HTTP幂等性

参见:幂等 – 术语表 | MDNRFC 7231 – Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content、《HTTP权威指南》的4.7.3连接关闭容限、重试以及幂等性。

一个HTTP方法是幂等的,指的是同样的请求被执行一次与连续执行多次的效果是一样的,服务器的状态也是一样的。换句话说就是,幂等方法不应该具有副作用(统计用途除外)。在正确实现的条件下,GET,HEAD,PUT和DELETE等方法都是幂等的,而POST方法不是。所有的safe方法也都是幂等的。

A request method is considered “idempotent” if the intended effect on the server of multiple identical requests with that method is the same as the effect for a single such request. Of the request methods defined by this specification, PUT, DELETE, and safe request methods are idempotent.

Like the definition of safe, the idempotent property only applies to what has been requested by the user; a server is free to log each request separately, retain a revision control history, or implement other non-idempotent side effects for each idempotent request.

Idempotent methods are distinguished because the request can be repeated automatically if a communication failure occurs before the client is able to read the server’s response. For example, if a client sends a PUT request and the underlying connection is closed before any response is received, then the client can establish a new connection and retry the idempotent request. It knows that repeating the request will have the same intended effect, even if the original request succeeded, though the response might differ.

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