看到了一篇讲幂等性的文章，我认为把幂等性说得十分清楚。如果你像我一样对幂等性还有困惑，可以仔细看下这篇文章。原文地址：https://www.restapitutorial.com/lessons/idempotency.html

【英文原文】

Idempotence is a funky word that often hooks people. Idempotence is sometimes a confusing concept, at least from the academic definition.

From a RESTful service standpoint, for an operation (or service call) to be idempotent, clients can make that same call repeatedly while producing the same result. In other words, making multiple identical requests has the same effect as making a single request. Note that while idempotent operations produce the same result on the server (no side effects), the response itself may not be the same (e.g. a resource's state may change between requests).

The PUT and DELETE methods are defined to be idempotent. However, there is a caveat on DELETE. The problem with DELETE, which if successful would normally return a 200 (OK) or 204 (No Content), will often return a 404 (Not Found) on subsequent calls, unless the service is configured to "mark" resources for deletion without actually deleting them. However, when the service actually deletes the resource, the next call will not find the resource to delete it and return a 404. However, the state on the server is the same after each DELETE call, but the response is different.

GET, HEAD, OPTIONS and TRACE methods are defined as safe, meaning they are only intended for retrieving data. This makes them idempotent as well since multiple, identical requests will behave the same.

【为了方便，我翻译一下】

“幂等性”是一个经常引人注意的时髦时汇。同时“幂等性”有时也是一个令人很困惑的慨念（至少从其学术定义来看）。

从RESTful服务的角度来说，如果一个operation（或service call）是幂等的，那么客户端以相同的方式重复调用服务就应该产生相同的结果。换句话说，发出多个相同的请求，与发出单个请求具有相同的效果。需要注意的是，虽然幂等性操作会在服务器端产生相同的结果（没有副作用），但操作的response本身可能并不相同（比如资源状态会在不同请求之间发生变化）。

PUT和DELETE方法被定义为幂等的。DELETE操作要特别注意的一个问题是，DELETE操作成功通常会返回200(OK)或204(No Content)，但后续相同的DELETE操作可能会返回404（Not Found)，除非服务被配置成只标记资源的删除状态而不是真正地删除资源。但是，当服务实际上已删除指定资源时，后续DELETE调用就会找不到该资源，并返回404。多次DELETE调用之后，服务器端的状态是相同的，但返回可以不同。

GET、HEAD、OPTIONS和TRACE方法被定义成安全的，意味着它们只是为了获取数据，多个相同请求时服务器端的表现是相同的，所以它们是天然幂等的。

[感想]

幂等性不同于返回值。