when_one_is_available_and_overload_retargeting_is_disabled(
+        self
+    ):
+        listener = OvertCommandListener()
+
+        # 1. Create a client `client` with `retryReads=true`, `readPreference=primaryPreferred`, and command event monitoring enabled.
+        client = await self.async_rs_or_single_client(
+            event_listeners=[listener],
+            retryReads=True,
+            readPreference="primaryPreferred",
+        )
+
+        # 2. Configure a fail point with the RetryableError and SystemOverloadedError error labels.
+        command_args = {
+            "configureFailPoint": "failCommand",
+            "mode": {"times": 1},
+            "data": {
+                "failCommands": ["find"],
+                "errorLabels": ["RetryableError", "SystemOverloadedError"],
+                "errorCode": 6,
+            },
+        }
+        await async_set_fail_point(client, command_args)
+
+        # 3. Reset the command event monitor to clear the fail point command from its stored events.
+        listener.reset()
+
+        # 4. Execute a `find` command with `client`.
+        await client.t.t.find_one({})
+
+        # 5. Assert that one failed command event and one successful command event occurred.
+        self.assertEqual(len(listener.failed_events), 1)
+        self.assertEqual(len(listener.succeeded_events), 1)
+
+        # 6. Assert that both events occurred on the same server.
+        assert listener.failed_events[0].connection_id == listener.succeeded_events[0].connection_id
+
 
 if __name__ == "__main__":
     unittest.main()

test/asynchronous/test_retryable_writes.py

@@ -43,14 +43,17 @@ from bson.codec_options import DEFAULT_CODEC_OPTIONS
 from bson.int64 import Int64
 from bson.raw_bson import RawBSONDocument
 from bson.son import SON
+from pymongo import MongoClient
 from pymongo.errors import (
     AutoReconnect,
     ConnectionFailure,
-    OperationFailure,
+    NotPrimaryError,
+    PyMongoError,
     ServerSelectionTimeoutError,
     WriteConcernError,
 )
 from pymongo.monitoring import (
+    CommandFailedEvent,
     CommandSucceededEvent,
     ConnectionCheckedOutEvent,
     ConnectionCheckOutFailedEvent,
@@ -601,5 +604,186 @@ class TestRetryableWritesTxnNumber(IgnoreDeprecationsTest):
                 self.assertEqual(sent_txn_id, final_txn_id, msg)
 
 
+class TestErrorPropagationAfterEncounteringMultipleErrors(AsyncIntegrationTest):
+    # Only run against replica sets as mongos does not propagate the NoWritesPerformed label to the drivers.
+    @async_client_context.require_replica_set
+    # Run against server versions 6.0 and above.
+    @async_client_context.require_version_min(6, 0)  # type: ignore[untyped-decorator]
+    async def asyncSetUp(self) -> None:
+        await super().asyncSetUp()
+        self.setup_client = MongoClient(**async_client_context.default_client_options)
+        self.addCleanup(self.setup_client.close)
+
+    # TODO: After PYTHON-4595 we can use async event handlers and remove this workaround.
+    def configure_fail_point_sync(self, command_args, off=False) -> None:
+        cmd = {"configureFailPoint": "failCommand"}
+        cmd.update(command_args)
+        if off:
+            cmd["mode"] = "off"
+            cmd.pop("data", None)
+        self.setup_client.admin.command(cmd)
+
+    async def test_01_drivers_return_the_correct_error_when_receiving_only_errors_without_NoWritesPerformed(
+        self
+    ) -> None:
+        # Create a client with retryWrites=true.
+        listener = OvertCommandListener()
+
+        # Configure a fail point with error code 91 (ShutdownInProgress) with the RetryableError and SystemOverloadedError error labels.
+        command_args = {
+            "configureFailPoint": "failCommand",
+            "mode": {"times": 1},
+            "data": {
+                "failCommands": ["insert"],
+                "errorLabels": ["RetryableError", "SystemOverloadedError"],
+                "errorCode": 91,
+            },
+        }
+
+        # Via the command monitoring CommandFailedEvent, configure a fail point with error code 10107 (NotWritablePrimary).
+        command_args_inner = {
+            "configureFailPoint": "failCommand",
+            "mode": "alwaysOn",
+            "data": {
+                "failCommands": ["insert"],
+                "errorCode": 10107,
+                "errorLabels": ["RetryableError", "SystemOverloadedError"],
+            },
+        }
+
+        def failed(event: CommandFailedEvent) -> None:
+            # Configure the 10107 fail point command only if the the failed event is for the 91 error configured in step 2.
+            if listener.failed_events:
+                return
+            assert event.failure["code"] == 91
+            self.configure_fail_point_sync(command_args_inner)
+            self.addCleanup(self.configure_fail_point_sync, {}, off=True)
+            listener.failed_events.append(event)
+
+        listener.failed = failed
+
+        client = await self.async_rs_client(retryWrites=True, event_listeners=[listener])
+
+        self.configure_fail_point_sync(command_args)
+        self.addCleanup(self.configure_fail_point_sync, {}, off=True)
+
+        # Attempt an insertOne operation on any record for any database and collection.
+        # Expect the insertOne to fail with a server error.
+        with self.assertRaises(NotPrimaryError) as exc:
+            await client.test.test.insert_one({})
+
+        # Assert that the error code of the server error is 10107.
+        assert exc.exception.errors["code"] == 10107  # type:ignore[call-overload]
+
+    async def test_02_drivers_return_the_correct_error_when_receiving_only_errors_with_NoWritesPerformed(
+        self
+    ) -> None:
+        # Create a client with retryWrites=true.
+        listener = OvertCommandListener()
+
+        # Configure a fail point with error code 91 (ShutdownInProgress) with the RetryableError and SystemOverloadedError error labels.
+        command_args = {
+            "configureFailPoint": "failCommand",
+            "mode": {"times": 1},
+            "data": {
+                "failCommands": ["insert"],
+                "errorLabels": ["RetryableError", "SystemOverloadedError", "NoWritesPerformed"],
+                "errorCode": 91,
+            },
+        }
+
+        # Via the command monitoring CommandFailedEvent, configure a fail point with error code `10107` (NotWritablePrimary)
+        # and a NoWritesPerformed label.
+        command_args_inner = {
+            "configureFailPoint": "failCommand",
+            "mode": "alwaysOn",
+            "data": {
+                "failCommands": ["insert"],
+                "errorCode": 10107,
+                "errorLabels": ["RetryableError", "SystemOverloadedError", "NoWritesPerformed"],
+            },
+        }
+
+        def failed(event: CommandFailedEvent) -> None:
+            if listener.failed_events:
+                return
+            # Configure the 10107 fail point command only if the the failed event is for the 91 error configured in step 2.
+            assert event.failure["code"] == 91
+            self.configure_fail_point_sync(command_args_inner)
+            self.addCleanup(self.configure_fail_point_sync, {}, off=True)
+            listener.failed_events.append(event)
+
+        listener.failed = failed
+
+        client = await self.async_rs_client(retryWrites=True, event_listeners=[listener])
+
+        self.configure_fail_point_sync(command_args)
+        self.addCleanup(self.configure_fail_point_sync, {}, off=True)
+
+        # Attempt an insertOne operation on any record for any database and collection.
+        # Expect the insertOne to fail with a server error.
+        with self.assertRaises(NotPrimaryError) as exc:
+            await client.test.test.insert_one({})
+
+        # Assert that the error code of the server error is 91.
+        assert exc.exception.errors["code"] == 91  # type:ignore[call-overload]
+
+    async def test_03_drivers_return_the_correct_error_when_receiving_some_errors_with_NoWritesPerformed_and_some_without_NoWritesPerformed(
+        self
+    ) -> None:
+        # Create a client with retryWrites=true.
+        listener = OvertCommandListener()
+
+        # Configure the client to listen to CommandFailedEvents. In the attached listener, configure a fail point with error
+        # code `91` (NotWritablePrimary) and the `NoWritesPerformed`, `RetryableError` and `SystemOverloadedError` labels.
+        command_args_inner = {
+            "configureFailPoint": "failCommand",
+            "mode": "alwaysOn",
+            "data": {
+                "failCommands": ["insert"],
+                "errorLabels": ["RetryableError", "SystemOverloadedError", "NoWritesPerformed"],
+                "errorCode": 91,
+            },
+        }
+
+        # Configure a fail point with error code `91` (ShutdownInProgress) with the `RetryableError` and
+        # `SystemOverloadedError` error labels but without the `NoWritesPerformed` error label.
+        command_args = {
+            "configureFailPoint": "failCommand",
+            "mode": {"times": 1},
+            "data": {
+                "failCommands": ["insert"],
+                "errorCode": 91,
+                "errorLabels": ["RetryableError", "SystemOverloadedError"],
+            },
+        }
+
+        def failed(event: CommandFailedEvent) -> None:
+            # Configure the fail point command only if the failed event is for the 91 error configured in step 2.
+            if listener.failed_events:
+                return
+            assert event.failure["code"] == 91
+            self.configure_fail_point_sync(command_args_inner)
+            self.addCleanup(self.configure_fail_point_sync, {}, off=True)
+            listener.failed_events.append(event)
+
+        listener.failed = failed
+
+        client = await self.async_rs_client(retryWrites=True, event_listeners=[listener])
+
+        self.configure_fail_point_sync(command_args)
+        self.addCleanup(self.configure_fail_point_sync, {}, off=True)
+
+        # Attempt an insertOne operation on any record for any database and collection.
+        # Expect the insertOne to fail with a server error.
+        with self.assertRaises(PyMongoError) as exc:
+            await client.test.test.insert_one({})
+
+        # Assert that the error code of the server error is 91.
+        assert exc.exception.errors["code"] == 91
+        # Assert that the error does not contain the error label `NoWritesPerformed`.
+        assert "NoWritesPerformed" not in exc.exception.errors["errorLabels"]
+
+
 if __name__ == "__main__":
     unittest.main()

test/asynchronous/test_transactions.py

@@ -16,9 +16,13 @@
 from __future__ import annotations
 
 import asyncio
+import random
 import sys
+import time
 from io import BytesIO
+from unittest.mock import patch
 
+import pymongo
 from gridfs.asynchronous.grid_file import AsyncGridFS, AsyncGridFSBucket
 from pymongo.asynchronous.pool import PoolState
 from pymongo.server_selectors import writable_server_selector
@@ -45,7 +49,9 @@ from pymongo.errors import (
     CollectionInvalid,
     ConfigurationError,
     ConnectionFailure,
+    ExecutionTimeout,
     InvalidOperation,
+    NetworkTimeout,
     OperationFailure,
 )
 from pymongo.operations import IndexModel, InsertOne
@@ -434,7 +440,7 @@ class TestTransactionsConvenientAPI(AsyncTransactionsBase):
             await self.configure_fail_point(client, command_args)
 
     @async_client_context.require_transactions
-    async def test_callback_raises_custom_error(self):
+    async def test_1_callback_raises_custom_error(self):
         class _MyException(Exception):
             pass
 
@@ -446,7 +452,7 @@ class TestTransactionsConvenientAPI(AsyncTransactionsBase):
                 await s.with_transaction(raise_error)
 
     @async_client_context.require_transactions
-    async def test_callback_returns_value(self):
+    async def test_2_callback_returns_value(self):
         async def callback(_):
             return "Foo"
 
@@ -474,7 +480,7 @@ class TestTransactionsConvenientAPI(AsyncTransactionsBase):
             self.assertEqual(await s.with_transaction(callback), "Foo")
 
     @async_client_context.require_transactions
-    async def test_callback_not_retried_after_timeout(self):
+    async def test_3_1_callback_not_retried_after_timeout(self):
         listener = OvertCommandListener()
         client = await self.async_rs_client(event_listeners=[listener])
         coll = client[self.db.name].test
@@ -495,14 +501,16 @@ class TestTransactionsConvenientAPI(AsyncTransactionsBase):
         listener.reset()
         async with client.start_session() as s:
             with PatchSessionTimeout(0):
-                with self.assertRaises(OperationFailure):
+                with self.assertRaises(NetworkTimeout) as context:
                     await s.with_transaction(callback)
 
         self.assertEqual(listener.started_command_names(), ["insert", "abortTransaction"])
+        # Assert that the timeout error has the same labels as the error it wraps.
+        self.assertTrue(context.exception.has_error_label("TransientTransactionError"))
 
     @async_client_context.require_test_commands
     @async_client_context.require_transactions
-    async def test_callback_not_retried_after_commit_timeout(self):
+    async def test_3_2_callback_not_retried_after_commit_timeout(self):
         listener = OvertCommandListener()
         client = await self.async_rs_client(event_listeners=[listener])
         coll = client[self.db.name].test
@@ -529,14 +537,16 @@ class TestTransactionsConvenientAPI(AsyncTransactionsBase):
 
         async with client.start_session() as s:
             with PatchSessionTimeout(0):
-                with self.assertRaises(OperationFailure):
+                with self.assertRaises(NetworkTimeout) as context:
                     await s.with_transaction(callback)
 
         self.assertEqual(listener.started_command_names(), ["insert", "commitTransaction"])
+        # Assert that the timeout error has the same labels as the error it wraps.
+        self.assertTrue(context.exception.has_error_label("TransientTransactionError"))
 
