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 error message, given when no more specific message is suitable.

502 - Bad Gateway（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
    - The server was acting as a gateway or proxy and received an invalid response from the upstream server.

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 overloaded or down for maintenance).

504 - Gateway 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.

生产环境常见的HTTP状态码列表