HTTP 504 Gateway Timeout with nginx

What the message looks like

2026/10/23 10:02:41 [error] 1234#1234: *9012 upstream timed out (110: Connection timed out) while reading response header from upstream, client: 198.51.100.7, server: cloud.beispiel.de, request: "POST /index.php/apps/files/ajax/upload.php HTTP/2.0"

Common causes

• Long-running requests: large uploads, exports, reports, image processing.
• The PHP-FPM pool is full (pm.max_children reached) and new requests wait in line.
• Slow or locked database queries.
• fastcgi_read_timeout or proxy_read_timeout too low for the use case.
• Backend on another host unreachable through network or firewall (connect runs into the timeout).

Check

1. Find timeouts in the nginx log
   grep 'upstream timed out' /var/log/nginx/error.log | tail -n 20Copy
2. Check whether PHP-FPM hits its limit
   grep -i 'max_children' /var/log/php8.3-fpm.logCopy
3. Measure response time precisely
   curl -s -o /dev/null -w 'connect=%{time_connect} ttfb=%{time_starttransfer} total=%{time_total}\n' https://cloud.example.com/status.phpCopy
4. Show running slow database queries
   mysqladmin -u root -p processlistCopy

Fix

• Fix the cause first: optimise slow queries, add indexes, move long jobs to cron or a queue.
• Raise pm.max_children according to RAM (RAM for PHP divided by memory per worker).
• Raise timeouts together: fastcgi_read_timeout 300s; in nginx plus max_execution_time and request_terminate_timeout in PHP-FPM.
• With reverse proxy chains (load balancer, Docker, CDN), check the timeout on every hop.
• After changes run nginx -t && systemctl reload nginx and systemctl reload php8.3-fpm.

FAQ

Should I just set the timeout to 3600 seconds?

No. It hides the problem and ties up workers. Raise timeouts only for specific locations such as upload or export paths.

Why do I get a 504 while PHP keeps running?

nginx only drops the connection to the client. The PHP script continues until max_execution_time or request_terminate_timeout kicks in.