>Yes, that's true. But this doesn't turn on what "delivery" means, it turns on what "you" means. If "you" are downstream of a de-duplication mechanism, then "you" can get exactly once-delivery. Why is that so absurd?

So in the case of, say, a network service on server A and a network client B, your solution to "exactly once delivery" is to re-define it as "deliver it from A to B multiple times and have B deduplicate"?

Do you not see how nonsensical that is to call that "exactly once delivery"?

If you have a reliable connection between “you” and the deduplicator, then “you” aren’t receiving messages over an unreliable connection at all and so the claim that you can’t have exactly once delivery over an unreliable connection isn’t applicable in the first place. You’re receiving messages over a reliable connection and what happens upstream of that is irrelevant.

If "you" are "downstream" of a "de-duplication mechanism" how do you ensure "exactly once delivery" between "you" and the "de-duplication mechanism"?

The same way I ensure any behavior in a digital system. There are boundaries inside of which processes are presumed to be reliable, typically consisting of a CPU, memory busses, and attached non-volatile storage. If you don't assume that those are reliable, then you can't guarantee anything.

Great! I agree 100%. We have to assume a "reliable network" within a "boundary" (i.e. a computers CPU, memory, busses etc...). Distributed systems (from which these rules are taken) are specifically systems where anything within one of these "boundaries" is considered a "single node" and treated the same, whether it's a NIC, a kernel module/driver, a user space process or anything else.

In our case if we were to take (for example) that the NIC would de-duplicate the messages for us, anyone writing the producer/sender and a user-space program for the receiver a would need to know that the NIC was doing this (or risk having messages dropped for failure of including a unique id).

This is a pedantic point, but I would strongly stress that the only reason these "delivery rules" are so popular (and evoke such a reaction) is because of the very large number of times that programmers mis-understand them.

Commonly they either assume that:

* the network is reliable

* something else will guarantee "exactly once delivery" for me (when in fact nothing will)

You're correct, but in my experience the vast majority of code written is not downstream of a usable de-duplication mechanism.

That may well be, but that's a very different question.

It explains why most people seem to disagree with you on what "delivery" and "you" mean in this context. For the majority of contexts, "delivery" means that a system responsible for de-duplication receives the message.

Yeah, but that just seems like a bizarre definition on which to base the claim that you cannot have exactly-once delivery. Obviously, if you define delivery to preclude de-duplication, then you can't have exactly-once delivery. But you can have something that delivers messages (for some reasonable definition of "delivers") exactly once. It seems weird to define delivery in such a way that such a system does not provide exactly-once delivery.

I'm not sure how to convince you to get over your hangup around how other people feel about the word "delivery", but this is exactly why others distinguish between "delivery" and "processing". If you think there are better terms than those two to describe "the system that receives a message and must be responsible for de-duplication" and "the system that can rely on messages being already de-duplicated", then feel free to propose them and have people debate, that I suppose. But because of what I noted earlier, this is a very useful distinction to maintain for people working on systems that are responsible for de-duplication (likely most people on this forum), and these words seem to make sense for most individuals.

I don't have any hangups about how other people feel about anything. My "hangup" is that I see no evidence that there is a consensus on a technical definition of the word "delivery". In the absence of such a definition, there is no basis for asserting that exactly-once delivery is impossible, particularly when there are existence proofs to the contrary based on reasonable informal definitions of the word "delivery".