HTTP 504 – Gateway Timeout
The server, acting as a gateway or proxy, did not get a response in time from the upstream server.
Common use case
Occurs when a proxy or load balancer is up and the backend is technically alive, but the backend is just too slow to respond within the configured timeout - e.g. a slow database query.
Example
HTTP/1.1 504 Gateway Timeout
History
Part of HTTP/1.1 (RFC 2068, 1997).
Did you know?
It is distinct from 502: a 502 means the upstream sent garbage, while a 504 means the upstream never answered in time at all.