Comment by warkdarrior
Comment by warkdarrior 5 hours ago
> The point is that it doesn't matter exactly where the deduplication matters.
Actually the point is that once deduplication is done at some layer, the layers above it will have to re-achieve exactly-once delivery.
"Yes, the TCP layer did deliver this message only once, but the receiving software crashed right after, so now the sender has to send it again."
Hmmm. Maybe this is the reason why the processing vs delivery distinction matters. Because my thought is, well of course: To fix that you only send the acknowledgement after processing succeeds.
But then again, once you do that, the processing code that is being wrapped really doesn't have to care about being idempotent anymore, as it is being handled a layer up. At that point, all it needs to care about is being atomic.
I'm not sure if it practically matters either way. I'd rather have my processing code be both atomic and idempotent regardless just to make things easier to reason about, as long as it's not too much of a burden. I've always been a fan of concepts like idempotency tokens.