Problem scenario
You want to change the main configuration file for Apache web server (e.g., httpd.conf). But you do not want to drop current web surfers viewing the website. How do you change a security module or listening port for an Apache web server and have the changes take effect without interrupting the service by doing a regular restart?
Solution
Use the "graceful" directive. The exact command depends on the distribution of Linux that is running apache.
To learn more about it, see one of the links below:
https://www.cyberciti.biz/faq/apache-2-reload-httpd-config-file-unix-linux-command/
https://www.liquidweb.com/kb/restarting-services-from-the-command-line/
Explanation
Behind the scenes signals are sent to a [web service] process. SIGHUP would interrupt the service.* SIGUSR1 would be a signal that would facilitate a graceful reloading and prevent interruption.*
*Taken from https://www.liquidweb.com/kb/restarting-services-from-the-command-line/
For further context and knowledge you may want to read this related posting.