     @async_client_context.require_test_commands
     @async_client_context.require_transactions
-    async def test_commit_not_retried_after_timeout(self):
+    async def test_3_3_commit_not_retried_after_timeout(self):
         listener = OvertCommandListener()
         client = await self.async_rs_client(event_listeners=[listener])
         coll = client[self.db.name].test
@@ -560,7 +570,7 @@ class TestTransactionsConvenientAPI(AsyncTransactionsBase):
 
         async with client.start_session() as s:
             with PatchSessionTimeout(0):
-                with self.assertRaises(ConnectionFailure):
+                with self.assertRaises(NetworkTimeout) as context:
                     await s.with_transaction(callback)
 
         # One insert for the callback and two commits (includes the automatic
@@ -568,6 +578,40 @@ class TestTransactionsConvenientAPI(AsyncTransactionsBase):
         self.assertEqual(
             listener.started_command_names(), ["insert", "commitTransaction", "commitTransaction"]
         )
+        # Assert that the timeout error has the same labels as the error it wraps.
+        self.assertTrue(context.exception.has_error_label("UnknownTransactionCommitResult"))
+
+    @async_client_context.require_transactions
+    async def test_callback_not_retried_after_csot_timeout(self):
+        listener = OvertCommandListener()
+        client = await self.async_rs_client(event_listeners=[listener])
+        coll = client[self.db.name].test
+
+        async def callback(session):
+            await coll.insert_one({}, session=session)
+            err: dict = {
+                "ok": 0,
+                "errmsg": "Transaction 7819 has been aborted.",
+                "code": 251,
+                "codeName": "NoSuchTransaction",
+                "errorLabels": ["TransientTransactionError"],
+            }
+            raise OperationFailure(err["errmsg"], err["code"], err)
+
+        # Create the collection.
+        await coll.insert_one({})
+        listener.reset()
+        async with client.start_session() as s:
+            with pymongo.timeout(1.0):
+                with self.assertRaises(ExecutionTimeout):
+                    await s.with_transaction(callback)
+
+        # At least two attempts: the original and one or more retries.
+        inserts = len([x for x in listener.started_command_names() if x == "insert"])
+        aborts = len([x for x in listener.started_command_names() if x == "abortTransaction"])
+
+        self.assertGreaterEqual(inserts, 2)
+        self.assertGreaterEqual(aborts, 2)
 
     # Tested here because this supports Motor's convenient transactions API.
     @async_client_context.require_transactions
@@ -606,6 +650,63 @@ class TestTransactionsConvenientAPI(AsyncTransactionsBase):
             await s.with_transaction(callback)
             self.assertFalse(s.in_transaction)
 
+    @async_client_context.require_test_commands
+    @async_client_context.require_transactions
+    async def test_4_retry_backoff_is_enforced(self):
+        client = async_client_context.client
+        coll = client[self.db.name].test
+        end = start = no_backoff_time = 0
+
+        # Make random.random always return 0 (no backoff)
+        with patch.object(random, "random", return_value=0):
+            # set fail point to trigger transaction failure and trigger backoff
+            await self.set_fail_point(
+                {
+                    "configureFailPoint": "failCommand",
+                    "mode": {"times": 13},
+                    "data": {
+                        "failCommands": ["commitTransaction"],
+                        "errorCode": 251,
+                    },
+                }
+            )
+            self.addAsyncCleanup(
+                self.set_fail_point, {"configureFailPoint": "failCommand", "mode": "off"}
+            )
+
+            async def callback(session):
+                await coll.insert_one({}, session=session)
+
+            start = time.monotonic()
+            async with self.client.start_session() as s:
+                await s.with_transaction(callback)
+            end = time.monotonic()
+            no_backoff_time = end - start
+
+        # Make random.random always return 1 (max backoff)
+        with patch.object(random, "random", return_value=1):
+            # set fail point to trigger transaction failure and trigger backoff
+            await self.set_fail_point(
+                {
+                    "configureFailPoint": "failCommand",
+                    "mode": {
+                        "times": 13
+                    },  # sufficiently high enough such that the time effect of backoff is noticeable
+                    "data": {
+                        "failCommands": ["commitTransaction"],
+                        "errorCode": 251,
+                    },
+                }
+            )
+            self.addAsyncCleanup(
+                self.set_fail_point, {"configureFailPoint": "failCommand", "mode": "off"}
+            )
+            start = time.monotonic()
+            async with self.client.start_session() as s:
+                await s.with_transaction(callback)
+            end = time.monotonic()
+        self.assertLess(abs(end - start - (no_backoff_time + 2.2)), 1)  # sum of 13 backoffs is 2.2
+
 
 class TestOptionsInsideTransactionProse(AsyncTransactionsBase):
     @async_client_context.require_transactions

test/client-backpressure/backpressure-connection-checkin.json

@@ -0,0 +1,111 @@
+{
+  "description": "tests that connections are returned to the pool on retry attempts for overload errors",
+  "schemaVersion": "1.3",
+  "runOnRequirements": [
+    {
+      "minServerVersion": "4.4",
+      "topologies": [
+        "replicaset",
+        "sharded",
+        "load-balanced"
+      ]
+    }
+  ],
+  "createEntities": [
+    {
+      "client": {
+        "id": "client",
+        "useMultipleMongoses": false,
+        "observeEvents": [
+          "connectionCheckedOutEvent",
+          "connectionCheckedInEvent"
+        ]
+      }
+    },
+    {
+      "client": {
+        "id": "fail_point_client",
+        "useMultipleMongoses": false
+      }
+    },
+    {
+      "database": {
+        "id": "database",
+        "client": "client",
+        "databaseName": "backpressure-connection-checkin"
+      }
+    },
+    {
+      "collection": {
+        "id": "collection",
+        "database": "database",
+        "collectionName": "coll"
+      }
+    }
+  ],
+  "tests": [
+    {
+      "description": "overload error retry attempts return connections to the pool",
+      "operations": [
+        {
+          "name": "failPoint",
+          "object": "testRunner",
+          "arguments": {
+            "client": "fail_point_client",
+            "failPoint": {
+              "configureFailPoint": "failCommand",
+              "mode": "alwaysOn",
+              "data": {
+                "failCommands": [
+                  "find"
+                ],
+                "errorLabels": [
+                  "RetryableError",
+                  "SystemOverloadedError"
+                ],
+                "errorCode": 2
+              }
+            }
+          }
+        },
+        {
+          "name": "find",
+          "object": "collection",
+          "arguments": {
+            "filter": {}
+          },
+          "expectError": {
+            "isError": true,
+            "isClientError": false
+          }
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client",
+          "eventType": "cmap",
+          "events": [
+            {
+              "connectionCheckedOutEvent": {}
+            },
+            {
+              "connectionCheckedInEvent": {}
+            },
+            {
+              "connectionCheckedOutEvent": {}
+            },
+            {
+              "connectionCheckedInEvent": {}
+            },
+            {
+              "connectionCheckedOutEvent": {}
+            },
+            {
+              "connectionCheckedInEvent": {}
+            }
+          ]
+        }
+      ]
+    }
+  ]
+}

test/client-backpressure/getMore-retried.json

@@ -0,0 +1,253 @@
+{
+  "description": "getMore-retried-backpressure",
+  "schemaVersion": "1.3",
+  "runOnRequirements": [
+    {
+      "minServerVersion": "4.4"
+    }
+  ],
+  "createEntities": [
+    {
+      "client": {
+        "id": "client0",
+        "useMultipleMongoses": false,
+        "observeEvents": [
+          "commandStartedEvent",
+          "commandFailedEvent",
+          "commandSucceededEvent"
+        ]
+      }
+    },
+    {
+      "client": {
+        "id": "failPointClient",
+        "useMultipleMongoses": false
+      }
+    },
+    {
+      "database": {
+        "id": "db",
+        "client": "client0",
+        "databaseName": "default"
+      }
+    },
+    {
+      "collection": {
+        "id": "coll",
+        "database": "db",
+        "collectionName": "default"
+      }
+    }
+  ],
+  "initialData": [
+    {
+      "databaseName": "default",
+      "collectionName": "default",
+      "documents": [
+        {
+          "a": 1
+        },
+        {
+          "a": 2
+        },
+        {
+          "a": 3
+        }
+      ]
+    }
+  ],
+  "tests": [
+    {
+      "description": "getMores are retried",
+      "operations": [
+        {
+          "name": "failPoint",
+          "object": "testRunner",
+          "arguments": {
+            "client": "failPointClient",
+            "failPoint": {
+              "configureFailPoint": "failCommand",
+              "mode": {
+                "times": 2
+              },
+              "data": {
+                "failCommands": [
+                  "getMore"
+                ],
+                "errorLabels": [
+                  "RetryableError",
+                  "SystemOverloadedError"
+                ],
+                "errorCode": 2
+              }
+            }
+          }
+        },
+        {
+          "name": "find",
+          "object": "coll",
+          "arguments": {
+            "batchSize": 2,
+            "filter": {},
+            "sort": {
+              "a": 1
+            }
+          },
+          "expectResult": [
+            {
+              "a": 1
+            },
+            {
+              "a": 2
+            },
+            {
+              "a": 3
+            }
+          ]
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client0",
+          "events": [
+            {
+              "commandStartedEvent": {
+                "commandName": "find"
+              }
+            },
+            {
+              "commandSucceededEvent": {
+                "commandName": "find"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "getMore"
+              }
+            },
+            {
+              "commandFailedEvent": {
+                "commandName": "getMore"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "getMore"
+              }
+            },
+            {
+              "commandFailedEvent": {
+                "commandName": "getMore"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "getMore"
+              }
+            },
+            {
+              "commandSucceededEvent": {
+                "commandName": "getMore"
+              }
+            }
+          ]
+        }
+      ]
+    },
+    {
+      "description": "getMores are retried maxAttempts=2 times",
+      "operations": [
+        {
+          "name": "failPoint",
+          "object": "testRunner",
+          "arguments": {
+            "client": "failPointClient",
+            "failPoint": {
+              "configureFailPoint": "failCommand",
+              "mode": "alwaysOn",
+              "data": {
+                "failCommands": [
+                  "getMore"
+                ],
+                "errorLabels": [
+                  "RetryableError",
+                  "SystemOverloadedError"
+                ],
+                "errorCode": 2
+              }
+            }
+          }
+        },
+        {
+          "name": "find",
+          "arguments": {
+            "batchSize": 2,
+            "filter": {}
+          },
+          "object": "coll",
+          "expectError": {
+            "isError": true,
+            "isClientError": false
+          }
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client0",
+          "events": [
+            {
+              "commandStartedEvent": {
+                "commandName": "find"
+              }
+            },
+            {
+              "commandSucceededEvent": {
+                "commandName": "find"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "getMore"
+              }
+            },
+            {
+              "commandFailedEvent": {
+                "commandName": "getMore"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "getMore"
+              }
+            },
+            {
+              "commandFailedEvent": {
+                "commandName": "getMore"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "getMore"
+              }
+            },
+            {
+              "commandFailedEvent": {
+                "commandName": "getMore"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "killCursors"
+              }
+            },
+            {
+              "commandSucceededEvent": {
+                "commandName": "killCursors"
+              }
+            }
+          ]
+        }
+      ]
+    }
+  ]
+}

test/connection_monitoring/pool-create-min-size-error.json

@@ -9,9 +9,7 @@
   ],
   "failPoint": {
     "configureFailPoint": "failCommand",
-    "mode": {
-      "times": 50
-    },
+    "mode": "alwaysOn",
     "data": {
       "failCommands": [
         "isMaster",

test/discovery_and_monitoring/errors/error_handling_handshake.json

@@ -97,14 +97,22 @@
       "outcome": {
         "servers": {
           "a:27017": {
-            "type": "Unknown",
-            "topologyVersion": null,
+            "type": "RSPrimary",
+            "setName": "rs",
+            "topologyVersion": {
+              "processId": {
+                "$oid": "000000000000000000000001"
+              },
+              "counter": {
+                "$numberLong": "1"
+              }
+            },
             "pool": {
-              "generation": 1
+              "generation": 0
             }
           }
         },
-        "topologyType": "ReplicaSetNoPrimary",
+        "topologyType": "ReplicaSetWithPrimary",
         "logicalSessionTimeoutMinutes": null,
         "setName": "rs"
       }

test/discovery_and_monitoring/unified/backpressure-network-error-fail-replicaset.json

