503 Service Unavailable
5xx Server Error
RFC 9110
The server is down or overwhelmed. Try again later.
What 503 Service Unavailable means {#what}
The server is temporarily unable to handle requests — either down for maintenance or overloaded. Should include a Retry-After header if the downtime is predictable. Responses should not be cached unless explicitly configured.
When you’ll see a 503 {#when}
- Planned maintenance window
- Traffic spike overwhelming the server
- Cascading failure in a dependent service
- Cold start timeout in serverless
When to use 503 {#use}
- Include
Retry-Afterheader - Return a user-friendly error page for browsers