定义幂等 [英] Defining Idempotence

问题描述

所以幂等可以定义为：

一个动作，如果执行N次，则只执行一次动作。

An action, that if performed N times has the same effect as performing the action only once.

知道了，很容易。

我的问题是关于这个定义的微妙之处 - 这个行为本身就是幂等的，或者你还必须考虑传递给行动的数据吗？

My question is about the subtlety of this definition -is an action considered idempotent by itself, or must you also consider the data being passed into the action?

让我用一个例子来澄清：

Let me clarify with an example:

假设我有一个更新某些资源的PUT方法，我们称之为 f（x）

Suppose I have a PUT method that updates some resource, we'll call it f(x)

显然， f（3）是幂等的，只要我提供3作为输入。同样显而易见的是， f（5）将改变资源的价值（即，它将不再是3或之前的任何值）

Obviously, f(3) is idempotent, as long as I supply 3 as the input. And equally obvious, f(5) will change the value of the resource (i.e., it will no longer be 3 or whatever value was there previously)

所以当我们谈论幂等性时，我们是指动作/函数的泛化（即 f（x）），或者我们是指动作/功能+传递给它的数据（即 f（3））？

So when we talk about idempotence, are we referring to the generalization of the action/function like (i.e., f(x)), or are we referring to action/function + the data being passed into it (i.e., f(3))?

推荐答案

假设我有一个更新某些资源的PUT方法，我们称之为
f（x）

Suppose I have a PUT method that updates some resource, we'll call it f(x)

显然，f（3）是幂等的，只要我提供3作为输入。并且
同样明显，f（5）将改变资源的价值（即，
将不再是3或之前的任何值）。

Obviously, f(3) is idempotent, as long as I supply 3 as the input. And equally obvious, f(5) will change the value of the resource (i.e., it will no longer be 3 or whatever value was there previously).

这是显而易见的，服务器实现是这样的 PUT 尊重这个幂等属性。在HTTP环境中， RFC 2616说：

This is only obvious is the server implementation is such that PUT respects this idempotent property. In the context of HTTP, RFC 2616 says:

方法也可以具有idempotence属性（除了错误或到期问题的
）之外的副作用N> 0 相同
请求与单个请求相同。

Methods can also have the property of "idempotence" in that (aside from error or expiration issues) the side-effects of N > 0 identical requests is the same as for a single request.

去咬掉主题...
在像网络这样的分布式系统中，您可能还需要考虑交换性和并发请求。例如，相同PUT（x1）请求的N + 1应具有相同的效果，但您不知道另一个客户端是否在您的之间发出了不同的PUT（x2）请求，因此当n PUT（x1）时= PUT（x1）和m PUT（x2）= PUT（x2），两组请求可以交错。

Going a bit off topic... In a distributed system like the web, you may also want to consider commutativity and concurrent requests. For example N+1 of the same PUT(x1) request should have the same effect, but you don't know if another client made a different PUT(x2) request in between yours, so while nPUT(x1)=PUT(x1) and mPUT(x2)=PUT(x2), the two sets of requests could be interleaved.

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