> But that's not because you built a system that successfully delivers messages exactly once... you build a system that successfully processes messages exactly once, even if delivery occurs multiple times.
I think the difference might be partly semantic. If processing at the messaging level is idempotent + at least once, then message delivery to the application level is exactly once. People mostly only care about the application level not the lower levels where they might just build on a library or system that handles that logic for them.
I'd say it's entirely semantic. I'm very much arguing for where to draw the definition lines in the terms we use. It won't change the code one bit (give or take a few different names on things). I definitely think understanding carefully the issues involved in delivery, and understanding the various solutions to that problem, is the way to go, not to blur the questions of delivery and handling into one atomic element. They're not atomic.
Alternatively we could come up with names for all the other combinations of delivery mechanism and handling mechanism, but since you can easily see we hit an NxM-type problem on that, this may well help elucidate why I think it's a bad idea to try to combine the two into one term. It visibly impairs people's ability to think about this topic clearly.
Well, my argument for erasing that line is that you generally don't care about TCP packets or SSL handshakes and such, so why is this one property relevant if it can be punted to a lower layer just like those others?
I'll grant that it matters if you're trying to debug some problem and trying to find at what layer it failed, but it's basically the same process you use to debug all of those other layers too, so I'm not sure why this layer deserves special consideration.
I think the difference might be partly semantic. If processing at the messaging level is idempotent + at least once, then message delivery to the application level is exactly once. People mostly only care about the application level not the lower levels where they might just build on a library or system that handles that logic for them.