@@ -0,0 +1,142 @@
+{
+  "description": "backpressure-network-error-fail-replicaset",
+  "schemaVersion": "1.17",
+  "runOnRequirements": [
+    {
+      "minServerVersion": "4.4",
+      "serverless": "forbid",
+      "topologies": [
+        "replicaset"
+      ]
+    }
+  ],
+  "createEntities": [
+    {
+      "client": {
+        "id": "setupClient",
+        "useMultipleMongoses": false
+      }
+    }
+  ],
+  "initialData": [
+    {
+      "collectionName": "backpressure-network-error-fail",
+      "databaseName": "sdam-tests",
+      "documents": [
+        {
+          "_id": 1
+        },
+        {
+          "_id": 2
+        }
+      ]
+    }
+  ],
+  "tests": [
+    {
+      "description": "apply backpressure on network connection errors during connection establishment",
+      "operations": [
+        {
+          "name": "createEntities",
+          "object": "testRunner",
+          "arguments": {
+            "entities": [
+              {
+                "client": {
+                  "id": "client",
+                  "useMultipleMongoses": false,
+                  "observeEvents": [
+                    "serverDescriptionChangedEvent",
+                    "poolClearedEvent"
+                  ],
+                  "uriOptions": {
+                    "retryWrites": false,
+                    "heartbeatFrequencyMS": 1000000,
+                    "serverMonitoringMode": "poll",
+                    "appname": "backpressureNetworkErrorFailTest"
+                  }
+                }
+              },
+              {
+                "database": {
+                  "id": "database",
+                  "client": "client",
+                  "databaseName": "sdam-tests"
+                }
+              },
+              {
+                "collection": {
+                  "id": "collection",
+                  "database": "database",
+                  "collectionName": "backpressure-network-error-fail"
+                }
+              }
+            ]
+          }
+        },
+        {
+          "name": "waitForEvent",
+          "object": "testRunner",
+          "arguments": {
+            "client": "client",
+            "event": {
+              "serverDescriptionChangedEvent": {
+                "newDescription": {
+                  "type": "RSPrimary"
+                }
+              }
+            },
+            "count": 1
+          }
+        },
+        {
+          "name": "failPoint",
+          "object": "testRunner",
+          "arguments": {
+            "client": "setupClient",
+            "failPoint": {
+              "configureFailPoint": "failCommand",
+              "mode": "alwaysOn",
+              "data": {
+                "failCommands": [
+                  "isMaster",
+                  "hello"
+                ],
+                "appName": "backpressureNetworkErrorFailTest",
+                "closeConnection": true
+              }
+            }
+          }
+        },
+        {
+          "name": "insertMany",
+          "object": "collection",
+          "arguments": {
+            "documents": [
+              {
+                "_id": 3
+              },
+              {
+                "_id": 4
+              }
+            ]
+          },
+          "expectError": {
+            "isError": true,
+            "errorLabelsContain": [
+              "SystemOverloadedError",
+              "RetryableError"
+            ]
+          }
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client",
+          "eventType": "cmap",
+          "events": []
+        }
+      ]
+    }
+  ]
+}

test/discovery_and_monitoring/unified/backpressure-network-error-fail-single.json

@@ -0,0 +1,142 @@
+{
+  "description": "backpressure-network-error-fail-single",
+  "schemaVersion": "1.17",
+  "runOnRequirements": [
+    {
+      "minServerVersion": "4.4",
+      "serverless": "forbid",
+      "topologies": [
+        "single"
+      ]
+    }
+  ],
+  "createEntities": [
+    {
+      "client": {
+        "id": "setupClient",
+        "useMultipleMongoses": false
+      }
+    }
+  ],
+  "initialData": [
+    {
+      "collectionName": "backpressure-network-error-fail",
+      "databaseName": "sdam-tests",
+      "documents": [
+        {
+          "_id": 1
+        },
+        {
+          "_id": 2
+        }
+      ]
+    }
+  ],
+  "tests": [
+    {
+      "description": "apply backpressure on network connection errors during connection establishment",
+      "operations": [
+        {
+          "name": "createEntities",
+          "object": "testRunner",
+          "arguments": {
+            "entities": [
+              {
+                "client": {
+                  "id": "client",
+                  "useMultipleMongoses": false,
+                  "observeEvents": [
+                    "serverDescriptionChangedEvent",
+                    "poolClearedEvent"
+                  ],
+                  "uriOptions": {
+                    "retryWrites": false,
+                    "heartbeatFrequencyMS": 1000000,
+                    "serverMonitoringMode": "poll",
+                    "appname": "backpressureNetworkErrorFailTest"
+                  }
+                }
+              },
+              {
+                "database": {
+                  "id": "database",
+                  "client": "client",
+                  "databaseName": "sdam-tests"
+                }
+              },
+              {
+                "collection": {
+                  "id": "collection",
+                  "database": "database",
+                  "collectionName": "backpressure-network-error-fail"
+                }
+              }
+            ]
+          }
+        },
+        {
+          "name": "waitForEvent",
+          "object": "testRunner",
+          "arguments": {
+            "client": "client",
+            "event": {
+              "serverDescriptionChangedEvent": {
+                "newDescription": {
+                  "type": "Standalone"
+                }
+              }
+            },
+            "count": 1
+          }
+        },
+        {
+          "name": "failPoint",
+          "object": "testRunner",
+          "arguments": {
+            "client": "setupClient",
+            "failPoint": {
+              "configureFailPoint": "failCommand",
+              "mode": "alwaysOn",
+              "data": {
+                "failCommands": [
+                  "isMaster",
+                  "hello"
+                ],
+                "appName": "backpressureNetworkErrorFailTest",
+                "closeConnection": true
+              }
+            }
+          }
+        },
+        {
+          "name": "insertMany",
+          "object": "collection",
+          "arguments": {
+            "documents": [
+              {
+                "_id": 3
+              },
+              {
+                "_id": 4
+              }
+            ]
+          },
+          "expectError": {
+            "isError": true,
+            "errorLabelsContain": [
+              "SystemOverloadedError",
+              "RetryableError"
+            ]
+          }
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client",
+          "eventType": "cmap",
+          "events": []
+        }
+      ]
+    }
+  ]
+}

test/discovery_and_monitoring/unified/backpressure-network-timeout-fail-replicaset.json

@@ -0,0 +1,145 @@
+{
+  "description": "backpressure-network-timeout-error-replicaset",
+  "schemaVersion": "1.17",
+  "runOnRequirements": [
+    {
+      "minServerVersion": "4.4",
+      "serverless": "forbid",
+      "topologies": [
+        "replicaset"
+      ]
+    }
+  ],
+  "createEntities": [
+    {
+      "client": {
+        "id": "setupClient",
+        "useMultipleMongoses": false
+      }
+    }
+  ],
+  "initialData": [
+    {
+      "collectionName": "backpressure-network-timeout-error",
+      "databaseName": "sdam-tests",
+      "documents": [
+        {
+          "_id": 1
+        },
+        {
+          "_id": 2
+        }
+      ]
+    }
+  ],
+  "tests": [
+    {
+      "description": "apply backpressure on network timeout error during connection establishment",
+      "operations": [
+        {
+          "name": "createEntities",
+          "object": "testRunner",
+          "arguments": {
+            "entities": [
+              {
+                "client": {
+                  "id": "client",
+                  "useMultipleMongoses": false,
+                  "observeEvents": [
+                    "serverDescriptionChangedEvent",
+                    "poolClearedEvent"
+                  ],
+                  "uriOptions": {
+                    "retryWrites": false,
+                    "heartbeatFrequencyMS": 1000000,
+                    "appname": "backpressureNetworkTimeoutErrorTest",
+                    "serverMonitoringMode": "poll",
+                    "connectTimeoutMS": 250,
+                    "socketTimeoutMS": 250
+                  }
+                }
+              },
+              {
+                "database": {
+                  "id": "database",
+                  "client": "client",
+                  "databaseName": "sdam-tests"
+                }
+              },
+              {
+                "collection": {
+                  "id": "collection",
+                  "database": "database",
+                  "collectionName": "backpressure-network-timeout-error"
+                }
+              }
+            ]
+          }
+        },
+        {
+          "name": "waitForEvent",
+          "object": "testRunner",
+          "arguments": {
+            "client": "client",
+            "event": {
+              "serverDescriptionChangedEvent": {
+                "newDescription": {
+                  "type": "RSPrimary"
+                }
+              }
+            },
+            "count": 1
+          }
+        },
+        {
+          "name": "failPoint",
+          "object": "testRunner",
+          "arguments": {
+            "client": "setupClient",
+            "failPoint": {
+              "configureFailPoint": "failCommand",
+              "mode": "alwaysOn",
+              "data": {
+                "failCommands": [
+                  "isMaster",
+                  "hello"
+                ],
+                "blockConnection": true,
+                "blockTimeMS": 500,
+                "appName": "backpressureNetworkTimeoutErrorTest"
+              }
+            }
+          }
+        },
+        {
+          "name": "insertMany",
+          "object": "collection",
+          "arguments": {
+            "documents": [
+              {
+                "_id": 3
+              },
+              {
+                "_id": 4
+              }
+            ]
+          },
+          "expectError": {
+            "isError": true,
+            "errorLabelsContain": [
+              "SystemOverloadedError",
+              "RetryableError"
+            ]
+          }
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client",
+          "eventType": "cmap",
+          "events": []
+        }
+      ]
+    }
+  ]
+}

test/discovery_and_monitoring/unified/backpressure-network-timeout-fail-single.json

@@ -0,0 +1,145 @@
+{
+  "description": "backpressure-network-timeout-error-single",
+  "schemaVersion": "1.17",
+  "runOnRequirements": [
+    {
+      "minServerVersion": "4.4",
+      "serverless": "forbid",
+      "topologies": [
+        "single"
+      ]
+    }
+  ],
+  "createEntities": [
+    {
+      "client": {
+        "id": "setupClient",
+        "useMultipleMongoses": false
+      }
+    }
+  ],
+  "initialData": [
+    {
+      "collectionName": "backpressure-network-timeout-error",
+      "databaseName": "sdam-tests",
+      "documents": [
+        {
+          "_id": 1
+        },
+        {
+          "_id": 2
+        }
+      ]
+    }
+  ],
+  "tests": [
+    {
+      "description": "apply backpressure on network timeout error during connection establishment",
+      "operations": [
+        {
+          "name": "createEntities",
+          "object": "testRunner",
+          "arguments": {
+            "entities": [
+              {
+                "client": {
+                  "id": "client",
+                  "useMultipleMongoses": false,
+                  "observeEvents": [
+                    "serverDescriptionChangedEvent",
+                    "poolClearedEvent"
+                  ],
+                  "uriOptions": {
+                    "retryWrites": false,
+                    "heartbeatFrequencyMS": 1000000,
+                    "appname": "backpressureNetworkTimeoutErrorTest",
+                    "serverMonitoringMode": "poll",
+                    "connectTimeoutMS": 250,
+                    "socketTimeoutMS": 250
+                  }
+                }
+              },
+              {
+                "database": {
+                  "id": "database",
+                  "client": "client",
+                  "databaseName": "sdam-tests"
+                }
+              },
+              {
+                "collection": {
+                  "id": "collection",
+                  "database": "database",
+                  "collectionName": "backpressure-network-timeout-error"
+                }
+              }
+            ]
+          }
+        },
+        {
+          "name": "waitForEvent",
+          "object": "testRunner",
+          "arguments": {
+            "client": "client",
+            "event": {
+              "serverDescriptionChangedEvent": {
+                "newDescription": {
+                  "type": "Standalone"
+                }
+              }
+            },
+            "count": 1
+          }
+        },
+        {
+          "name": "failPoint",
+          "object": "testRunner",
+          "arguments": {
+            "client": "setupClient",
+            "failPoint": {
+              "configureFailPoint": "failCommand",
+              "mode": "alwaysOn",
+              "data": {
+                "failCommands": [
+                  "isMaster",
+                  "hello"
+                ],
+                "blockConnection": true,
+                "blockTimeMS": 500,
+                "appName": "backpressureNetworkTimeoutErrorTest"
+              }
+            }
+          }
+        },
+        {
+          "name": "insertMany",
+          "object": "collection",
+          "arguments": {
+            "documents": [
+              {
+                "_id": 3
+              },
+              {
+                "_id": 4
+              }
+            ]
+          },
+          "expectError": {
+            "isError": true,
+            "errorLabelsContain": [
+              "SystemOverloadedError",
+              "RetryableError"
+            ]
+          }
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client",
+          "eventType": "cmap",
+          "events": []
+        }
+      ]
+    }
+  ]
+}

test/discovery_and_monitoring/unified/backpressure-server-description-unchanged-on-min-pool-size-population-error.json

@@ -0,0 +1,106 @@
+{
+  "description": "backpressure-server-description-unchanged-on-min-pool-size-population-error",
+  "schemaVersion": "1.17",
+  "runOnRequirements": [
+    {
+      "minServerVersion": "4.4",
+      "serverless": "forbid",
+      "topologies": [
+        "single"
+      ]
+    }
+  ],
+  "createEntities": [
+    {
+      "client": {
+        "id": "setupClient",
+        "useMultipleMongoses": false
+      }
+    }
+  ],
+  "tests": [
+    {
+      "description": "the server description is not changed on handshake error during minPoolSize population",
+      "operations": [
+        {
+          "name": "failPoint",
+          "object": "testRunner",
+          "arguments": {
+            "client": "setupClient",
+            "failPoint": {
+              "configureFailPoint": "failCommand",
+              "mode": {
+                "skip": 1
+              },
+              "data": {
+                "failCommands": [
+                  "hello",
+                  "isMaster"
+                ],
+                "appName": "authErrorTest",
+                "closeConnection": true
+              }
+            }
+          }
+        },
+        {
+          "name": "createEntities",
+          "object": "testRunner",
+          "arguments": {
+            "entities": [
+              {
+                "client": {
+                  "id": "client",
+                  "observeEvents": [
+                    "serverDescriptionChangedEvent",
+                    "connectionClosedEvent"
+                  ],
+                  "uriOptions": {
+                    "appname": "authErrorTest",
+                    "minPoolSize": 5,
+                    "maxConnecting": 1,
+                    "serverMonitoringMode": "poll",
+                    "heartbeatFrequencyMS": 1000000
+                  }
+                }
+              }
+            ]
+          }
+        },
+        {
+          "name": "waitForEvent",
+          "object": "testRunner",
+          "arguments": {
+            "client": "client",
+            "event": {
+              "serverDescriptionChangedEvent": {}
+            },
+            "count": 1
+          }
+        },
+        {
+          "name": "waitForEvent",
+          "object": "testRunner",
+          "arguments": {
+            "client": "client",
+            "event": {
+              "connectionClosedEvent": {}
+            },
+            "count": 1
+          }
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client",
+          "eventType": "sdam",
+          "events": [
+            {
+              "serverDescriptionChangedEvent": {}
+            }
+          ]
+        }
+      ]
+    }
+  ]
+}

test/test_client.py

@@ -645,6 +645,38 @@ class ClientUnitTest(UnitTest):
             with self.assertWarns(UserWarning):
                 self.simple_client(multi_host)
 
+    def test_max_adaptive_retries(self):
+        # Assert that max adaptive retries defaults to 2.
+        c = self.simple_client(connect=False)
+        self.assertEqual(c.options.max_adaptive_retries, 2)
+
+        # Assert that max adaptive retries can be configured through connection or client options.
+        c = self.simple_client(connect=False, max_adaptive_retries=10)
+        self.assertEqual(c.options.max_adaptive_retries, 10)
+
+        c = self.simple_client(connect=False, maxAdaptiveRetries=10)
+        self.assertEqual(c.options.max_adaptive_retries, 10)
+
+        c = self.simple_client(host="mongodb://localhost/?maxAdaptiveRetries=10", connect=False)
+        self.assertEqual(c.options.max_adaptive_retries, 10)
+
+    def test_enable_overload_retargeting(self):
+        # Assert that overload retargeting defaults to false.
+        c = self.simple_client(connect=False)
+        self.assertFalse(c.options.enable_overload_retargeting)
+
+        # Assert that overload retargeting can be enabled through connection or client options.
+        c = self.simple_client(connect=False, enable_overload_retargeting=True)
+        self.assertTrue(c.options.enable_overload_retargeting)
+
+        c = self.simple_client(connect=False, enableOverloadRetargeting=True)
+        self.assertTrue(c.options.enable_overload_retargeting)
+
+        c = self.simple_client(
+            host="mongodb://localhost/?enableOverloadRetargeting=true", connect=False
+        )
+        self.assertTrue(c.options.enable_overload_retargeting)
+
 
 class TestClient(IntegrationTest):
     def test_multiple_uris(self):

test/test_client_backpressure.py

@@ -0,0 +1,310 @@
+# Copyright 2025-present MongoDB, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Test Client Backpressure spec."""
+from __future__ import annotations
+
+import os
+import pathlib
+import sys
+from time import perf_counter
+from unittest.mock import patch
+
+from pymongo.common import MAX_ADAPTIVE_RETRIES
+
+sys.path[0:0] = [""]
+
+from test import (
+    IntegrationTest,
+    client_context,
+    unittest,
+)
+from test.unified_format import generate_test_classes
+from test.utils_shared import EventListener, OvertCommandListener
+
+from pymongo.errors import OperationFailure, PyMongoError
+
+_IS_SYNC = True
+
+# Mock a system overload error.
+mock_overload_error = {
+    "configureFailPoint": "failCommand",
+    "mode": {"times": 1},
+    "data": {
+        "failCommands": ["find", "insert", "update"],
+        "errorCode": 462,  # IngressRequestRateLimitExceeded
+        "errorLabels": ["RetryableError", "SystemOverloadedError"],
+    },
+}
+
+
+def get_mock_overload_error(times: int):
+    error = mock_overload_error.copy()
+    error["mode"] = {"times": times}
+    return error
+
+
+class TestBackpressure(IntegrationTest):
+    RUN_ON_LOAD_BALANCER = True
+
+    @client_context.require_failCommand_appName
+    def test_retry_overload_error_command(self):
+        self.db.t.insert_one({"x": 1})
+
+        # Ensure command is retried on overload error.
+        fail_many = get_mock_overload_error(MAX_ADAPTIVE_RETRIES)
+        with self.fail_point(fail_many):
+            self.db.command("find", "t")
+
+        # Ensure command stops retrying after MAX_ADAPTIVE_RETRIES.
+        fail_too_many = get_mock_overload_error(MAX_ADAPTIVE_RETRIES + 1)
+        with self.fail_point(fail_too_many):
+            with self.assertRaises(PyMongoError) as error:
+                self.db.command("find", "t")
+
+        self.assertIn("RetryableError", str(error.exception))
+        self.assertIn("SystemOverloadedError", str(error.exception))
+
+    @client_context.require_failCommand_appName
+    def test_retry_overload_error_find(self):
+        self.db.t.insert_one({"x": 1})
+
+        # Ensure command is retried on overload error.
+        fail_many = get_mock_overload_error(MAX_ADAPTIVE_RETRIES)
+        with self.fail_point(fail_many):
+            self.db.t.find_one()
+
+        # Ensure command stops retrying after MAX_ADAPTIVE_RETRIES.
+        fail_too_many = get_mock_overload_error(MAX_ADAPTIVE_RETRIES + 1)
+        with self.fail_point(fail_too_many):
+            with self.assertRaises(PyMongoError) as error:
+                self.db.t.find_one()
+
+        self.assertIn("RetryableError", str(error.exception))
+        self.assertIn("SystemOverloadedError", str(error.exception))
+
+    @client_context.require_failCommand_appName
+    def test_retry_overload_error_insert_one(self):
+        # Ensure command is retried on overload error.
+        fail_many = get_mock_overload_error(MAX_ADAPTIVE_RETRIES)
+        with self.fail_point(fail_many):
+            self.db.t.insert_one({"x": 1})
+
+        # Ensure command stops retrying after MAX_ADAPTIVE_RETRIES.
+        fail_too_many = get_mock_overload_error(MAX_ADAPTIVE_RETRIES + 1)
+        with self.fail_point(fail_too_many):
+            with self.assertRaises(PyMongoError) as error:
+                self.db.t.insert_one({"x": 1})
+
+        self.assertIn("RetryableError", str(error.exception))
+        self.assertIn("SystemOverloadedError", str(error.exception))
+
+    @client_context.require_failCommand_appName
+    def test_retry_overload_error_update_many(self):
+        # Even though update_many is not a retryable write operation, it will
+        # still be retried via the "RetryableError" error label.
+        self.db.t.insert_one({"x": 1})
+
+        # Ensure command is retried on overload error.
+        fail_many = get_mock_overload_error(MAX_ADAPTIVE_RETRIES)
+        with self.fail_point(fail_many):
+            self.db.t.update_many({}, {"$set": {"x": 2}})
+
+        # Ensure command stops retrying after MAX_ADAPTIVE_RETRIES.
+        fail_too_many = get_mock_overload_error(MAX_ADAPTIVE_RETRIES + 1)
+        with self.fail_point(fail_too_many):
+            with self.assertRaises(PyMongoError) as error:
+                self.db.t.update_many({}, {"$set": {"x": 2}})
+
+        self.assertIn("RetryableError", str(error.exception))
+        self.assertIn("SystemOverloadedError", str(error.exception))
+
+    @client_context.require_failCommand_appName
+    def test_retry_overload_error_getMore(self):
+        coll = self.db.t
+        coll.insert_many([{"x": 1} for _ in range(10)])
+
+        # Ensure command is retried on overload error.
+        fail_many = {
+            "configureFailPoint": "failCommand",
+            "mode": {"times": MAX_ADAPTIVE_RETRIES},
+            "data": {
+                "failCommands": ["getMore"],
+                "errorCode": 462,  # IngressRequestRateLimitExceeded
+                "errorLabels": ["RetryableError", "SystemOverloadedError"],
+            },
+        }
+        cursor = coll.find(batch_size=2)
+        cursor.next()
+        with self.fail_point(fail_many):
+            cursor.to_list()
+
+        # Ensure command stops retrying after MAX_ADAPTIVE_RETRIES.
+        fail_too_many = fail_many.copy()
+        fail_too_many["mode"] = {"times": MAX_ADAPTIVE_RETRIES + 1}
+        cursor = coll.find(batch_size=2)
+        cursor.next()
+        with self.fail_point(fail_too_many):
+            with self.assertRaises(PyMongoError) as error:
+                cursor.to_list()
+
+        self.assertIn("RetryableError", str(error.exception))
+        self.assertIn("SystemOverloadedError", str(error.exception))
+
+
+# Prose tests.
+class TestClientBackpressure(IntegrationTest):
+    listener: EventListener
+
+    @classmethod
+    def setUpClass(cls) -> None:
+        cls.listener = OvertCommandListener()
+
+    @client_context.require_connection
+    def setUp(self) -> None:
+        super().setUp()
+        self.listener.reset()
+        self.app_name = self.__class__.__name__.lower()
+        self.client = self.rs_or_single_client(
+            event_listeners=[self.listener], appName=self.app_name
+        )
+
+    @patch("random.random")
+    @client_context.require_failCommand_appName
+    def test_01_operation_retry_uses_exponential_backoff(self, random_func):
+        # Drivers should test that retries do not occur immediately when a SystemOverloadedError is encountered.
+
+        # 1. let `client` be a `MongoClient`
+        client = self.client
+
+        # 2. let `collection` be a collection
+        collection = client.test.test
+
+        # 3. Now, run transactions without backoff:
+
+        # a. Configure the random number generator used for jitter to always return `0` -- this effectively disables backoff.
+        random_func.return_value = 0
+
+        # b. Configure the following failPoint:
+        fail_point = dict(
+            mode="alwaysOn",
+            data=dict(
+                failCommands=["insert"],
+                errorCode=2,
+                errorLabels=["SystemOverloadedError", "RetryableError"],
+                appName=self.app_name,
+            ),
+        )
+        with self.fail_point(fail_point):
+            # c. Execute the following command. Expect that the command errors. Measure the duration of the command execution.
+            start0 = perf_counter()
+            with self.assertRaises(OperationFailure):
+                collection.insert_one({"a": 1})
+            end0 = perf_counter()
+
+            # d. Configure the random number generator used for jitter to always return `1`.
+            random_func.return_value = 1
+
+            # e. Execute step c again.
+            start1 = perf_counter()
+            with self.assertRaises(OperationFailure):
+                collection.insert_one({"a": 1})
+            end1 = perf_counter()
+
+            # f. Compare the times between the two runs.
+            # The sum of 2 backoffs is 0.3 seconds. There is a 0.3-second window to account for potential variance between the two
+            # runs.
+            self.assertTrue(abs((end1 - start1) - (end0 - start0 + 0.3)) < 0.3)
+
+    @client_context.require_failCommand_appName
+    def test_03_overload_retries_limited(self):
+        # Drivers should test that overload errors are retried a maximum of two times.
+
+        # 1. Let `client` be a `MongoClient`.
+        client = self.client
+        # 2. Let `coll` be a collection.
+        coll = client.pymongo_test.coll
+
+        # 3. Configure the following failpoint:
+        failpoint = {
+            "configureFailPoint": "failCommand",
+            "mode": "alwaysOn",
+            "data": {
+                "failCommands": ["find"],
+                "errorCode": 462,  # IngressRequestRateLimitExceeded
+                "errorLabels": ["RetryableError", "SystemOverloadedError"],
+            },
+        }
+
+        # 4. Perform a find operation with `coll` that fails.
+        with self.fail_point(failpoint):
+            with self.assertRaises(PyMongoError) as error:
+                coll.find_one({})
+
+        # 5. Assert that the raised error contains both the `RetryableError` and `SystemOverloadedError` error labels.
+        self.assertIn("RetryableError", str(error.exception))
+        self.assertIn("SystemOverloadedError", str(error.exception))
+
+        # 6. Assert that the total number of started commands is MAX_ADAPTIVE_RETRIES + 1.
+        self.assertEqual(len(self.listener.started_events), MAX_ADAPTIVE_RETRIES + 1)
+
+    @client_context.require_failCommand_appName
+    def test_04_overload_retries_limited_configured(self):
+        # Drivers should test that overload errors are retried a maximum of maxAdaptiveRetries times.
+        max_retries = 1
+
+        # 1. Let `client` be a `MongoClient` with `maxAdaptiveRetries=1` and command event monitoring enabled.
+        client = self.single_client(maxAdaptiveRetries=max_retries, event_listeners=[self.listener])
+        # 2. Let `coll` be a collection.
+        coll = client.pymongo_test.coll
+
+        # 3. Configure the following failpoint:
+        failpoint = {
+            "configureFailPoint": "failCommand",
+            "mode": "alwaysOn",
+            "data": {
+                "failCommands": ["find"],
+                "errorCode": 462,  # IngressRequestRateLimitExceeded
+                "errorLabels": ["RetryableError", "SystemOverloadedError"],
+            },
+        }
+
+        # 4. Perform a find operation with `coll` that fails.
+        with self.fail_point(failpoint):
+            with self.assertRaises(PyMongoError) as error:
+                coll.find_one({})
+
+        # 5. Assert that the raised error contains both the `RetryableError` and `SystemOverloadedError` error labels.
+        self.assertIn("RetryableError", str(error.exception))
+        self.assertIn("SystemOverloadedError", str(error.exception))
+
+        # 6. Assert that the total number of started commands is max_retries + 1.
+        self.assertEqual(len(self.listener.started_events), max_retries + 1)
+
+
+# Location of JSON test specifications.
+if _IS_SYNC:
+    _TEST_PATH = os.path.join(pathlib.Path(__file__).resolve().parent, "client-backpressure")
+else:
+    _TEST_PATH = os.path.join(pathlib.Path(__file__).resolve().parent.parent, "client-backpressure")
+
+globals().update(
+    generate_test_classes(
+        _TEST_PATH,
+        module=__name__,
+    )
+)
+
+if __name__ == "__main__":
+    unittest.main()

test/test_client_metadata.py

@@ -219,6 +219,19 @@ class TestClientMetadataProse(IntegrationTest):
         # add same metadata again
         self.check_metadata_added(client, "Framework", None, None)
 
+    def test_handshake_documents_include_backpressure(self):
+        # Create a `MongoClient` that is configured to record all handshake documents sent to the server as a part of
+        # connection establishment.
+        client = self.rs_or_single_client("mongodb://" + self.server.address_string)
+
+        # Send a `ping` command to the server and verify that the command succeeds. This ensure that a connection is
+        # established on all topologies.  Note: MockupDB only supports standalone servers.
+        client.admin.command("ping")
+
+        # Assert that for every handshake document intercepted:
+        # the document has a field `backpressure` whose value is `true`.
+        self.assertEqual(self.handshake_req["backpressure"], True)
+
 
 if __name__ == "__main__":
     unittest.main()

test/test_discovery_and_monitoring.py

@@ -25,7 +25,9 @@ from asyncio import StreamReader, StreamWriter
 from pathlib import Path
 from test.helpers import ConcurrentRunner
 from test.utils import flaky
+from test.utils_shared import delay
 
+from pymongo.errors import ConnectionFailure
 from pymongo.operations import _Op
 from pymongo.server_selectors import writable_server_selector
 from pymongo.synchronous.pool import Connection
@@ -67,7 +69,12 @@ from pymongo.errors import (
 )
 from pymongo.hello import Hello, HelloCompat
 from pymongo.helpers_shared import _check_command_response, _check_write_command_response
-from pymongo.monitoring import ServerHeartbeatFailedEvent, ServerHeartbeatStartedEvent
+from pymongo.monitoring import (
+    ConnectionCheckOutFailedEvent,
+    PoolClearedEvent,
+    ServerHeartbeatFailedEvent,
+    ServerHeartbeatStartedEvent,
+)
 from pymongo.server_description import SERVER_TYPE, ServerDescription
 from pymongo.synchronous.settings import TopologySettings
 from pymongo.synchronous.topology import Topology, _ErrorContext
@@ -131,6 +138,9 @@ def got_app_error(topology, app_error):
         raise AssertionError
     except (AutoReconnect, NotPrimaryError, OperationFailure) as e:
         if when == "beforeHandshakeCompletes":
+            # The pool would have added the SystemOverloadedError in this case.
+            if isinstance(e, AutoReconnect):
+                e._add_error_label("SystemOverloadedError")
             completed_handshake = False
         elif when == "afterHandshakeCompletes":
             completed_handshake = True
@@ -437,6 +447,57 @@ class TestPoolManagement(IntegrationTest):
             Connection.close_conn = original_close
 
 
+class TestPoolBackpressure(IntegrationTest):
+    @client_context.require_version_min(7, 0, 0)
+    def test_connection_pool_is_not_cleared(self):
+        listener = CMAPListener()
+
+        # Create a client that listens to CMAP events, with maxConnecting=100.
+        client = self.rs_or_single_client(maxConnecting=100, event_listeners=[listener])
+
+        # Enable the ingress rate limiter.
+        client.admin.command(
+            "setParameter", 1, ingressConnectionEstablishmentRateLimiterEnabled=True
+        )
+        client.admin.command("setParameter", 1, ingressConnectionEstablishmentRatePerSec=20)
+        client.admin.command("setParameter", 1, ingressConnectionEstablishmentBurstCapacitySecs=1)
+        client.admin.command("setParameter", 1, ingressConnectionEstablishmentMaxQueueDepth=1)
+
+        # Disable the ingress rate limiter on teardown.
+        # Sleep for 1 second before disabling to avoid the rate limiter.
+        def teardown():
+            time.sleep(1)
+            client.admin.command(
+                "setParameter", 1, ingressConnectionEstablishmentRateLimiterEnabled=False
+            )
+
+        self.addCleanup(teardown)
+
+        # Make sure the collection has at least one document.
+        client.test.test.delete_many({})
+        client.test.test.insert_one({})
+
+        # Run a slow operation to tie up the connection.
+        def target():
+            try:
+                client.test.test.find_one({"$where": delay(0.1)})
+            except ConnectionFailure:
+                pass
+
+        # Run 100 parallel operations that contend for connections.
+        tasks = []
+        for _ in range(100):
+            tasks.append(ConcurrentRunner(target=target))
+        for t in tasks:
+            t.start()
+        for t in tasks:
+            t.join()
+
+        # Verify there were at least 10 connection checkout failed event but no pool cleared events.
+        self.assertGreater(len(listener.events_by_type(ConnectionCheckOutFailedEvent)), 10)
+        self.assertEqual(len(listener.events_by_type(PoolClearedEvent)), 0)
+
+
 class TestServerMonitoringMode(IntegrationTest):
     @client_context.require_no_load_balancer
     def setUp(self):

test/test_pooling.py

@@ -511,6 +511,39 @@ class TestPooling(_TestPoolingBase):
             str(error.exception),
         )
 
+    @client_context.require_failCommand_appName
+    def test_pool_backpressure_preserves_existing_connections(self):
+        client = self.rs_or_single_client()
+        coll = client.pymongo_test.t
+        pool = get_pool(client)
+        coll.insert_many([{"x": 1} for _ in range(10)])
+        t = SocketGetter(self.c, pool)
+        t.start()
+        while t.state != "connection":
+            time.sleep(0.1)
+
+        assert not t.sock.conn_closed()
+
+        # Mock a session establishment overload.
+        mock_connection_fail = {
+            "configureFailPoint": "failCommand",
+            "mode": {"times": 1},
+            "data": {
+                "closeConnection": True,
+            },
+        }
+
+        with self.fail_point(mock_connection_fail):
+            coll.find_one({})
+
+        # Make sure the existing socket was not affected.
+        assert not t.sock.conn_closed()
+
+        # Cleanup
+        t.release_conn()
+        t.join()
+        pool.close()
+
 
 class TestPoolMaxSize(_TestPoolingBase):
     def test_max_pool_size(self):

test/test_retryable_reads.py

@@ -263,14 +263,17 @@ class TestRetryableReads(IntegrationTest):
     @client_context.require_secondaries_count(1)
     @client_context.require_failCommand_fail_point
     @client_context.require_version_min(4, 4, 0)
-    def test_03_01_retryable_reads_caused_by_overload_errors_are_retried_on_a_different_replicaset_server_when_one_is_available(
+    def test_03_01_retryable_reads_caused_by_overload_errors_are_retried_on_a_different_replicaset_server_when_one_is_available_and_overload_retargeting_is_enabled(
         self
     ):
         listener = OvertCommandListener()
 
-        # 1. Create a client `client` with `retryReads=true`, `readPreference=primaryPreferred`, and command event monitoring enabled.
+        # 1. Create a client `client` with `retryReads=true`, `readPreference=primaryPreferred`, `enableOverloadRetargeting=True`, and command event monitoring enabled.
         client = self.rs_or_single_client(
-            event_listeners=[listener], retryReads=True, readPreference="primaryPreferred"
+            event_listeners=[listener],
+            retryReads=True,
+            readPreference="primaryPreferred",
+            enableOverloadRetargeting=True,
         )
 
         # 2. Configure a fail point with the RetryableError and SystemOverloadedError error labels.
@@ -337,6 +340,47 @@ class TestRetryableReads(IntegrationTest):
         # 6. Assert that both events occurred the same server.
         assert listener.failed_events[0].connection_id == listener.succeeded_events[0].connection_id
 
+    @client_context.require_replica_set
+    @client_context.require_secondaries_count(1)
+    @client_context.require_failCommand_fail_point
+    @client_context.require_version_min(4, 4, 0)
+    def test_03_03_retryable_reads_caused_by_overload_errors_are_retried_on_the_same_replicaset_server_when_one_is_available_and_overload_retargeting_is_disabled(
+        self
+    ):
+        listener = OvertCommandListener()
+
+        # 1. Create a client `client` with `retryReads=true`, `readPreference=primaryPreferred`, and command event monitoring enabled.
+        client = self.rs_or_single_client(
+            event_listeners=[listener],
+            retryReads=True,
+            readPreference="primaryPreferred",
+        )
+
+        # 2. Configure a fail point with the RetryableError and SystemOverloadedError error labels.
+        command_args = {
+            "configureFailPoint": "failCommand",
+            "mode": {"times": 1},
+            "data": {
+                "failCommands": ["find"],
+                "errorLabels": ["RetryableError", "SystemOverloadedError"],
+                "errorCode": 6,
+            },
+        }
+        set_fail_point(client, command_args)
+
+        # 3. Reset the command event monitor to clear the fail point command from its stored events.
+        listener.reset()
+
+        # 4. Execute a `find` command with `client`.
+        client.t.t.find_one({})
+
+        # 5. Assert that one failed command event and one successful command event occurred.
+        self.assertEqual(len(listener.failed_events), 1)
+        self.assertEqual(len(listener.succeeded_events), 1)
+
+        # 6. Assert that both events occurred on the same server.
+        assert listener.failed_events[0].connection_id == listener.succeeded_events[0].connection_id
+
 
 if __name__ == "__main__":
     unittest.main()

test/test_retryable_writes.py

@@ -43,14 +43,17 @@ from bson.codec_options import DEFAULT_CODEC_OPTIONS
 from bson.int64 import Int64
 from bson.raw_bson import RawBSONDocument
 from bson.son import SON
+from pymongo import MongoClient
 from pymongo.errors import (
     AutoReconnect,
     ConnectionFailure,
-    OperationFailure,
+    NotPrimaryError,
+    PyMongoError,
     ServerSelectionTimeoutError,
     WriteConcernError,
 )
 from pymongo.monitoring import (
+    CommandFailedEvent,
     CommandSucceededEvent,
     ConnectionCheckedOutEvent,
     ConnectionCheckOutFailedEvent,
@@ -597,5 +600,186 @@ class TestRetryableWritesTxnNumber(IgnoreDeprecationsTest):
                 self.assertEqual(sent_txn_id, final_txn_id, msg)
 
 
+class TestErrorPropagationAfterEncounteringMultipleErrors(IntegrationTest):
+    # Only run against replica sets as mongos does not propagate the NoWritesPerformed label to the drivers.
+    @client_context.require_replica_set
+    # Run against server versions 6.0 and above.
+    @client_context.require_version_min(6, 0)  # type: ignore[untyped-decorator]
+    def setUp(self) -> None:
+        super().setUp()
+        self.setup_client = MongoClient(**client_context.default_client_options)
+        self.addCleanup(self.setup_client.close)
+
+    # TODO: After PYTHON-4595 we can use async event handlers and remove this workaround.
+    def configure_fail_point_sync(self, command_args, off=False) -> None:
+        cmd = {"configureFailPoint": "failCommand"}
+        cmd.update(command_args)
+        if off:
+            cmd["mode"] = "off"
+            cmd.pop("data", None)
+        self.setup_client.admin.command(cmd)
+
+    def test_01_drivers_return_the_correct_error_when_receiving_only_errors_without_NoWritesPerformed(
+        self
+    ) -> None:
+        # Create a client with retryWrites=true.
+        listener = OvertCommandListener()
+
+        # Configure a fail point with error code 91 (ShutdownInProgress) with the RetryableError and SystemOverloadedError error labels.
+        command_args = {
+            "configureFailPoint": "failCommand",
+            "mode": {"times": 1},
+            "data": {
+                "failCommands": ["insert"],
+                "errorLabels": ["RetryableError", "SystemOverloadedError"],
+                "errorCode": 91,
+            },
+        }
+
+        # Via the command monitoring CommandFailedEvent, configure a fail point with error code 10107 (NotWritablePrimary).
+        command_args_inner = {
+            "configureFailPoint": "failCommand",
+            "mode": "alwaysOn",
+            "data": {
+                "failCommands": ["insert"],
+                "errorCode": 10107,
+                "errorLabels": ["RetryableError", "SystemOverloadedError"],
+            },
+        }
+
+        def failed(event: CommandFailedEvent) -> None:
+            # Configure the 10107 fail point command only if the the failed event is for the 91 error configured in step 2.
+            if listener.failed_events:
+                return
+            assert event.failure["code"] == 91
+            self.configure_fail_point_sync(command_args_inner)
+            self.addCleanup(self.configure_fail_point_sync, {}, off=True)
+            listener.failed_events.append(event)
+
+        listener.failed = failed
+
+        client = self.rs_client(retryWrites=True, event_listeners=[listener])
+
+        self.configure_fail_point_sync(command_args)
+        self.addCleanup(self.configure_fail_point_sync, {}, off=True)
+
+        # Attempt an insertOne operation on any record for any database and collection.
+        # Expect the insertOne to fail with a server error.
+        with self.assertRaises(NotPrimaryError) as exc:
+            client.test.test.insert_one({})
+
+        # Assert that the error code of the server error is 10107.
+        assert exc.exception.errors["code"] == 10107  # type:ignore[call-overload]
+
+    def test_02_drivers_return_the_correct_error_when_receiving_only_errors_with_NoWritesPerformed(
+        self
+    ) -> None:
+        # Create a client with retryWrites=true.
+        listener = OvertCommandListener()
+
+        # Configure a fail point with error code 91 (ShutdownInProgress) with the RetryableError and SystemOverloadedError error labels.
+        command_args = {
+            "configureFailPoint": "failCommand",
+            "mode": {"times": 1},
+            "data": {
+                "failCommands": ["insert"],
+                "errorLabels": ["RetryableError", "SystemOverloadedError", "NoWritesPerformed"],
+                "errorCode": 91,
+            },
+        }
+
+        # Via the command monitoring CommandFailedEvent, configure a fail point with error code `10107` (NotWritablePrimary)
+        # and a NoWritesPerformed label.
+        command_args_inner = {
+            "configureFailPoint": "failCommand",
+            "mode": "alwaysOn",
+            "data": {
+                "failCommands": ["insert"],
+                "errorCode": 10107,
+                "errorLabels": ["RetryableError", "SystemOverloadedError", "NoWritesPerformed"],
+            },
+        }
+
+        def failed(event: CommandFailedEvent) -> None:
+            if listener.failed_events:
+                return
+            # Configure the 10107 fail point command only if the the failed event is for the 91 error configured in step 2.
+            assert event.failure["code"] == 91
+            self.configure_fail_point_sync(command_args_inner)
+            self.addCleanup(self.configure_fail_point_sync, {}, off=True)
+            listener.failed_events.append(event)
+
+        listener.failed = failed
+
+        client = self.rs_client(retryWrites=True, event_listeners=[listener])
+
+        self.configure_fail_point_sync(command_args)
+        self.addCleanup(self.configure_fail_point_sync, {}, off=True)
+
+        # Attempt an insertOne operation on any record for any database and collection.
+        # Expect the insertOne to fail with a server error.
+        with self.assertRaises(NotPrimaryError) as exc:
+            client.test.test.insert_one({})
+
+        # Assert that the error code of the server error is 91.
+        assert exc.exception.errors["code"] == 91  # type:ignore[call-overload]
+
+    def test_03_drivers_return_the_correct_error_when_receiving_some_errors_with_NoWritesPerformed_and_some_without_NoWritesPerformed(
+        self
+    ) -> None:
+        # Create a client with retryWrites=true.
+        listener = OvertCommandListener()
+
+        # Configure the client to listen to CommandFailedEvents. In the attached listener, configure a fail point with error
+        # code `91` (NotWritablePrimary) and the `NoWritesPerformed`, `RetryableError` and `SystemOverloadedError` labels.
+        command_args_inner = {
+            "configureFailPoint": "failCommand",
+            "mode": "alwaysOn",
+            "data": {
+                "failCommands": ["insert"],
+                "errorLabels": ["RetryableError", "SystemOverloadedError", "NoWritesPerformed"],
+                "errorCode": 91,
+            },
+        }
+
+        # Configure a fail point with error code `91` (ShutdownInProgress) with the `RetryableError` and
+        # `SystemOverloadedError` error labels but without the `NoWritesPerformed` error label.
+        command_args = {
+            "configureFailPoint": "failCommand",
+            "mode": {"times": 1},
+            "data": {
+                "failCommands": ["insert"],
+                "errorCode": 91,
+                "errorLabels": ["RetryableError", "SystemOverloadedError"],
+            },
+        }
+
+        def failed(event: CommandFailedEvent) -> None:
+            # Configure the fail point command only if the failed event is for the 91 error configured in step 2.
+            if listener.failed_events:
+                return
+            assert event.failure["code"] == 91
+            self.configure_fail_point_sync(command_args_inner)
+            self.addCleanup(self.configure_fail_point_sync, {}, off=True)
+            listener.failed_events.append(event)
+
+        listener.failed = failed
+
+        client = self.rs_client(retryWrites=True, event_listeners=[listener])
+
+        self.configure_fail_point_sync(command_args)
+        self.addCleanup(self.configure_fail_point_sync, {}, off=True)
+
+        # Attempt an insertOne operation on any record for any database and collection.
+        # Expect the insertOne to fail with a server error.
+        with self.assertRaises(PyMongoError) as exc:
+            client.test.test.insert_one({})
+
+        # Assert that the error code of the server error is 91.
+        assert exc.exception.errors["code"] == 91
+        # Assert that the error does not contain the error label `NoWritesPerformed`.
+        assert "NoWritesPerformed" not in exc.exception.errors["errorLabels"]
+
+
 if __name__ == "__main__":
     unittest.main()

test/test_transactions.py

@@ -16,9 +16,13 @@
 from __future__ import annotations
 
 import asyncio
+import random
 import sys
+import time
 from io import BytesIO
+from unittest.mock import patch
 
+import pymongo
 from gridfs.synchronous.grid_file import GridFS, GridFSBucket
 from pymongo.server_selectors import writable_server_selector
 from pymongo.synchronous.pool import PoolState
@@ -40,7 +44,9 @@ from pymongo.errors import (
     CollectionInvalid,
     ConfigurationError,
     ConnectionFailure,
+    ExecutionTimeout,
     InvalidOperation,
+    NetworkTimeout,
     OperationFailure,
 )
 from pymongo.operations import IndexModel, InsertOne
@@ -426,7 +432,7 @@ class TestTransactionsConvenientAPI(TransactionsBase):
             self.configure_fail_point(client, command_args)
 
     @client_context.require_transactions
-    def test_callback_raises_custom_error(self):
+    def test_1_callback_raises_custom_error(self):
         class _MyException(Exception):
             pass
 
@@ -438,7 +444,7 @@ class TestTransactionsConvenientAPI(TransactionsBase):
                 s.with_transaction(raise_error)
 
     @client_context.require_transactions
-    def test_callback_returns_value(self):
+    def test_2_callback_returns_value(self):
         def callback(_):
             return "Foo"
 
@@ -466,7 +472,7 @@ class TestTransactionsConvenientAPI(TransactionsBase):
             self.assertEqual(s.with_transaction(callback), "Foo")
 
     @client_context.require_transactions
-    def test_callback_not_retried_after_timeout(self):
+    def test_3_1_callback_not_retried_after_timeout(self):
         listener = OvertCommandListener()
         client = self.rs_client(event_listeners=[listener])
         coll = client[self.db.name].test
@@ -487,14 +493,16 @@ class TestTransactionsConvenientAPI(TransactionsBase):
         listener.reset()
         with client.start_session() as s:
             with PatchSessionTimeout(0):
-                with self.assertRaises(OperationFailure):
+                with self.assertRaises(NetworkTimeout) as context:
                     s.with_transaction(callback)
 
         self.assertEqual(listener.started_command_names(), ["insert", "abortTransaction"])
+        # Assert that the timeout error has the same labels as the error it wraps.
+        self.assertTrue(context.exception.has_error_label("TransientTransactionError"))
 
     @client_context.require_test_commands
     @client_context.require_transactions
-    def test_callback_not_retried_after_commit_timeout(self):
+    def test_3_2_callback_not_retried_after_commit_timeout(self):
         listener = OvertCommandListener()
         client = self.rs_client(event_listeners=[listener])
         coll = client[self.db.name].test
@@ -519,14 +527,16 @@ class TestTransactionsConvenientAPI(TransactionsBase):
 
         with client.start_session() as s:
             with PatchSessionTimeout(0):
-                with self.assertRaises(OperationFailure):
+                with self.assertRaises(NetworkTimeout) as context:
                     s.with_transaction(callback)
 
         self.assertEqual(listener.started_command_names(), ["insert", "commitTransaction"])
+        # Assert that the timeout error has the same labels as the error it wraps.
+        self.assertTrue(context.exception.has_error_label("TransientTransactionError"))
 
     @client_context.require_test_commands
     @client_context.require_transactions
-    def test_commit_not_retried_after_timeout(self):
+    def test_3_3_commit_not_retried_after_timeout(self):
         listener = OvertCommandListener()
         client = self.rs_client(event_listeners=[listener])
         coll = client[self.db.name].test
@@ -548,7 +558,7 @@ class TestTransactionsConvenientAPI(TransactionsBase):
 
         with client.start_session() as s:
             with PatchSessionTimeout(0):
-                with self.assertRaises(ConnectionFailure):
+                with self.assertRaises(NetworkTimeout) as context:
                     s.with_transaction(callback)
 
         # One insert for the callback and two commits (includes the automatic
@@ -556,6 +566,40 @@ class TestTransactionsConvenientAPI(TransactionsBase):
         self.assertEqual(
             listener.started_command_names(), ["insert", "commitTransaction", "commitTransaction"]
         )
+        # Assert that the timeout error has the same labels as the error it wraps.
+        self.assertTrue(context.exception.has_error_label("UnknownTransactionCommitResult"))
+
+    @client_context.require_transactions
+    def test_callback_not_retried_after_csot_timeout(self):
+        listener = OvertCommandListener()
+        client = self.rs_client(event_listeners=[listener])
+        coll = client[self.db.name].test
+
+        def callback(session):
+            coll.insert_one({}, session=session)
+            err: dict = {
+                "ok": 0,
+                "errmsg": "Transaction 7819 has been aborted.",
+                "code": 251,
+                "codeName": "NoSuchTransaction",
+                "errorLabels": ["TransientTransactionError"],
+            }
+            raise OperationFailure(err["errmsg"], err["code"], err)
+
+        # Create the collection.
+        coll.insert_one({})
+        listener.reset()
+        with client.start_session() as s:
+            with pymongo.timeout(1.0):
+                with self.assertRaises(ExecutionTimeout):
+                    s.with_transaction(callback)
+
+        # At least two attempts: the original and one or more retries.
+        inserts = len([x for x in listener.started_command_names() if x == "insert"])
+        aborts = len([x for x in listener.started_command_names() if x == "abortTransaction"])
+
+        self.assertGreaterEqual(inserts, 2)
+        self.assertGreaterEqual(aborts, 2)
 
     # Tested here because this supports Motor's convenient transactions API.
     @client_context.require_transactions
@@ -594,6 +638,63 @@ class TestTransactionsConvenientAPI(TransactionsBase):
             s.with_transaction(callback)
             self.assertFalse(s.in_transaction)
 
