I have upgraded several servers from Ubuntu 14.04 to 18.04 with no issue. However, on a fresh Ubuntu 18.04 install, I have encountered a problem that seems strange.
I have edited the systemd service file located at /etc/systemd/system/multi-user.target.wants/tomcat9.service but my changes made no difference, systemctl wouldn't even ask me to reload the daemon.
I even tried to delete the file which did not prevent me from restarting the tomcat service.
After searching for a while, I discovered that there was another service file located in /lib/systemd/system/tomcat9.service and after making the required changes (adding a ReadWritePaths entry), my application was working again.
I found out that systemd scans different directories looking for service files but it seems that /etc/systemd/system should have priority.
How come servers upgraded from Ubuntu 14.04 to 18.04 read the file from /etc/systemd/system but not fresh installs? Is there a place where this behaviour is configured?