HTTP 503 Service Unavailable
What a 503 Service Unavailable response means, why it happens, and the fastest ways to fix it.
The server is temporarily unable to handle the request — overloaded, in maintenance, or shedding load. Retry-After may indicate when to try again.
- Maintenance mode or rolling deploy.
- Autoscaling lag under traffic spikes.
- Upstream health checks failing (all backends marked down).
- Circuit breakers shedding load.
- Retry with exponential backoff, honoring Retry-After.
- Check backend health-check status on the load balancer.
- Scale out or fix the failing dependency that tripped the breaker.
Notes
HTTP status codes are defined in RFC 9110 (which replaced RFC 7231). The first digit is the class: 1xx informational, 2xx success, 3xx redirection, 4xx client error, 5xx server error.
Same class: 500 · 501 · 502 · 504 · 505 · 507
All codes: HTTP status code reference
Frequently asked questions
What does HTTP 503 mean?
HTTP 503 Service Unavailable: The server is temporarily unable to handle the request — overloaded, in maintenance, or shedding load. Retry-After may indicate when to try again.
How do I fix a 503 error?
Retry with exponential backoff, honoring Retry-After. Check backend health-check status on the load balancer. Scale out or fix the failing dependency that tripped the breaker.
Is a 503 my fault or the server's?
503 is a server error — something on the server side (or between the proxy and the app) failed.