+    @client_context.require_test_commands
+    @client_context.require_transactions
+    def test_4_retry_backoff_is_enforced(self):
+        client = client_context.client
+        coll = client[self.db.name].test
+        end = start = no_backoff_time = 0
+
+        # Make random.random always return 0 (no backoff)
+        with patch.object(random, "random", return_value=0):
+            # set fail point to trigger transaction failure and trigger backoff
+            self.set_fail_point(
+                {
+                    "configureFailPoint": "failCommand",
+                    "mode": {"times": 13},
+                    "data": {
+                        "failCommands": ["commitTransaction"],
+                        "errorCode": 251,
+                    },
+                }
+            )
+            self.addCleanup(
+                self.set_fail_point, {"configureFailPoint": "failCommand", "mode": "off"}
+            )
+
+            def callback(session):
+                coll.insert_one({}, session=session)
+
+            start = time.monotonic()
+            with self.client.start_session() as s:
+                s.with_transaction(callback)
+            end = time.monotonic()
+            no_backoff_time = end - start
+
+        # Make random.random always return 1 (max backoff)
+        with patch.object(random, "random", return_value=1):
+            # set fail point to trigger transaction failure and trigger backoff
+            self.set_fail_point(
+                {
+                    "configureFailPoint": "failCommand",
+                    "mode": {
+                        "times": 13
+                    },  # sufficiently high enough such that the time effect of backoff is noticeable
+                    "data": {
+                        "failCommands": ["commitTransaction"],
+                        "errorCode": 251,
+                    },
+                }
+            )
+            self.addCleanup(
+                self.set_fail_point, {"configureFailPoint": "failCommand", "mode": "off"}
+            )
+            start = time.monotonic()
+            with self.client.start_session() as s:
+                s.with_transaction(callback)
+            end = time.monotonic()
+        self.assertLess(abs(end - start - (no_backoff_time + 2.2)), 1)  # sum of 13 backoffs is 2.2
+
 
 class TestOptionsInsideTransactionProse(TransactionsBase):
     @client_context.require_transactions

test/transactions/unified/backpressure-retryable-abort.json

@@ -0,0 +1,342 @@
+{
+  "description": "backpressure-retryable-abort",
+  "schemaVersion": "1.3",
+  "runOnRequirements": [
+    {
+      "minServerVersion": "4.4",
+      "topologies": [
+        "replicaset",
+        "sharded",
+        "load-balanced"
+      ]
+    }
+  ],
+  "createEntities": [
+    {
+      "client": {
+        "id": "client0",
+        "useMultipleMongoses": false,
+        "observeEvents": [
+          "commandStartedEvent"
+        ]
+      }
+    },
+    {
+      "database": {
+        "id": "database0",
+        "client": "client0",
+        "databaseName": "transaction-tests"
+      }
+    },
+    {
+      "collection": {
+        "id": "collection0",
+        "database": "database0",
+        "collectionName": "test"
+      }
+    },
+    {
+      "session": {
+        "id": "session0",
+        "client": "client0"
+      }
+    }
+  ],
+  "initialData": [
+    {
+      "collectionName": "test",
+      "databaseName": "transaction-tests",
+      "documents": []
+    }
+  ],
+  "tests": [
+    {
+      "description": "abortTransaction retries if backpressure labels are added",
+      "operations": [
+        {
+          "object": "testRunner",
+          "name": "failPoint",
+          "arguments": {
+            "client": "client0",
+            "failPoint": {
+              "configureFailPoint": "failCommand",
+              "mode": {
+                "times": 2
+              },
+              "data": {
+                "failCommands": [
+                  "abortTransaction"
+                ],
+                "errorLabels": [
+                  "RetryableError",
+                  "SystemOverloadedError"
+                ],
+                "errorCode": 112
+              }
+            }
+          }
+        },
+        {
+          "object": "session0",
+          "name": "startTransaction"
+        },
+        {
+          "object": "collection0",
+          "name": "insertOne",
+          "arguments": {
+            "session": "session0",
+            "document": {
+              "_id": 1
+            }
+          },
+          "expectResult": {
+            "$$unsetOrMatches": {
+              "insertedId": {
+                "$$unsetOrMatches": 1
+              }
+            }
+          }
+        },
+        {
+          "object": "session0",
+          "name": "abortTransaction"
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client0",
+          "events": [
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "insert": "test",
+                  "documents": [
+                    {
+                      "_id": 1
+                    }
+                  ],
+                  "ordered": true,
+                  "readConcern": {
+                    "$$exists": false
+                  },
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "startTransaction": true,
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "insert",
+                "databaseName": "transaction-tests"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "abortTransaction": 1,
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "startTransaction": {
+                    "$$exists": false
+                  },
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "abortTransaction",
+                "databaseName": "admin"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "abortTransaction": 1,
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "startTransaction": {
+                    "$$exists": false
+                  },
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "abortTransaction",
+                "databaseName": "admin"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "abortTransaction": 1,
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "startTransaction": {
+                    "$$exists": false
+                  },
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "abortTransaction",
+                "databaseName": "admin"
+              }
+            }
+          ]
+        }
+      ],
+      "outcome": [
+        {
+          "collectionName": "test",
+          "databaseName": "transaction-tests",
+          "documents": []
+        }
+      ]
+    },
+    {
+      "description": "abortTransaction is retried maxAttempts=2 times if backpressure labels are added",
+      "operations": [
+        {
+          "object": "testRunner",
+          "name": "failPoint",
+          "arguments": {
+            "client": "client0",
+            "failPoint": {
+              "configureFailPoint": "failCommand",
+              "mode": "alwaysOn",
+              "data": {
+                "failCommands": [
+                  "abortTransaction"
+                ],
+                "errorLabels": [
+                  "RetryableError",
+                  "SystemOverloadedError"
+                ],
+                "errorCode": 112
+              }
+            }
+          }
+        },
+        {
+          "object": "session0",
+          "name": "startTransaction"
+        },
+        {
+          "object": "collection0",
+          "name": "insertOne",
+          "arguments": {
+            "session": "session0",
+            "document": {
+              "_id": 1
+            }
+          },
+          "expectResult": {
+            "$$unsetOrMatches": {
+              "insertedId": {
+                "$$unsetOrMatches": 1
+              }
+            }
+          }
+        },
+        {
+          "object": "session0",
+          "name": "abortTransaction"
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client0",
+          "events": [
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "insert": "test",
+                  "documents": [
+                    {
+                      "_id": 1
+                    }
+                  ],
+                  "ordered": true,
+                  "readConcern": {
+                    "$$exists": false
+                  },
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "startTransaction": true,
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "insert",
+                "databaseName": "transaction-tests"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "abortTransaction": 1,
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "startTransaction": {
+                    "$$exists": false
+                  },
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "abortTransaction",
+                "databaseName": "admin"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "abortTransaction"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "abortTransaction"
+              }
+            }
+          ]
+        }
+      ],
+      "outcome": [
+        {
+          "collectionName": "test",
+          "databaseName": "transaction-tests",
+          "documents": []
+        }
+      ]
+    }
+  ]
+}

test/transactions/unified/backpressure-retryable-commit.json

@@ -0,0 +1,359 @@
+{
+  "description": "backpressure-retryable-commit",
+  "schemaVersion": "1.4",
+  "runOnRequirements": [
+    {
+      "minServerVersion": "4.4",
+      "topologies": [
+        "sharded",
+        "replicaset",
+        "load-balanced"
+      ]
+    }
+  ],
+  "createEntities": [
+    {
+      "client": {
+        "id": "client0",
+        "useMultipleMongoses": false,
+        "observeEvents": [
+          "commandStartedEvent"
+        ]
+      }
+    },
+    {
+      "database": {
+        "id": "database0",
+        "client": "client0",
+        "databaseName": "transaction-tests"
+      }
+    },
+    {
+      "collection": {
+        "id": "collection0",
+        "database": "database0",
+        "collectionName": "test"
+      }
+    },
+    {
+      "session": {
+        "id": "session0",
+        "client": "client0"
+      }
+    }
+  ],
+  "initialData": [
+    {
+      "collectionName": "test",
+      "databaseName": "transaction-tests",
+      "documents": []
+    }
+  ],
+  "tests": [
+    {
+      "description": "commitTransaction retries if backpressure labels are added",
+      "runOnRequirements": [
+        {
+          "serverless": "forbid"
+        }
+      ],
+      "operations": [
+        {
+          "object": "testRunner",
+          "name": "failPoint",
+          "arguments": {
+            "client": "client0",
+            "failPoint": {
+              "configureFailPoint": "failCommand",
+              "mode": {
+                "times": 2
+              },
+              "data": {
+                "failCommands": [
+                  "commitTransaction"
+                ],
+                "errorLabels": [
+                  "RetryableError",
+                  "SystemOverloadedError"
+                ],
+                "errorCode": 112
+              }
+            }
+          }
+        },
+        {
+          "object": "session0",
+          "name": "startTransaction"
+        },
+        {
+          "object": "collection0",
+          "name": "insertOne",
+          "arguments": {
+            "session": "session0",
+            "document": {
+              "_id": 1
+            }
+          },
+          "expectResult": {
+            "$$unsetOrMatches": {
+              "insertedId": {
+                "$$unsetOrMatches": 1
+              }
+            }
+          }
+        },
+        {
+          "object": "session0",
+          "name": "commitTransaction"
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client0",
+          "events": [
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "insert": "test",
+                  "documents": [
+                    {
+                      "_id": 1
+                    }
+                  ],
+                  "ordered": true,
+                  "readConcern": {
+                    "$$exists": false
+                  },
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "startTransaction": true,
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "insert",
+                "databaseName": "transaction-tests"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "commitTransaction": 1,
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "startTransaction": {
+                    "$$exists": false
+                  },
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "commitTransaction",
+                "databaseName": "admin"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "commitTransaction": 1,
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "startTransaction": {
+                    "$$exists": false
+                  },
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "commitTransaction",
+                "databaseName": "admin"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "commitTransaction": 1,
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "startTransaction": {
+                    "$$exists": false
+                  },
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "commitTransaction",
+                "databaseName": "admin"
+              }
+            }
+          ]
+        }
+      ],
+      "outcome": [
+        {
+          "collectionName": "test",
+          "databaseName": "transaction-tests",
+          "documents": [
+            {
+              "_id": 1
+            }
+          ]
+        }
+      ]
+    },
+    {
+      "description": "commitTransaction is retried maxAttempts=2 times if backpressure labels are added",
+      "runOnRequirements": [
+        {
+          "serverless": "forbid"
+        }
+      ],
+      "operations": [
+        {
+          "object": "testRunner",
+          "name": "failPoint",
+          "arguments": {
+            "client": "client0",
+            "failPoint": {
+              "configureFailPoint": "failCommand",
+              "mode": "alwaysOn",
+              "data": {
+                "failCommands": [
+                  "commitTransaction"
+                ],
+                "errorLabels": [
+                  "RetryableError",
+                  "SystemOverloadedError"
+                ],
+                "errorCode": 112
+              }
+            }
+          }
+        },
+        {
+          "object": "session0",
+          "name": "startTransaction"
+        },
+        {
+          "object": "collection0",
+          "name": "insertOne",
+          "arguments": {
+            "session": "session0",
+            "document": {
+              "_id": 1
+            }
+          },
+          "expectResult": {
+            "$$unsetOrMatches": {
+              "insertedId": {
+                "$$unsetOrMatches": 1
+              }
+            }
+          }
+        },
+        {
+          "object": "session0",
+          "name": "commitTransaction",
+          "expectError": {
+            "isError": true
+          }
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client0",
+          "events": [
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "insert": "test",
+                  "documents": [
+                    {
+                      "_id": 1
+                    }
+                  ],
+                  "ordered": true,
+                  "readConcern": {
+                    "$$exists": false
+                  },
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "startTransaction": true,
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "insert",
+                "databaseName": "transaction-tests"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "commitTransaction": 1,
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "startTransaction": {
+                    "$$exists": false
+                  },
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "commitTransaction",
+                "databaseName": "admin"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "commitTransaction"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "commitTransaction"
+              }
+            }
+          ]
+        }
+      ],
+      "outcome": [
+        {
+          "collectionName": "test",
+          "databaseName": "transaction-tests",
+          "documents": []
+        }
+      ]
+    }
+  ]
+}

