Cron jobs and daemons

Cron#

Cron runs commands on a schedule. Each entry becomes a systemd timer:

• a run never overlaps the previous one,
• a Timeout (seconds) stops runaway commands,
• the output is kept in the system journal (Last run and logs).

Add schedule with a Name, the user to Run as, a Schedule (cron) such as */5 * * * * (every five minutes) or 30 3 * * * (03:30 every day), and the Command. Run now starts it immediately; Enabled pauses or resumes it. Commands are stored encrypted. Entries that run as root (or a sudo user) need your passkey when you save them.

Some apps add their own entries: the Laravel scheduler, