Why is the Inbox pattern needed if the Outbox already guarantees delivery?

The Outbox pattern guarantees that messages or events related to database changes are reliably recorded and sent, even if the sending process is interrupted. However, it does not solve the problem of processing incoming messages.

The Inbox pattern is needed for reliable reception and processing of incoming messages to avoid duplication and ensure idempotency. It stores received messages and their processing status, allowing the system to correctly respond to redeliveries or failures.

Thus, the Outbox is responsible for reliable sending, and the Inbox for reliable reception and processing of messages, together ensuring end-to-end reliability of data exchange in distributed systems.