Comment by lukeasrodgers

Here is my understanding, roughly:

- say you need a messaging system to communicate between different components - that messaging system is a 3rd party library or tool, it has no knowledge of your needs or architecture - therefore it can have no knowledge of what counts as a duplicate message, it either just blasts your message off once, or blasts them off until it gets an ack, it is up to the software you build around this component to avoid duplicate processing - so yes of course you can build "exactly once processing" on top of an "at least once delivery" system - but it still makes sense to talk about the distinction between delivery and processing, and "exactly once delivery is impossible" is still (in OP's terms) a "useful" claim

I haven't personally used kafka but it and similar systems (I vaguely recall some work by Pat Helland that may fall into a similar bucket) could possibly be said to a) constitute messaging systems, b) provide exactly once delivery semantics, in that they are less of a library and more of a framework that provide a concept of "duplicate message" that you basically buy into by using those systems.

You could then argue that "if it provides exactly once delivery it is not a messaging system", maybe there's a good argument there or maybe it's just pedantry.