test/transactions/unified/backpressure-retryable-reads.json

@@ -0,0 +1,313 @@
+{
+  "description": "backpressure-retryable-reads",
+  "schemaVersion": "1.3",
+  "runOnRequirements": [
+    {
+      "minServerVersion": "4.4",
+      "topologies": [
+        "replicaset",
+        "sharded",
+        "load-balanced"
+      ]
+    }
+  ],
+  "createEntities": [
+    {
+      "client": {
+        "id": "client0",
+        "useMultipleMongoses": false,
+        "observeEvents": [
+          "commandStartedEvent"
+        ]
+      }
+    },
+    {
+      "database": {
+        "id": "database0",
+        "client": "client0",
+        "databaseName": "transaction-tests"
+      }
+    },
+    {
+      "collection": {
+        "id": "collection0",
+        "database": "database0",
+        "collectionName": "test"
+      }
+    },
+    {
+      "session": {
+        "id": "session0",
+        "client": "client0"
+      }
+    }
+  ],
+  "initialData": [
+    {
+      "collectionName": "test",
+      "databaseName": "transaction-tests",
+      "documents": []
+    }
+  ],
+  "tests": [
+    {
+      "description": "reads are retried if backpressure labels are added",
+      "operations": [
+        {
+          "object": "session0",
+          "name": "startTransaction"
+        },
+        {
+          "object": "collection0",
+          "name": "insertOne",
+          "arguments": {
+            "session": "session0",
+            "document": {
+              "_id": 1
+            }
+          },
+          "expectResult": {
+            "$$unsetOrMatches": {
+              "insertedId": {
+                "$$unsetOrMatches": 1
+              }
+            }
+          }
+        },
+        {
+          "object": "testRunner",
+          "name": "failPoint",
+          "arguments": {
+            "client": "client0",
+            "failPoint": {
+              "configureFailPoint": "failCommand",
+              "mode": {
+                "times": 1
+              },
+              "data": {
+                "failCommands": [
+                  "find"
+                ],
+                "errorLabels": [
+                  "RetryableError",
+                  "SystemOverloadedError"
+                ],
+                "errorCode": 112
+              }
+            }
+          }
+        },
+        {
+          "object": "collection0",
+          "name": "find",
+          "arguments": {
+            "filter": {},
+            "session": "session0"
+          }
+        },
+        {
+          "object": "session0",
+          "name": "commitTransaction"
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client0",
+          "events": [
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "insert": "test",
+                  "documents": [
+                    {
+                      "_id": 1
+                    }
+                  ],
+                  "ordered": true,
+                  "readConcern": {
+                    "$$exists": false
+                  },
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "startTransaction": true,
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "insert",
+                "databaseName": "transaction-tests"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "find": "test",
+                  "readConcern": {
+                    "$$exists": false
+                  },
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "find",
+                "databaseName": "transaction-tests"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "find": "test",
+                  "readConcern": {
+                    "$$exists": false
+                  },
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "find",
+                "databaseName": "transaction-tests"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "abortTransaction": {
+                    "$$exists": false
+                  },
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "startTransaction": {
+                    "$$exists": false
+                  },
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "commitTransaction",
+                "databaseName": "admin"
+              }
+            }
+          ]
+        }
+      ]
+    },
+    {
+      "description": "reads are retried maxAttempts=2 times if backpressure labels are added",
+      "operations": [
+        {
+          "object": "session0",
+          "name": "startTransaction"
+        },
+        {
+          "object": "collection0",
+          "name": "insertOne",
+          "arguments": {
+            "session": "session0",
+            "document": {
+              "_id": 1
+            }
+          },
+          "expectResult": {
+            "$$unsetOrMatches": {
+              "insertedId": {
+                "$$unsetOrMatches": 1
+              }
+            }
+          }
+        },
+        {
+          "object": "testRunner",
+          "name": "failPoint",
+          "arguments": {
+            "client": "client0",
+            "failPoint": {
+              "configureFailPoint": "failCommand",
+              "mode": "alwaysOn",
+              "data": {
+                "failCommands": [
+                  "find"
+                ],
+                "errorLabels": [
+                  "RetryableError",
+                  "SystemOverloadedError"
+                ],
+                "errorCode": 112
+              }
+            }
+          }
+        },
+        {
+          "object": "collection0",
+          "name": "find",
+          "arguments": {
+            "filter": {},
+            "session": "session0"
+          },
+          "expectError": {
+            "isError": true
+          }
+        },
+        {
+          "object": "session0",
+          "name": "abortTransaction"
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client0",
+          "events": [
+            {
+              "commandStartedEvent": {
+                "commandName": "insert"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "find"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "find"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "find"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "abortTransaction"
+              }
+            }
+          ]
+        }
+      ]
+    }
+  ]
+}

test/transactions/unified/backpressure-retryable-writes.json

@@ -0,0 +1,439 @@
+{
+  "description": "backpressure-retryable-writes",
+  "schemaVersion": "1.3",
+  "runOnRequirements": [
+    {
+      "minServerVersion": "4.4",
+      "topologies": [
+        "replicaset",
+        "sharded",
+        "load-balanced"
+      ]
+    }
+  ],
+  "createEntities": [
+    {
+      "client": {
+        "id": "client0",
+        "useMultipleMongoses": false,
+        "observeEvents": [
+          "commandStartedEvent"
+        ]
+      }
+    },
+    {
+      "database": {
+        "id": "database0",
+        "client": "client0",
+        "databaseName": "transaction-tests"
+      }
+    },
+    {
+      "collection": {
+        "id": "collection0",
+        "database": "database0",
+        "collectionName": "test"
+      }
+    },
+    {
+      "session": {
+        "id": "session0",
+        "client": "client0"
+      }
+    }
+  ],
+  "initialData": [
+    {
+      "collectionName": "test",
+      "databaseName": "transaction-tests",
+      "documents": []
+    }
+  ],
+  "tests": [
+    {
+      "description": "writes are retried if backpressure labels are added",
+      "operations": [
+        {
+          "object": "session0",
+          "name": "startTransaction"
+        },
+        {
+          "object": "collection0",
+          "name": "insertOne",
+          "arguments": {
+            "session": "session0",
+            "document": {
+              "_id": 1
+            }
+          },
+          "expectResult": {
+            "$$unsetOrMatches": {
+              "insertedId": {
+                "$$unsetOrMatches": 1
+              }
+            }
+          }
+        },
+        {
+          "object": "testRunner",
+          "name": "failPoint",
+          "arguments": {
+            "client": "client0",
+            "failPoint": {
+              "configureFailPoint": "failCommand",
+              "mode": {
+                "times": 1
+              },
+              "data": {
+                "failCommands": [
+                  "insert"
+                ],
+                "errorLabels": [
+                  "RetryableError",
+                  "SystemOverloadedError"
+                ],
+                "errorCode": 112
+              }
+            }
+          }
+        },
+        {
+          "object": "collection0",
+          "name": "insertOne",
+          "arguments": {
+            "session": "session0",
+            "document": {
+              "_id": 2
+            }
+          }
+        },
+        {
+          "object": "session0",
+          "name": "commitTransaction"
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client0",
+          "events": [
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "insert": "test",
+                  "documents": [
+                    {
+                      "_id": 1
+                    }
+                  ],
+                  "ordered": true,
+                  "readConcern": {
+                    "$$exists": false
+                  },
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "startTransaction": true,
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "insert",
+                "databaseName": "transaction-tests"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "insert": "test",
+                  "documents": [
+                    {
+                      "_id": 2
+                    }
+                  ],
+                  "ordered": true,
+                  "readConcern": {
+                    "$$exists": false
+                  },
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "insert",
+                "databaseName": "transaction-tests"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "insert": "test",
+                  "documents": [
+                    {
+                      "_id": 2
+                    }
+                  ],
+                  "ordered": true,
+                  "readConcern": {
+                    "$$exists": false
+                  },
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "insert",
+                "databaseName": "transaction-tests"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "abortTransaction": {
+                    "$$exists": false
+                  },
+                  "lsid": {
+                    "$$sessionLsid": "session0"
+                  },
+                  "txnNumber": {
+                    "$numberLong": "1"
+                  },
+                  "startTransaction": {
+                    "$$exists": false
+                  },
+                  "autocommit": false,
+                  "writeConcern": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "commitTransaction",
+                "databaseName": "admin"
+              }
+            }
+          ]
+        }
+      ],
+      "outcome": [
+        {
+          "collectionName": "test",
+          "databaseName": "transaction-tests",
+          "documents": [
+            {
+              "_id": 1
+            },
+            {
+              "_id": 2
+            }
+          ]
+        }
+      ]
+    },
+    {
+      "description": "writes are retried maxAttempts=2 times if backpressure labels are added",
+      "operations": [
+        {
+          "object": "session0",
+          "name": "startTransaction"
+        },
+        {
+          "object": "collection0",
+          "name": "insertOne",
+          "arguments": {
+            "session": "session0",
+            "document": {
+              "_id": 1
+            }
+          },
+          "expectResult": {
+            "$$unsetOrMatches": {
+              "insertedId": {
+                "$$unsetOrMatches": 1
+              }
+            }
+          }
+        },
+        {
+          "object": "testRunner",
+          "name": "failPoint",
+          "arguments": {
+            "client": "client0",
+            "failPoint": {
+              "configureFailPoint": "failCommand",
+              "mode": "alwaysOn",
+              "data": {
+                "failCommands": [
+                  "insert"
+                ],
+                "errorLabels": [
+                  "RetryableError",
+                  "SystemOverloadedError"
+                ],
+                "errorCode": 112
+              }
+            }
+          }
+        },
+        {
+          "object": "collection0",
+          "name": "insertOne",
+          "arguments": {
+            "session": "session0",
+            "document": {
+              "_id": 2
+            }
+          },
+          "expectError": {
+            "isError": true
+          }
+        },
+        {
+          "object": "session0",
+          "name": "abortTransaction"
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client0",
+          "events": [
+            {
+              "commandStartedEvent": {
+                "commandName": "insert"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "insert"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "insert"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "insert"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "commandName": "abortTransaction"
+              }
+            }
+          ]
+        }
+      ],
+      "outcome": [
+        {
+          "collectionName": "test",
+          "databaseName": "transaction-tests",
+          "documents": []
+        }
+      ]
+    },
+    {
+      "description": "retry succeeds if backpressure labels are added to the first operation in a transaction",
+      "operations": [
+        {
+          "object": "session0",
+          "name": "startTransaction"
+        },
+        {
+          "object": "testRunner",
+          "name": "failPoint",
+          "arguments": {
+            "client": "client0",
+            "failPoint": {
+              "configureFailPoint": "failCommand",
+              "mode": {
+                "times": 1
+              },
+              "data": {
+                "failCommands": [
+                  "insert"
+                ],
+                "errorLabels": [
+                  "RetryableError",
+                  "SystemOverloadedError"
+                ],
+                "errorCode": 112
+              }
+            }
+          }
+        },
+        {
+          "object": "collection0",
+          "name": "insertOne",
+          "arguments": {
+            "session": "session0",
+            "document": {
+              "_id": 2
+            }
+          }
+        },
+        {
+          "object": "session0",
+          "name": "abortTransaction"
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client0",
+          "events": [
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "startTransaction": true
+                },
+                "commandName": "insert",
+                "databaseName": "transaction-tests"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "startTransaction": true
+                },
+                "commandName": "insert",
+                "databaseName": "transaction-tests"
+              }
+            },
+            {
+              "commandStartedEvent": {
+                "command": {
+                  "startTransaction": {
+                    "$$exists": false
+                  }
+                },
+                "commandName": "abortTransaction",
+                "databaseName": "admin"
+              }
+            }
+          ]
+        }
+      ],
+      "outcome": [
+        {
+          "collectionName": "test",
+          "databaseName": "transaction-tests",
+          "documents": []
+        }
+      ]
+    }
+  ]
+}

test/uri_options/client-backpressure-options.json

@@ -0,0 +1,66 @@
+{
+  "tests": [
+    {
+      "description": "maxAdaptiveRetries is parsed correctly",
+      "uri": "mongodb://example.com/?maxAdaptiveRetries=3",
+      "valid": true,
+      "warning": false,
+      "hosts": null,
+      "auth": null,
+      "options": {
+        "maxAdaptiveRetries": 3
+      }
+    },
+    {
+      "description": "maxAdaptiveRetries=0 is parsed correctly",
+      "uri": "mongodb://example.com/?maxAdaptiveRetries=0",
+      "valid": true,
+      "warning": false,
+      "hosts": null,
+      "auth": null,
+      "options": {
+        "maxAdaptiveRetries": 0
+      }
+    },
+    {
+      "description": "maxAdaptiveRetries with invalid value causes a warning",
+      "uri": "mongodb://example.com/?maxAdaptiveRetries=-5",
+      "valid": true,
+      "warning": true,
+      "hosts": null,
+      "auth": null,
+      "options": null
+    },
+    {
+      "description": "enableOverloadRetargeting is parsed correctly",
+      "uri": "mongodb://example.com/?enableOverloadRetargeting=true",
+      "valid": true,
+      "warning": false,
+      "hosts": null,
+      "auth": null,
+      "options": {
+        "enableOverloadRetargeting": true
+      }
+    },
+    {
+      "description": "enableOverloadRetargeting=false is parsed correctly",
+      "uri": "mongodb://example.com/?enableOverloadRetargeting=false",
+      "valid": true,
+      "warning": false,
+      "hosts": null,
+      "auth": null,
+      "options": {
+        "enableOverloadRetargeting": false
+      }
+    },
+    {
+      "description": "enableOverloadRetargeting with invalid value causes a warning",
+      "uri": "mongodb://example.com/?enableOverloadRetargeting=invalid",
+      "valid": true,
+      "warning": true,
+      "hosts": null,
+      "auth": null,
+      "options": null
+    }
+  ]
+}

tools/synchro.py

@@ -213,6 +213,7 @@ converted_tests = [
     "test_bulk.py",
     "test_change_stream.py",
     "test_client.py",
+    "test_client_backpressure.py",
     "test_client_bulk_write.py",
     "test_client_context.py",
     "test_client_metadata.py",
@@ -350,7 +351,7 @@ def translate_async_sleeps(lines: list[str]) -> list[str]:
     sleeps = [line for line in lines if "asyncio.sleep" in line]
 
     for line in sleeps:
-        res = re.search(r"asyncio.sleep\(([^()]*)\)", line)
+        res = re.search(r"asyncio\.sleep\(\s*(.*?)\)", line)
         if res:
             old = res[0]
             index = lines.index(line)