feat(stats): add whole key cardinality limit#2158
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This was referenced Jun 24, 2026
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| pub grpc_status_code: Option<u8>, | ||
| pub is_synthetics_request: bool, | ||
| pub is_trace_root: bool, | ||
| pub is_trace_root: pb::Trilean, |
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Changing FixedAggregationKey<StringRef> from a bool to pb::Trilean changes the in-memory size/alignment of ShmKeyHeader, which is embedded in the shared-memory ShmEntry layout. ShmSpanConcentrator::open only rejects incompatible mappings via SHM_VERSION, but that constant remains 1, so during a mixed old/new sidecar or PHP-worker rollout an old process can open a new mapping (or vice versa) and read/write slots using different offsets, corrupting or dropping stats instead of falling back/reopening.
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| .is_some_and(|origin| origin.starts_with(TAG_SYNTHETICS)), | ||
| is_trace_root: span.is_trace_root(), | ||
| is_trace_root: if span.is_trace_root() { | ||
| pb::Trilean::True |
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nit (And maybe a dumb idea since I'm not great at rust): Should we have something like Trilean::From(bool)->Trilean to make stuff like this a little nicer to read?
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What does this PR do?
Add whole-key cardinality limit to the SpanConcentrator. See this RFC for details
The telemetry metrics specified in the RFC are implemented in #2159
The per fields limit from the RFC is out of the scope of this PR
Breaking change
The cardinality limit is applied to all users of the SpanConcentrator however the limit can be configured manually.
Motivation
Avoid unbounded bucket growth when dealing with high cardinality.
Additional Notes
Limit default value
The limit of 7000 entries has been chosen based on the following results
Single Bucket
SpanConcentrator
Trilean change
The overflow sentinel key requires us to use a trilean to represent the
is_trace_rootin the aggregation key.