Server Selector Example
=======================

Users can exert fine-grained control over the `server selection algorithm`_
by setting the ``server_selector`` option on the :class:`~pymongo.MongoClient`
to an appropriate callable. This example shows how to use this functionality
to prefer servers running on ``localhost``.


.. warning::

   Use of custom server selector functions is a power user feature. Misusing
   custom server selectors can have unintended consequences such as degraded
   read/write performance.


.. testsetup::

   from pymongo import MongoClient


.. _server selection algorithm: https://mongodb.com/docs/manual/core/read-preference-mechanics/


Example: Selecting Servers Running on ``localhost``
---------------------------------------------------

To start, we need to write the server selector function that will be used.
The server selector function should accept a list of
:class:`~pymongo.server_description.ServerDescription` objects and return a
list of server descriptions that are suitable for the read or write operation.
A server selector must not create or modify
:class:`~pymongo.server_description.ServerDescription` objects, and must return
the selected instances unchanged.

In this example, we write a server selector that prioritizes servers running on
``localhost``. This can be desirable when using a sharded cluster with multiple
``mongos``, as locally run queries are likely to see lower latency and higher
throughput. Please note, however, that it is highly dependent on the
application if preferring ``localhost`` is beneficial or not.

In addition to comparing the hostname with ``localhost``, our server selector
function accounts for the edge case when no servers are running on
``localhost``. In this case, we allow the default server selection logic to
prevail by passing through the received server description list unchanged.
Failure to do this would render the client unable to communicate with MongoDB
in the event that no servers were running on ``localhost``.


The described server selection logic is implemented in the following server
selector function:


.. doctest::

   >>> def server_selector(server_descriptions):
   ...     servers = [
   ...         server for server in server_descriptions if server.address[0] == "localhost"
   ...     ]
   ...     if not servers:
   ...         return server_descriptions
   ...     return servers
   ...



Finally, we can create a :class:`~pymongo.MongoClient` instance with this
server selector.


.. doctest::

   >>> client = MongoClient(server_selector=server_selector)



Server Selection Process
------------------------

This section dives deeper into the server selection process for reads and
writes. In the case of a write, the driver performs the following operations
(in order) during the selection process:


#. Select all writeable servers from the list of known hosts. For a replica set
   this is the primary, while for a sharded cluster this is all the known mongoses.

#. Apply the user-defined server selector function. Note that the custom server
   selector is **not** called if there are no servers left from the previous
   filtering stage.

#. Apply the ``localThresholdMS`` setting to the list of remaining hosts. This
   whittles the host list down to only contain servers whose latency is at most
   ``localThresholdMS`` milliseconds higher than the lowest observed latency.

#. Select a server at random from the remaining host list. The desired
   operation is then performed against the selected server.


In the case of **reads** the process is identical except for the first step.
Here, instead of selecting all writeable servers, we select all servers
matching the user's :class:`~pymongo.read_preferences.ReadPreference` from the
list of known hosts. As an example, for a 3-member replica set with a
:class:`~pymongo.read_preferences.Secondary` read preference, we would select
all available secondaries.


.. _server selection algorithm: https://mongodb.com/docs/manual/core/read-preference-mechanics/
