> You're calling people out by name
I am? Where?
> Getting "exactly once" processing usually requires making various kinds of resilience tradeoffs based on timing windows, storage requirements, etc
Yes, of course. But that's not the same as "impossible".
The whole refusal to accept that a field could legitimately define something differently than how you prefer, and then running off to blog about it and name names... and then coming around for round II of flamewar... with ever more splitting of hairs in definitions... is not awesome.
You are especially well-answered here, I think: https://news.ycombinator.com/item?id=41599131
One reason the delivery / processing distinction exists because very often the application needs to atomically persist "I have received this message" with any other state changes made as a result of processing that message for correctness. You can't generally solve this with a layer put on top, even on the same machine. If it's not atomic, then you can still deliver duplicates to the application or end up never delivering to the application. (Power goes out when one side has written but not the other).
So, the state change to "already received" and the changes you want to make in response to the message being received have to happen together. TCP or even a message queueing implementation with a persistence layer cannot solve this problem for you. Thus, the application needs to deal with multiple delivery.
Imagine a "subtract $5 from my bank account" message with no ID on the message itself, and a layer "on top" that gives IDs and tries to ensure exactly once delivery. If the layer "on top" does not change state at the exact time $5 is deducted from the account, bad things can happen-- and in practice this is impossible. Hence, the application needs to be able to cope with the "subtract $5" being delivered to it multiple times, and this deduping has to be intimately tied to it subtracting the $5 (processing).
> I was reading Hacker News a few days ago and stumbled on a comment posted by ...