OutboxPattern

Outbox Pattern is a software architectural pattern used to ensure reliable integration between a domain model and an external or internal event-driven messaging system. It achieves transactional integrity by persisting domain events in a dedicated storage area—typically an outbox table—within the same database transaction as the domain write. After the transaction commits, a separate process reads the outbox entries, publishes the events to the message broker (such as Kafka or RabbitMQ), and marks them as processed. This decouples the act of updating the domain model from the act of delivering messages, while keeping guarantees against message loss in the face of service outages.

Implementation usually involves: 1) writing an event record to the outbox as part of the same commit that updates the domain model; 2) a background poller or processor that queries unprocessed entries in batches; 3) publishing the payload to the messaging system with idempotency safeguards; 4) updating the entry to reflect delivery status and timestamps. Common fields include id, aggregate_id, event_type, payload, created_at, processed_at, status, and attempt_count.

Advantages include stronger consistency guarantees, resilience to temporary messaging failures, and simpler reasoning about transactional boundaries. Limitations involve added storage and processing latency, eventual consistency for consumers, and the need for idempotent consumer logic or deduplication. Variants include direct polling versus CDC-based approaches and integration with Saga or CQRS patterns. It is commonly used in microservice architectures to enable reliable cross-service event publication.