作者：张岩峰，转载请注明出处 笔名：云烟旧梦
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生产环境常见HTTP状态码列表（List of HTTP status codes）为：
200 ok，服务器成功返回网页
Standard response for successful HTTP requests。
301 Moved Permanently（永久跳转），请求的网页已永久跳转到新位置。
This and all future requests should be directed to the given。
403 Forbidden（禁止访问），服务器拒绝请求
forbidden request （matches a deny filter） => HTTP 403
The request was a legal request，but the server is refusing to respond to it。
404 Not Found，服务器找不到请求的页面。
The requested resource could not be found but may be available again in the future。
500 Internal Server Error（内部服务器错误）
internal error in haproxy => HTTP 500
A generic message，given when no more specific message is suitable。
502 Bad Gatway（坏的网关），一般是网关服务器请求后端服务时，后端服务没有按照http协议正确返回结果。
the server returned an invalid or incomplete response => HTTP 502
503 Service Unavailable （服务当前不可用），可能因为超载或停机维护
no server was available to handle the request => HTTP 503
The server is currently unavailable （because it is overload or down for maintenance）
504 Getway Timeout（网关超时），一般是网关服务器请求后端服务时，后端服务没有在特定的时间内完成服务。
the server failed to reply in time => HTTP 504
The server was acting as a gateway or proxy and did not receive a timely response from the upstream server。