最近在试着做RESTful API，学了很多平时用不到的HTTP知识。在一个API上，我想用409 Conflict表示要创建的资源已经存在。这是一个挺冷门的状态码，在B/S结构中基本上用不到。RFC 2616对它的解释是：
409 Conflict
The request could not be completed due to a conflict with the current
state of the resource. This code is only allowed in situations where
it is expected that the user might be able to resolve the conflict and
resubmit the request. The response body SHOULD include enough
information for the user to recognize the source of the conflict.
Ideally, the response entity would include enough information for the
user or user agent to fix the problem; however, that might not be
possible and is not required.
《RESTful Web APIs》这本书在409 Conflict的解释中写到：
响应报头：如果该冲突是由于某些其他资源的存在（比如，客户端尝试创建的某个特定的资源已经存在了）而造成的话，那么Location报头应该链接到该资源的URL：也就是说，冲突的来源。
