> All you need to do is discard duplicates at the receiver.
...yes, which would be exactly once _processing_ but not exactly once _delivery_.
Unless you're wanting to redefine "exactly once delivery" to mean "at least once delivery but I'm calling it exactly once because I have a strategy to cope with duplicate messages"
Delivery is communication between two actors. Processing is what one actor (the receiver) does with a message.
Communication is when two actors exchange a message. Communication is generally done over an unreliable medium because in practice there is no way to communicate without the potential of failure.
1. Exactly once communication between two actors over an unreliable medium is impossible. At the very least you have to account for the possibility of failure of the medium, so you might need to re-send messages.
2. At least once communication between two actors is possible -- just re-send a message until the receive acknowledges the message.
3. Because a message might be re-sent, the receiver must be able to cope with duplicate messages. This is what you're describing. This might be done by making message processing idempotent or tracking which messages you've seen before. In either case, you have achieve exactly once processing. That is, if a receiver is given the same message multiple times, only the first receive of the messages changes the state of the receiver.
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> But exactly-once delivery is not only possible, it's trivial.
Considering that many in the field consider this problem to be impossible (or, at best, extremely difficult, e.g. https://www.confluent.io/blog/exactly-once-semantics-are-pos...), this should be a huge red flag to yourself that you're missing something. Everyone has blind spots and that's okay, but hopefully you understand that there's a pretty big mismatch here.
Alternatively, it's possible that this problem _really_ is trivial and you have some unique insight which means there's a great opportunity for you to write a paper or blog post.
> hopefully you understand that there's a pretty big mismatch here
Yep. But on the other hand, 1) I have a Ph.D. in CS, and 2) I have yet to see anyone in this thread actually produce a reference to a reliable source to back up the assertion that exactly-one delivery is impossible. Indeed, the one reference you provided has a headline "Exactly-Once Semantics Are Possible" so you are actually supporting my position here.
Finally, I will point out something that should be obvious but apparently isn't: "exchanging a message" between computers over a network is a metaphor. There is not anything that is actually exchanged, no material transferred from one computer to another. There is only information sent in the form of electrical signals which results in state changes in the receiving system, so there is no clean boundary between "communication" and "what the receiver does with a message". Receiving a message in the context of a computer network is necessarily "doing something" with that message. There is no other way to "receive" a message.
...yes, which would be exactly once _processing_ but not exactly once _delivery_.
Unless you're wanting to redefine "exactly once delivery" to mean "at least once delivery but I'm calling it exactly once because I have a strategy to cope with duplicate messages"