The answer
Only when Nginx is completely stuck and reload doesn't work, after installing a new Nginx module, or after a system upgrade. Restart causes a brief downtime as all connections are dropped. For routine config changes, always use reload.
Updating Nginx Without Downtime Zero-Downtime Configuration Changes
When should I use restart instead of reload?
More FAQs in Updating Nginx Without Downtime Zero-Downtime Configuration Changes
Use sudo systemctl reload nginx (not restart). Reload starts new worker processes with the new config while old workers finish their current requests. No connections are dropped. Always test with nginx -t first, and back up the config before changes.
Reload: master reads new config, starts new workers, old workers finish requests and shut down. Zero downtime. Restart: stops and starts Nginx. All active connections dropped. Brief downtime. Always use reload for config changes in production.
Master receives SIGHUP, reads new config, starts new workers with new config, sends shutdown signal to old workers, old workers stop accepting new connections but finish current requests, old workers shut down, new workers handle all traffic. Takes milliseconds.
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