What is a 504 Gateway Timeout Error?

Question

What is a 504 Gateway Timeout Error?

Answer

A 504 Gateway Timeout error means that one server didn't receive a timely response from another server it was accessing while attempting to load a web page or fulfill another browser request.  Or more precisely, a different computer besides the web server, isn't able to communicate with another server it needs to.  

This could happen if a specific application or plugin needs to access something on another server and that server is overloaded or down.  Then you would get a 504 Gateway Timeout error.

RFC-7231 defines a 504 status code as:

CONFIG_TEXT:    The 504 (Gateway Timeout) status code indicates that the server,
   while acting as a gateway or proxy, did not receive a timely response
   from an upstream server it needed to access in order to complete the
   request.

The other server does not need to be external. It can also be another service or server-like entity within the main web server, such as a reverse proxy, mail server, or database server. 

These errors are usually temporary. A qualified system administrator should be able to check which site or service is unable to connect to the other server or service.