All HTTP status codes

HTTP 503 Service Unavailable

Server Error

The server is temporarily unable to handle the request — overloaded, in maintenance, or deliberately shedding load. Retry-After may say when to come back.

Defined in RFC 9110

Common causes

  • Deploy/maintenance windows
  • Overload: connection pools, workers or CPU exhausted
  • Circuit breakers or autoscaling lag shedding traffic
  • Paused/suspended deployments on PaaS platforms

How to fix it

  1. Send Retry-After and a maintenance page for planned work

  2. Scale out / fix the bottleneck for overload cases

  3. Use backoff in clients instead of hammering a recovering service

Related status codes

502 Bad Gateway

A gateway/reverse proxy got an invalid response from the upstream server — the proxy is fine, the thing behind it isn't answering properly.

429 Too Many Requests

The client exceeded the rate limit. The Retry-After header (when present) says how long to back off.

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