The Transactional Outbox Pattern

Concepts covered: paTransactionalOutbox, paIdempotency

The transactional outbox is the canonical fix for the dual-write problem. The application writes only to the database. Inside the same transaction that writes the business row, it inserts a row into an outbox table that describes the event to be published. A separate publisher process reads from the outbox table and publishes events to the broker. The atomic boundary is the database transaction; the broker is downstream of that boundary. Either both rows commit or neither does, and the publisher's job becomes the strictly easier problem of draining a queue of pending events. The Outbox Table The table holds enough state to publish each event later. The aggregate_type and aggregate_id identify the entity the event concerns. The event_type names the event. The payload carries the body. publi

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