Can you explain the concept of Transaction Outbox and its application?

Transaction Outbox is a pattern used to ensure reliable transmission of events or messages from a database to other systems, especially in microservices architecture.

The idea is that when data in the database changes and there is a need to send an event (for example, to a message queue), these operations are performed within a single transaction. Instead of directly sending the message, the event is recorded in a special table — the "outbox".

After the transaction commits successfully, a separate process or service reads the records from the outbox table and sends them to the target system (for example, Kafka, RabbitMQ). This guarantees that the event is not lost and will not be sent twice.

Example of application:

1. The service updates an order and simultaneously records an "OrderUpdated" event in the outbox table.
2. The transaction commits — data and event are saved atomically.
3. A separate worker reads from the outbox and publishes the event to the queue.

This way, consistency between the database and the message exchange system is achieved without distributed transactions.