Since I have been using Icinga DB (instead of just PostgreSQL) as the backend for my Icinga2 instance, it has happened to me twice that Icinga no longer worked correctly after an update of the Icinga DB package. On closer inspection, it also becomes clear why:
× icingadb.service - Icinga DB
Loaded: loaded (/lib/systemd/system/icingadb.service; enabled; preset: enabled)
Active: failed (Result: exit-code) since Wed 2025-01-22 21:42:50 CET; 27s ago
Duration: 13ms
Main PID: 1853805 (code=exited, status=1/FAILURE)
CPU: 13ms
Jan 22 21:42:50 monitor systemd[1]: Starting icingadb.service - Icinga DB…
Jan 22 21:42:50 monitor systemd[1]: Started icingadb.service - Icinga DB.
Jan 22 21:42:50 monitor icingadb[1853805]: Starting Icinga DB daemon (v1.2.1)
Jan 22 21:42:50 monitor icingadb[1853805]: Connecting to database at 'pgsql://icingadb@localhost:5432/icingadb'
Jan 22 21:42:50 monitor icingadb[1853805]: unexpected database schema version: v3 (expected v4), please make sure you have applied all database migrations after upgrading Icinga DB
Jan 22 21:42:50 monitor systemd[1]: icingadb.service: Main process exited, code=exited, status=1/FAILURE
Jan 22 21:42:50 monitor systemd[1]: icingadb.service: Failed with result 'exit-code'.