When accessing a webpage, why does it display a 503 Service Unavailable error?

503 General Description The server is currently unable to process requests normally. This is typically not a page issue, but rather caused by server-side pressure or abnormal process states. The most common scenarios include: - The server experiencing momentary overload or maxed-out connection limits. - PHP-FPM or web service processes crashing or restarting. If this occurs sporadically but consistently during peak hours, it's likely related to resource exhaustion—such as maxed-out CPU usage, depleted memory, or concurrent connections exceeding limits. Another common cause is unreachable backend APIs or databases, resulting in frontend requests returning a 503 error directly. Recommended troubleshooting steps: Use `top` or `htop` to check for abnormal CPU/memory usage Verify if Nginx/Apache/PHP-FPM is still running Check error logs and access logs for connection timeouts or process termination entries Review recent plugin installations, software updates, or new API deployments; test disabling suspicious modules to restore normal operation Typically, the root cause can be quickly identified by examining resource usage and logs.