HTTP 503 Service Unavailable — Retry-After Explained

Overview

HTTP 503 Service Unavailable with Retry-After header tells clients when to retry. The server is temporarily overloaded or in maintenance.

Key Details

• 503 means the server is alive but cannot handle the request right now
• Retry-After header specifies seconds or a date when the service will be available
• Different from 500 (server error/bug) — 503 is intentional and temporary
• Load balancers return 503 when all backend servers are unhealthy
• Used during planned maintenance windows

Common Causes

• Server under heavy load exceeding capacity
• Planned maintenance with server intentionally returning 503
• All backend servers behind a load balancer are down
• Rate limiting at the infrastructure level
• Deployment in progress with rolling restart

Steps

1. 1Check the Retry-After response header for when to try again
2. 2If you manage the server, check backend health and scale up resources
3. 3Review load balancer health checks — backends may be failing
4. 4Check server logs for the underlying cause (memory, CPU, connections)
5. 5If during deployment, wait for the rollout to complete

Tags

More in 5xx Server Error

Frequently Asked Questions