fix(analytics): surface dropped exposure when distinct_id missing from context (SDK-84)

[tylerjroach]

Summary

When a flag uses a non-default Variant Assignment Key (e.g. device_id) and the evaluation context doesn't include distinct_id, local eval succeeds — flag definitions are cached, bucketing only needs the configured key — but trackLocalExposure then silently returns because it can only attribute the exposure event to a distinct_id. Caller gets the right flag value with no signal that analytics were dropped.

Add a Level.WARNING log so the drop becomes visible.

Context

Linear: SDK-84. Only Java and Ruby are actually affected of the five SDKs the audit listed — Python / Go / Node already log when this happens.

This bug only affects local evaluation. Remote eval requires distinct_id at the eval step itself, so it errors out before reaching exposure tracking.

Behavior when distinct_id IS present (alongside the bucketing key) is unchanged — exposure still fires correctly attributed to the user.

Test plan

• All 54 LocalFlagsProviderTest tests pass (mvn test)
• New testWarnsWhenExposureDroppedDueToMissingDistinctId exercises a device_id-bucketed flag with a context that has device_id but no distinct_id: asserts the variant value is returned, no exposure event is sent, AND a WARNING log is emitted

🤖 Generated with Claude Code

[linear-code]

SDK-84

[tylerjroach]

@greptileai

[greptile-apps]

Confidence Score: 5/5

Safe to merge — one-line production change adds a warning log; no behavior change on the happy path.

The production change is a single parameterized log statement before an existing early return. The test correctly exercises the new path and properly restores JUL logger state in a finally block. No logic regressions are possible on the normal (distinct_id present) code path.

No files require special attention.

Important Files Changed

Filename	Overview
src/main/java/com/mixpanel/mixpanelapi/featureflags/provider/LocalFlagsProvider.java	Adds a parameterized WARNING log before the silent return when distinct_id is missing; no logic changes to the happy path.
src/test/java/com/mixpanel/mixpanelapi/featureflags/provider/LocalFlagsProviderTest.java	New test covers device_id-bucketed flag without distinct_id; correctly captures originalLevel and restores it in a finally block alongside handler removal.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant Caller
    participant LocalFlagsProvider
    participant Logger
    participant EventSender

    Caller->>LocalFlagsProvider: getVariantValue(flagKey, fallback, context)
    Note over LocalFlagsProvider: Local eval succeeds using device_id bucket key
    LocalFlagsProvider->>LocalFlagsProvider: trackLocalExposure(context, flagKey, ...)
    alt distinct_id present in context
        LocalFlagsProvider->>EventSender: trackExposure(distinctId, flagKey, ...)
    else distinct_id missing
        LocalFlagsProvider->>Logger: WARNING — exposure dropped, distinct_id missing
        LocalFlagsProvider-->>LocalFlagsProvider: return (exposure dropped)
    end
    LocalFlagsProvider-->>Caller: variant value

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sequenceDiagram
    participant Caller
    participant LocalFlagsProvider
    participant Logger
    participant EventSender

    Caller->>LocalFlagsProvider: getVariantValue(flagKey, fallback, context)
    Note over LocalFlagsProvider: Local eval succeeds using device_id bucket key
    LocalFlagsProvider->>LocalFlagsProvider: trackLocalExposure(context, flagKey, ...)
    alt distinct_id present in context
        LocalFlagsProvider->>EventSender: trackExposure(distinctId, flagKey, ...)
    else distinct_id missing
        LocalFlagsProvider->>Logger: WARNING — exposure dropped, distinct_id missing
        LocalFlagsProvider-->>LocalFlagsProvider: return (exposure dropped)
    end
    LocalFlagsProvider-->>Caller: variant value

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_{Reviews (2): Last reviewed commit: "fix(flags): parameterize JUL warning + r..." | Re-trigger Greptile}

[tylerjroach]

Pushed 8a8ae1f addressing both Greptile threads.

P2 — parameterized JUL logging (LocalFlagsProvider.java:674): switched from "...'" + flagKey + "'..." string concatenation to logger.log(Level.WARNING, "...''{0}''...", flagKey). The template is only formatted when WARNING is actually enabled.

P2 — logger level not restored (LocalFlagsProviderTest.java:917): captured logger.getLevel() before the setLevel(Level.ALL) call and restore it alongside removeHandler in finally. Also switched the assertion to SimpleFormatter().formatMessage(record) — with parameterized logging LogRecord.getMessage() returns the raw {0} template, so the previous .contains("device-flag") check would no longer match.

All 54 LocalFlagsProviderTest tests pass.