To add: In a distributed system, an ordering guarantee by producer (and optionally partition "key" which in the end is like an extension of the topic key) is p

To add: In a distributed system, an ordering guarantee by producer (and optionally partition "key" which in the end is like an extension of the topic key) is pretty much all you need and all you can get. If you have two producers then how would one decide which one sent the message first? Go by some timestamp? Clocks are unreliable for these purposes so it comes down to consesus. And letting the queue decide which one came first is equivalent. Once a message got acknowledged by the queue, ordering cannot change anymore.