With idempotence, you shift the problem from "deliver X exactly once" to "make it seem like X was delivered exactly once". In most systems, exactly-once is really "effectively exactly once".

That's my point. You are simply converting the problem to a new form, not actually solving it.

Hey here's a solution to the halting problem – always assume yes, and then figure out the edge cases. How do you do that? Well that's on you, I did my job.

Idempotency - famously complex. No one has ever successfully implemented it, great point.

If you don't think idempotency can be complex then you haven't really worked on serious distributed computing problems.

If you don't think your analogy is a miss then you haven't really read any serious literature.

Converting a problem to a new form that you know how to better solve, or at least hope is more tractable, is a time honored mathematical and CS tradition

Not exactly. If you have a business problem where you’re thinking “But I really, really need the effect of exactly-once; what can I do?”, GP’s post has the answer.

OP's idea should be

idempotence + at least once

idempotence isn't necessarily commutative.

No, if your datastore is online (the only way you're functioning anyway), store an idempotency key, vector clock, etc.

For single consumers, exactly once delivery may as well be a thing.

In active / active setups, there are other strategies.

It can be exactly once at the application level just not exactly once at the more fine-grained message level. The fact that it's not exactly once at that lower level doesn't really matter, the semantics at the application level is what we care about.

Exactly. In practice there are probably a bunch of other things happening over the wire we also don't care about, handshakes and keepalives and negotiation and splitting and encryption and latency measuring and backpressure... It doesn't matter, in a variety of systems, at the application layer it is fine for the user to assume they will see a delivery to their code exactly once and that's what the user cares about. A delivery didn't mean some internal bytes travelled across the wire, it means your clients received a call.

That's why if you search for exactly once delivery you'll see a bunch of products claiming to have it (e.g kafka).