503 Service Unavailable
The server is not ready to handle the request, often because of overload or maintenance.
| Class | Server error (5xx) |
|---|---|
| Defined in | RFC9110, Section 15.6.4 |
| Cacheable by default | No, unless explicit freshness headers allow it |
Common causes
- Overload or maintenance mode.
- The service is starting up, or all backends are unhealthy.
What to do
Retry later. Servers should send a Retry-After header. For planned maintenance use 503 rather than 200 or 404 so search engines treat it as temporary.
Related codes
- 502 Bad Gateway: A gateway or proxy received an invalid response from the upstream server.
- 504 Gateway Timeout: A gateway or proxy did not get a response from the upstream server in time.
- 429 Too Many Requests: The client has sent too many requests in a given amount of time (rate limiting).