HTTP 503 Service Unavailable

Plain-English meaning

The service is temporarily unable to process requests.

Why it happens

Used during overload, maintenance mode, or dependency exhaustion events.

Common causes

  • Autoscaling lag under sudden load
  • Database connection pool saturation
  • Maintenance flag left enabled
  • Circuit breaker open after repeated upstream failures

What to check first

Check CPU/memory/queue depth/DB pool metrics, maintenance toggles, and dependency status dashboards.

How to fix it

Scale capacity, shed non-critical traffic, return Retry-After where possible, and reduce synchronous hot-path work.

Example signal

HTTP/1.1 503 Service Unavailable
Retry-After: 120

Related links

404 · 429 · 500 · 502 · 503 · 504 · Cloudflare 522 · Nginx 499 · ERR_CONNECTION_REFUSED

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