5xx Server errors
503 Service Unavailable
The server can't handle the request right now, usually because it's overloaded or in maintenance.
Common causes
- Planned maintenance.
- Too much traffic for the available servers.
- The app is starting up or has no healthy instances behind the load balancer.
How to fix it
- Add capacity or autoscaling if traffic spikes cause it.
- Check the load balancer's health checks: are all instances failing?
- During maintenance, send a Retry-After header and a friendly page.
Find out the moment your site returns 503
Spot Downtime monitors count this as down and alert you, since visitors see an error. (The public “Is it down?” checker treats 4xx answers from big sites as online, because many refuse automated requests.)