Post vs Put

Post:创建，不安全，不幂等

Put：更新或创建，幂等

比较容易混淆的是HTTP POST和PUT。POST和PUT的区别容易被简单地误认为“POST表示创建资源，PUT表示更新资源”；而实际上，二者均可用于创建资源，更为本质的差别是在幂等性方面。在HTTP规范中对POST和PUT是这样定义的：

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

The PUT methodrequests that the enclosed entity be stored under the supplied Request-URI. 
If the Request-URI refers to an already existing resource, 
the enclosed entitySHOULD be considered as a modified version of the one residing on the originserver. 
If the Request-URI does not point to an existing resource,
and that URIis 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 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。

而PUT所对应的URI是要创建或更新的资源本身。比如：PUT http://www.forum/articles/4231的语义是创建或更新ID为4231的帖子。对同一URI进行多次PUT的副作用和一次PUT是相同的；因此，PUT方法具有幂等性。