Outbox pattern -- reliable event publishing with database transactions

Publishing an event after a database write can fail, leading to inconsistency. The database is updated but the event is never published. Or the event is published but the database transaction rolls back.

Write events to an outbox table in the same database transaction as the business data. A separate process reads the outbox table and publishes events to the message broker. This guarantees atomicity: either both the data and event are committed, or neither is. Poll the outbox table or use change data capture (CDC) tools like Debezium to read the outbox.

You cannot atomically write to a database and publish to a message broker in one transaction (they are different systems). The outbox pattern makes event publishing part of the database transaction.