If you zoom out enough then every system looks like a black box and the only thing that matters is inputs and outputs. Messaging systems are no different in tha

If you zoom out enough then every system looks like a black box and the only thing that matters is inputs and outputs. Messaging systems are no different in that respect from literally everything else. If you're building a messaging system, there's a world of difference in terms of how other people integrate with your software if you say "exactly-once" vs "at-least-once."