Outbox pattern

The outbox pattern writes domain changes and outgoing messages in one local database transaction, then a relay process publishes to the broker. This avoids dual-write races where the DB commits but the message never sends, or vice versa.

Trade-off: added table, relay lag, and deduplication at consumers — but strong reliability within the service boundary.

On interviews: contrast outbox with naive "save then publish"; explain relay failure handling and ordering guarantees.

Common pitfalls: publishing before commit; relay without backoff; consumers that are not idempotent.

Checklist:

• Store outbox rows in the same transaction as domain writes.
• Run a reliable relay with retries and metrics.
• Mark published rows and prune safely.
• Pair with idempotent consumers or inbox.