Scheduled Tasks

Demi can create, manage, and monitor scheduled tasks using NixOS systemd timers. These are declarative, survive reboots, and can be rolled back like any other NixOS change.

Creating a Timer

You: back up my postgres database every night at 2am

Demi: Here's the plan:
      - Timer: nurvus-backup-postgres
      - Schedule: *-*-* 02:00:00 (daily at 2am)
      - Command: podman exec postgres pg_dump -U postgres mydb > /srv/backups/postgres/$(date +%Y%m%d).sql
      - Creates /srv/backups/postgres/ directory

      Ready to proceed?

You: yes

Demi: Timer added. Rebuilding to activate.
      ★ nixos_rebuild (action=switch)
      Backup scheduled. First run at 2am tonight.

Schedule Formats

Demi uses systemd calendar notation:

Schedule	Meaning
*-*-* 02:00:00	Daily at 2am
hourly	Every hour
daily	Every day at midnight
weekly	Every Monday at midnight
*-*-* *:00/15:00	Every 15 minutes
Mon *-*-* 09:00:00	Every Monday at 9am
*-*-01 00:00:00	First of every month

Managing Timers

You: what timers are running?

Demi: ★ timer (action=list)
      Scheduled tasks (2):
        backup-postgres [enabled]
          Schedule: *-*-* 02:00:00
        update-blocklists [enabled]
          Schedule: weekly

You: disable the postgres backup

Demi: Disabling timer nurvus-backup-postgres. Ready to proceed?

You: yes

Demi: Timer disabled. Rebuild complete.

You: remove the blocklist updater

Demi: Removing timer nurvus-update-blocklists. This will delete the scheduled task.
      Ready to proceed?

You: yes

Demi: Timer removed. Rebuild complete.

Running Tasks Immediately

You can run any scheduled task right now without waiting for its schedule:

You: run the postgres backup now
Demi: ★ timer (action=run, name=backup-postgres)
      Started nurvus-backup-postgres.service.

One-Off Scheduled Tasks

Need to run something once at a specific time? Use run_at for a transient timer that doesn't require a rebuild:

You: restart the minecraft server at midnight tonight
Demi: ★ timer (action=run_at, name=restart-minecraft,
        schedule=*-*-* 00:00:00, command=systemctl restart podman-minecraft)
      Scheduled one-off task for midnight. This won't survive a reboot.

You: back up postgres in an hour
Demi: ★ timer (action=run_at, name=backup-postgres-now,
        schedule=+1h, command=podman exec postgres pg_dump ...)
      Scheduled. Will run in 1 hour.

One-off tasks are transient and won't survive a reboot. Use add for recurring tasks that need to persist.

Common Use Cases

Database backups: - "back up postgres every night" - "dump the nextcloud database weekly"

Container maintenance: - "restart the minecraft server every Sunday at 4am" - "update pi-hole's gravity list daily"

Monitoring: - "check if nginx is responding every 5 minutes and restart it if not"

Cleanup: - "delete postgres backups older than 30 days every week" - "clean up old container images monthly"

How It Works

Under the hood, Demi adds two blocks to configuration.nix:

1. A systemd service (oneshot) that runs the command
2. A systemd timer that triggers the service on schedule

All managed timers are prefixed with nurvus- so they don't conflict with system timers. Disabling a timer sets its wantedBy to an empty list so it stays in the config but doesn't activate.