Web Caching

5xx Server Errors

500 Internal Server Error

This code is the default for an error condition when none of the other 5xx codes apply.

501 Not Implemented

This code indicates that the server does not implement the necessary features to satisfy the request.

502 Bad Gateway

This code indicates that the server received an invalid response from an upstream server.

503 Service Unavailable

This code indicates that the server is temporarily unable to process the client’s request. A server that becomes overloaded may use this code to let the client know that it can retry the request later.

504 Gateway Time-out

This code is used by proxies and some servers to indicate a timeout when forwarding the client’s request. It’s also used when a request with the only-if-cached directive would result in a cache miss.

505 HTTP Version Not Supported

This code indicates that the server refuses to handle this request because of the HTTP version in the request line.