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But `GET(GET(x))` doesn't make sense, in general (and if it did, then you would not expect it to be idempotent), so clearly idempotency in this context is meant
by smaddox 8y ago
But `GET(GET(x))` doesn't make sense, in general (and if it did, then you would not expect it to be idempotent), so clearly idempotency in this context is meant to mean side-effect free. They should probably just say side-effect free, though, to avoid the confusion.
- Ajedi32 8y agoIt's possible to be idempotent without being side-effect free. If you PUT some record, for example, then that operation _will_ have side effects (modifying the record). If you then PUT that same data again, the result will be the same (it's idempotent).
- jiveturkey 8y agothat’s not a side effect. in common usage, a side effect is an extra action, not the desired action itself. if you PUT some record, and some other record or state changes, that’s a side effect.
- __david__ 8y agoI suppose logging is a side effect and that makes practically everything technically non-idempotent (though on purpose).
- jholman 8y agoI don't disagree with common usage, but common usage is not what's being discussed here. It is a side effect in programming usage (not just HTTP). Something is side-effect-free if and only if the only result of it running is that you get an answer. If you ignore the answer, then you cannot tell you ran the function/method/call/whatever. PUT is not side-effect-free. That said, side-effect-free-ness is an incomplete paraphrasing of the HTTP spec (RFC7231); you'll notice that the only mentions of the phrase "side effect" are giving examples of legal side effects.