503 🌐 HTTP

HTTP 503 Service Unavailable

The server is temporarily unable to handle the request — overloaded, in maintenance, or has no healthy backends.

Seen on: PHP WordPress Kubernetes AWS

Meaning

503 signals a temporary condition. Servers may include Retry-After. It’s returned by maintenance modes (Laravel php artisan down, WordPress updates), by load balancers with no healthy targets, and by servers that have exhausted worker processes.

Common causes

  • Maintenance mode enabled (and not disabled after a deploy)
  • No healthy targets behind the load balancer
  • All worker processes busy (PHP-FPM pm.max_children, Apache MaxRequestWorkers)
  • Rate limiting returning 503 (Nginx limit_req default)
  • Dependency outage with a circuit breaker open
  • Kubernetes service with no ready pods

⚡ Quick fix

  1. Check whether maintenance mode is on (php artisan up, delete WordPress .maintenance file)
  2. Check load balancer target health
  3. Look for worker exhaustion in server logs and scale or raise limits
  4. Retry after the Retry-After interval

Detailed fix by platform

PHP

  1. PHP-FPM log "server reached pm.max_children setting": raise pm.max_children based on available RAM ÷ average process size.

WordPress

  1. Delete the .maintenance file in the site root if an update was interrupted.

Kubernetes

  1. kubectl get endpoints <svc> — empty endpoints means no ready pods; check readiness probes and pod status.

AWS

  1. ALB returns 503 when the target group has no registered/healthy targets — check health check path and security groups.

How to diagnose

  1. Maintenance — Is maintenance mode on?
  2. Capacity — Are workers/pods saturated?
  3. Health — Are backends passing health checks?
  4. Dependencies — Is a downstream service failing?

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