You can install the frontend and backend on separate systems.
This article describes a case in which both frontend and backend are installed on the same system.
When you installed the Admin Diary application from the app center and are missing some entries from system changes that were made, you should check the backend variable on all systems that should report events to the Admin Diary backend:
ucr search --brief admin/diary/backend
If the backend variable is empty or set to the wrong server, then system changes are not (correctly) reported to the Admin Diary backend. In this example the backend is installed on the backup server, so we should get an output like:
admin/diary/backend: backup.dom.ain
When you changed this UCR variable you have to restart the rsyslog service to make those changes effective:
systemctl restart rsyslog