HTTP 503 Service Unavailable

In short: The server is temporarily unavailable, for example due to overload or maintenance.

In more detail: Often with Retry-After. The service is running but cannot accept requests right now.

In Depth

Possible causes

• Overload (DDoS, load spike).
• Maintenance mode.
• All workers busy.

Solutions

1. Try again later.
2. Operators: scale, use caching, limit load, serve a maintenance page correctly with 503.

See also: HTTP 500 Internal Server Error, DDoS, Load Balancing