HTTP status codes / 5xx — Server error
503 Service Unavailable
What it means
The server is temporarily unable to serve — overloaded, in maintenance, or dependencies are down. Explicitly a temporary condition; Retry-After may say when to come back.
What causes it
Deliberate maintenance mode, traffic spikes beyond capacity, backend queue saturation, failing health checks pulling instances out of rotation.
How to fix it
Users: wait and retry. Operators: if not intentional maintenance, this is a capacity or health-check signal — scale out or find what made instances unhealthy.