# HTTP幂等性

· 2018-7-16 · 次阅读

HTTP协议是一种分布式的面向资源的网络应用层协议，Web API如此流行很大程度上应归功于简单有效的HTTP协议。然而，正如简单的Java语言并不意味着高质量的Java程序，

### http幂等性定义

http1.1规范中幂等性的定义是：

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.

HTTP方法的幂等性是指一次和多次请求某一个资源应该具有同样的副作用。幂等性属于语义范畴，正如编译器只能帮助检查语法错误一样，HTTP规范也没有办法通过消息格式等语法手段来定

### 分布式事务 vs 幂等性

bool withdraw(account_id, amount)


withdraw是从账号account_id中扣除amount数额的钱，扣除成功则返回true，否则返回false账户余额不变。一种可能的情况是，服务器已经成功处理请求并返回，但是由于网络等原因可能

int create_ticket()
bool idempotent_withdraw(ticket_id, account_id, amount)


create_ticket的语义是获取一个服务器端生成的唯一的处理号ticket_id，它将用于标识后续的操作。idempotent_withdraw和withdraw的区别在于关联了一个ticket_id，一个ticket_id

create_ticket不是幂等的，但在这种设计下，它对系统状态的影响可以忽略，加上idempotent_withdraw是幂等的，所以任何一步由于网络等原因失败或超时，客户端都可以重试，直到获得

### HTTP的幂等性

HTTP GET方法用于获取资源，不应有副作用，所以是幂等的。

HTTP DELETE方法用于删除资源，有副作用，但它应该满足幂等性。比如：DELETE http://www.forum.com/article/4231 ，调用一次和N次对系统产生的副作用是相同的，即删掉id为4231

The POST method is used to request that the origin server accept the entity enclosed in the request as a new subordinate of the resource identified
by the Request-URI in the Request-Line. …… If a resource has been created on the origin server, the response SHOULD be 201 (Created) and contain an
entity which describes the status of the request and refers to the new resource, and a Location header.

The PUT method requests that the enclosed entity be stored under the supplied Request-URI. If the Request-URI refers to an already existing
resource, the enclosed entity SHOULD be considered as a modified version of the one residing on the origin server. If the Request-URI does not point
to an existing resource, and that URI is capable of being defined as a new resource by the requesting user agent, the origin server can create the
resource with that URI.

POST所对应的URI并非创建的资源本身，而是资源的接收者。比如：POST http://www.forum.com/articles的语义是在 http://www.forum.com/articles 下创建一篇帖子，
HTTP响应中应包含帖子的创建状态以及帖子的URI。两次相同的POST请求会在服务器端创建两份资源，它们具有不同的URI；所以，POST方法不具备幂等性。