Transactional Outbox

Problem: a service needs to update its own database AND publish an event about that change — but a DB write and a broker publish can't share one transaction (the dual write problem). If the commit succeeds and the publish fails (or vice versa), the two systems drift out of sync.

The Solution

• Write the event to an outbox table, in the same local transaction as the business data change
• A separate process reads the outbox table and publishes to the broker, then marks the row as sent
• Polling publisher — a worker polls the outbox table on an interval
• Change Data Capture (CDC) — a tool (e.g. Debezium) tails the DB transaction log and publishes new rows automatically

Guarantees

• The business change and the outbox row commit atomically — either both happen or neither does
• Delivery to the broker is at-least-once — pair it with an idempotent consumer to handle duplicates

Trade-offs

• Extra table and publishing process to operate
• Consumers see the event slightly after the write commits, not synchronously