alt Hacker NewsWith an inbox/outbox pattern it's possible. The incoming message might be processed more than once, and an outgoing message might be sent more than once. That's the limitation, and the system needs to be able to handle it.
If you can't de-duplicate messages it's not possible, that's true.
I'm not following. Doesn't the outbox pattern just pass the buck?
The motive seems to be a naive process that enqueues a message and then commits to a database - two independent actions. But a well-behaved process would commit to a database, and then only if successful enqueue a message. That's better but still not atomic - commit, crash, and no message queued.
So the solution is a two-table write - the outbox pattern. But the process that reads the outbox must commit both a query and delete before sending the message. That's the same risk as the agreement well-behaved program - commit, crash, and no message queued. Except now you introduced another pipeline element so your overall complexity increases, and so too risk.
What if you never delete messages from the outbox? Well, what you have now is no longer an outbox nor a database nor useful for large volumes. What if you implement a database to track procesed messages. Return to square one - that's the same problem you were initially trying to solve.
What if you fetch, enqueue, and then delete? Ohh... that works. In case of a crash the message remains in the outbox. It may be processed in duplicate, but eventually if successfully it will be deleted from the outbox.
The message broker then receives a possibly duplicate message. It must consult its internal database, and if the message is unique, route it. So right back at square one. Can't have atomicity and uniqueness.