Not that I disagree with getting terms exactly right, but if the "processing" that happens exactly once in your messaging system is firing a "delivery" event to a higher level of the stack (over a local channel), that's observationally indistinguishable from exactly-once delivery to that higher level, isn't it? What am I missing?
Over a local channel? Like same machine or even same process? I mean, yeah, you might have a bug that messes up the local "processing", but that's not a deep truth about distributed systems anymore, it's just a normal bug that you can and should fix.
In the systems I'm used to, that "passing up" is a function call within the same codebase. There isn't a second delivery system for a message to be lost/duplicated, though sometimes they obscure it by making it look more like such a system.
Power gets cut. Now what? A process can fail between two function calls. When your power is restored, what happens with that mid-processing request? Lost or retried?
