The 5xx class indicates the server failed to fulfil an apparently valid request. These drive SLOs and on-call paging.
Table of 5xx Codes (selected)
| Code | Name | Summary |
|---|---|---|
| 500 | Internal Server Error | Generic server failure. |
| 501 | Not Implemented | Method not supported at server. |
| 502 | Bad Gateway | Upstream error at proxy/gateway. |
| 503 | Service Unavailable | Overloaded or maintenance. |
| 504 | Gateway Timeout | Upstream timeout. |
| 507 | Insufficient Storage | WebDAV storage failure. |
| 508 | Loop Detected | WebDAV loop. |
| 510 | Not Extended | Further extensions required. |
| 511 | Network Authentication Required | Captive portal/auth needed. |
500 Internal Server Error
Catch-all for unhandled exceptions. Log with correlation IDs; prefer specific 5xx when possible.
501 Not Implemented
Server does not support the method or feature. Different from 405 (method known but not allowed on resource).
502 Bad Gateway
A reverse proxy/CDN got an invalid response from upstream. Investigate upstream health, TLS, and timeouts.
503 Service Unavailable
Temporary overload or maintenance. Include Retry-After. Can be used for brownouts.
504 Gateway Timeout
Proxy/CDN timed out waiting for upstream. Review origin latency, keepalives, and connection pools.
507/508/510/511
Less common; mostly enterprise or network control contexts.
CDN notes: RFC 5861 permits stale-if-error for errors that would produce 500, 502, 503, or 504. Provider-specific controls can differ. Verify retry and failover triggers, and restrict retries to safe or idempotent operations to avoid request storms.