The database is down since morning, nothing has been changed — how to diagnose?

If the database is "frozen" without visible changes, diagnostics should be systematic and step-by-step:

1. Check server status and resources:

   • Use top, htop, vmstat, iostat to assess CPU, memory, and disk load.
   • Check free disk space (df -h).

2. Database logs:

   • Review error and event logs (e.g., postgresql.log, mysqld.log).
   • Pay attention to lock errors, buffer overflows, network issues.

3. Locks and waits:

   • Run queries to identify locks (e.g., in PostgreSQL SELECT * FROM pg_locks WHERE NOT granted;).
   • Check for long transactions that may be blocking others.

4. Network issues:

   • Ensure the network between the application and the database is functioning correctly.

5. Query monitoring:

   • Enable slow query log if not already enabled.
   • Use query profiling tools.

6. Configuration check:

   • Ensure database parameters have not changed.

7. External factors:

   • Check if heavy backups or maintenance operations are running.

This systematic approach will help identify the cause of the downtime and take appropriate measures.