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Comment by polio | Hacker News Reader

Beyond the pedantic distinction, is there any real point to not calling "at-least-once delivery with idempotent processing" exactly-once processing? I can't imagine that any external observers would be able to tell.

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It conveys a false sense of correctness. Usually the system doing the processing has to use higher level or external methods of providing idempotency.

For example TCP implements "exactly once processing" by your definition but you probably still want Stripe to include idempotency keys in their charge API so you don't pay twice.

It’s a problem for those of us who build consumer APIs, because people without a deep distributed systems background quite reasonably expect that “exactly once” means their downstream application will receive each event exactly once. I’ve had multiple incredibly frustrating conversations where I had to find a diplomatic way to explain that yes, your connector needs to handle idempotency, and no, it’s not because there’s some “exactly once” technology our system is missing.

Yeah, 100%. I've had a tier-one telco complain to me - very seriously - that my API was too complex because I wanted them to include a user-generated UUID in an API call, so we could ensure we only processed it once. It was very frustrating to be told that we're making things too complex when the whole point was to make it more reliable. A lot of people - including telco software engineers, it turns out - just treat computers as magical.

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