feat(llmobs): propagate session_id across distributed traces [MLOB-7756]#5011
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feat(llmobs): propagate session_id across distributed traces [MLOB-7756]#5011ncybul wants to merge 5 commits into
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Add session_id to the LLMObs propagating tag set (_dd.p.llmobs_session_id), mirroring ml_app: inject the active span's resolved session, extract it into PropagatedLLMSpan, and fall back to the propagated session in propagatedSessionID. A downstream service's LLM spans now inherit the session_id set upstream; an explicit session_id still takes precedence. MLOB-7756 Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Matches the wire contract agreed in dd-trace-py#18952: keep the llmobs_ namespace, shorten session_id -> sid to conserve x-datadog-tags budget. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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LLMObs propagation tags are trace-scoped, so a session-bearing span's _dd.p.llmobs_sid lingered when a later active LLM span had no session, causing downstream services to inherit an unrelated session. Unset the tag when the active span has no session, mirroring keyPropagatedUserID. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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What Does This Do
Adds LLMObs
session_idto the cross-service propagation set (_dd.p.llmobs_sid), so a downstream service's LLM spans inherit thesession_idset on an upstream service's span. Mirrors the existingml_apppropagation and the dd-trace-py change in dd-trace-py#18952.Changes
internal/llmobs/context.go— addSessionIDtoPropagatedLLMSpan.internal/llmobs/span.go— addSessionID()accessor;propagatedSessionIDfalls back tos.propagated.SessionID(mirrorspropagatedMLApp).ddtrace/tracer/span.go— addkeyPropagatedLLMObsSessionID(_dd.p.llmobs_sid); inject the active span's resolved session insetLLMObsPropagatingTags(only when non-empty, to conservex-datadog-tagsbudget).ddtrace/tracer/context.go— extract_dd.p.llmobs_sidinpropagatedLLMSpanFromTags.The tag key
_dd.p.llmobs_sidmatches the wire contract agreed across SDKs (dd-trace-py, dd-trace-js): thellmobs_namespace is kept (consistent with_dd.p.llmobs_parent_id/_ml_app/_trace_id) whilesession_idis shortened tosidto conserve thex-datadog-tagsbudget.An explicit
session_idon a span still takes precedence over a propagated one.Scope note
This PR is cross-service propagation only. It does not add the in-process "trace-level default" (a sibling under a session-less parent inheriting the trace's session), which py/js get via their trace-shared propagating bag. In go,
internal/llmobscan't reach the tracer's trace-level propagating tags (one-way import boundary), so a trace default would need a separate trace-scoped store with lifecycle management — deferred as a follow-up. Go already covers the common case (session on the root span → all descendants inherit via the parent-chain walk).Testing
Simulating a distributed session (trace)
session_id:my-sessionset in service A which calls service B and inherits the same session IDSimulating session inheritance in a trace unless explicitly annotated (trace)
session_id:my-sessionwhich is inherited by its children spans except those explicitly annotated withsession_id:other-sessionMLOB-7756