503 Service Unavailable
The server is temporarily unable to handle requests
Seen in Web servers, load balancers, CDNs
The server is up but refuses work right now: overloaded, in maintenance, or with no healthy backends.
Common causes
- No healthy targets behind the load balancer.
- Overload, rate limiting or a maintenance mode.
- The app is starting up or out of workers.
How to fix it
- Check load-balancer health checks and target status.
- Scale up or fix the bottleneck; honour Retry-After.
- Check deploy and maintenance settings.
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Related errors
- 502 Bad Gateway · The proxy got a bad response from the server behind it
- 504 Gateway Timeout · The server behind the proxy took too long
- Error 520: Web server is returning an unknown error · Cloudflare got an empty or invalid response
- Error 521: Web server is down · Cloudflare could not connect to your origin
- Error 522: Connection timed out · Cloudflare timed out connecting to your origin
- Error 525: SSL handshake failed · Cloudflare could not complete TLS with your origin
- Error 526: Invalid SSL certificate · Cloudflare doesn't trust your origin's certificate
- 429 Too Many Requests · You've hit a rate limit
- 400 Bad Request · The server could not understand the request
- 401 Unauthorized · The request needs valid credentials
- 403 Forbidden · The server refuses to allow this request
- 404 Not Found · Nothing exists at this URL