502 vs 503 vs 504: what each one means and where to look
All three come from something in front of your application (a proxy, a load balancer or a CDN) or from the server itself. The number tells you which kind of failure it is, and so where to look first.
| Code | What happened | Usually | Check first |
|---|---|---|---|
| 502 Bad Gateway | The proxy reached the application and got a bad answer or none. | The application process is down or restarting; wrong upstream port. | Is the application process running? Read the proxy's error log. |
| 503 Service Unavailable | Something is refusing work on purpose, for now. | Maintenance mode, overload, no healthy backend behind the load balancer. | Maintenance switch, load balancer target health, worker usage. |
| 504 Gateway Timeout | The proxy waited for the application and gave up. | A slow query or a hanging call to another service. | Slow-query log and traces for the failing request. |
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