Gateway Timeout

The server, acting as a gateway, didn't get a timely upstream response.

504 Gateway Timeout means a proxy or gateway waited for an upstream server and gave up before it answered. The upstream isn't necessarily down - it's just too slow, often because of a long query, a stuck external call, or an undersized timeout. It differs from 502, where the upstream replied but the reply was invalid.

What 504 looks like on the wire

GET /api/reports/annual HTTP/1.1
Host: example.com

HTTP/1.1 504 Gateway Timeout
Server: nginx
Content-Type: text/html

About 5xx Server Error

The server failed to fulfill a valid request. The problem is on the server side, not the client.

How to fix 504 Gateway Timeout

Find the slow upstream operation - a query, lock, or external call.
Raise the proxy's upstream timeout if the work is legitimately long.
Move long jobs to a background queue instead of the request path.
Check upstream health and capacity under load.

Where it's defined

504 Gateway Timeout is defined in RFC 9110 §15.6.5. MDN's 504 reference covers browser behavior and examples.