Nginx 503 Service Temporarily Unavailable

Hey team,

We’ve all seen this one before—the dreaded 503 Service Temporarily Unavailable error. In short, this means Nginx is up and running, but it can’t forward requests to the upstream server (e.g., your application backend, API, or proxied service).

Common Causes:

• The upstream server(s) are down or unreachable.

• Resource exhaustion (e.g., maxed-out connections, memory, or CPU on the backend).

• Misconfigured proxy_pass or upstream block in the Nginx config.

• Firewall rules or network issues blocking communication between Nginx and upstream.

• Temporary maintenance or intentional downtime.

Quick Checks:

1. Verify upstream services are running:

   curl -I http://upstream-server:port

2. Check Nginx error logs:

   tail -f /var/log/nginx/error.log

3. Review upstream config:

   upstream backend {  
       server 10.0.1.42:3000 fail_timeout=30s;  
       server 10.0.1.43:3000 backup;  
   }

   Ensure servers are correctly defined and responsive.

Typical Fixes:

• Restart or scale up backend services.

• Adjust timeouts and retry logic in nginx.conf:

  proxy_connect_timeout 5s;  
  proxy_next_upstream error timeout invalid_header;

• Validate network connectivity (e.g., DNS, firewalls, security groups).

• If using load balancing, ensure health checks are configured properly.

Let me know if you need help digging into the config or logs!