alt Hacker NewsIdempotency means f(x) = f(f(x)).*
Here x is interpreted as state and f an action acting on the state.
State is in practice always subjected to side effects and concurrency. That's why if x is state then f can never be purely idempotent and the term has to be interpreted in a hand-wavy fashion which leads to confusions regarding attempts to handle that mismatch which again leads to rather meandering and confusing and way too long blog posts as the one we are seeing here.
*: I wonder how you can write such a lengthy text and not once even mention this. If you want to understand idempotency in a meaningful way then you have to reduce the scenario to a mathematical function. If you don't then you are left with a fuzzy concept and there isn't much point about philosophizing over just accepting how something is practically implemented; like this idempotency-key.
Replies
> That's why if x is state then f can never be purely idempotent
That is simply not true. f could be, for example, “set x.variable to 7”, which is definitely idempotent.
> State is in practice always subjected to side effects and concurrency.
In that mathematical notation typically there is no side effects and those are meant to be pure functions.
> *
I wondered about this too. Also, why was it framed in the context of JSON based RPC over HTTP ?
Idempotence is a semantically overloaded term in computer science where in functional programming it refers to the same concept as mathematical idempotence it refers to any function leading to the same state in multiple calls as the first.
And yes, in real machines we can't ever have true same states between multiple calls as system time, heat and other effects will differ but we define the state over the abstracted system model of whatever we are modelling and we define idempotency as the same state over multiple calls in that system.