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SERVER-119095 Document rate-limiting logs with SeveritySuppressor (#48106)

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Didier Nadeau 2026-02-17 13:18:51 -05:00 committed by MongoDB Bot
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@ -640,6 +640,39 @@ Evergreen patch builds.
UserDefinedType t; // Defined in previous example
logd("this is a debug log, value 1: {} and value 2: {}", 1, t);
## Rate limiting
Rate limiting logs is useful to reduce the impact of logging on database throughput. At high
rate and concurrency, logging can be expensive and reduce performance. Attention should be paid
specifically to logs that can occur on every operation, whether they fail or succeed.
The rate limiting feature is implemented by `SeveritySuppressor` (see
`src/mongo/logv2/log_severity_suppressor.h`). It works by changing the severity level of logs
dynamically: within a configurable time interval, only the first log is emitted at the "normal"
severity; subsequent logs within that interval are emitted at a "quiet" severity (typically a debug
level). This ensures logs are not always written unless the logging level is increased for the
component.
`SeveritySuppressor` is typically used with `StaticImmortal` for static storage. The interval can
be configured with a server parameter when constructing SeveritySuppressor.
##### Example
static StaticImmortal<logv2::SeveritySuppressor> logSuppressor{
gSlowNetworkLogRate.loadRelaxed(), logv2::LogSeverity::Info(), logv2::LogSeverity::Debug(2)};
LOGV2_DEBUG(6983000,
(*logSuppressor)(),
"Slow network response send time",
"elapsed"_attr = bob.obj());
In this example, the first log within each gSlowNetworkLogRate-second window is emitted at Info level;
subsequent logs within that window are emitted at Debug(2), which requires increasing the component's
log level to be visible.
For per-key rate limiting (e.g., one log per key per interval), use `KeyedSeveritySuppressor`
instead.
# JSON output format
Produces structured logs of the [Relaxed Extended JSON 2.0.0][relaxed_json_2]