What is the Outbox pattern and how does it work?

Outbox pattern is an architectural approach to ensure reliable message delivery between services in distributed systems, especially when using microservices.

The idea is that changes in the database and message sending to the queue are performed within a single transaction. Instead of immediately sending a message to the broker, the service records an event (message) in a special table — "outbox" — within the same transaction where it modifies business data.

Then, a separate process or component periodically reads new messages from the outbox table and sends them to the queue (Kafka, RabbitMQ, etc.). After successful sending, the message is marked as sent or deleted.

Advantages:

• Guarantees that the message will not be lost or sent twice.
• Avoids inconsistency issues between the database and the messaging system.

Example workflow:

1. A transaction begins in the service.
2. Business data is updated.
3. A record with the event is added to the outbox table.
4. The transaction is committed.
5. A separate process reads from the outbox and sends messages to the broker.
6. After successful sending, the record from the outbox is deleted or marked.

Thus, the Outbox pattern helps implement reliable asynchronous communication between services with data consistency.