event-driven-messaging-patternslisted

# Event-Driven Messaging Patterns ## When to Use - Events stop flowing after Redis reconnect. - UI messages lose fields (`name`, `reason`, counts). - Legacy transport migration leaves stale realtime references. - Duplicate or reordered side effects appear in consumers. ## Quick Reference | Symptom | Cause | Verify | Fix | |---|---|---|---| | Pub/sub quiet after reconnect | Subscription not restored | Simulate Redis reconnect | Re-subscribe on reconnect-ready hook | | UI text degraded | Bridge dropped payload fields | Compare source event vs emitted frame | Enforce field-fidelity contract tests | | Duplicate actions | Missing idempotency key handling | Replay same message | Consumer dedup + side-effect guard | | Migration regressions | Deprecated transport paths still active | grep code/docs/tests/config | Complete cleanup checklist below | ## Patterns | Pattern | Rule | Checkpoint | |---|---|---| | Reconnect-safe subscription | On client reconnect-ready, explicitly subscribe all channels again | Channel count restored after reconnect test | | Field fidelity | Bridge mappings are additive-only; no dropping/renaming without versioning | Contract test compares required fields end-to-end | | Source attribution | Every emitted event includes normalized `source` and `transport` metadata | Debug traces can identify origin path | | Outbox discipline | Publish from outbox/transaction boundary, not ad-hoc post-commit fire-and-forget | No lost events on partial failures | | Idempotent