What it means
Maintenance windows and overload - the code that says 'come back'. Send Retry-After. Prefer this over 500 for a deliberate outage, because it tells crawlers not to de-index the page.
- Class
- 5xx Server error
- Defined in
- RFC 9110 §15.6.4
- Cached by default
- No
When you receive a 503
- The server is overloaded or down for maintenance. Honour
Retry-Afterif it is there. - Retry with backoff - a 503 is meant to be temporary.
- From a platform or CDN, a 503 can mean the origin has no healthy instances left.
When you send a 503
- For planned maintenance and for overload.
- Include
Retry-After. - Search engines treat a short run of 503s as temporary and keep the page indexed; weeks of them are read as the page being gone.
On the wire
HTTP/1.1 503 Service Unavailable Retry-After: 600 Content-Type: text/html <p>Down for maintenance. Back in about ten minutes.</p>