The Transactional Outbox Pattern: Solving Dual-Write in Distributed Systems

Most distributed systems have this bug hidden somewhere in their codebase: save the order to the database, then publish an order.created event to Kafka. When the publish fails — and eventually it will — the order exists but every downstream service is permanently blind to it. You can add retry logic, but that just moves the problem. The real fix is architectural: stop treating the database write and the broker publish as two separate operations.

The transactional outbox pattern solves this at the infrastructure level. It trades the illusion of atomicity across two systems for a genuine atomic write to one system, then propagates asynchronously with a separate relay process. The result is consistent, debuggable, and horizontally scalable.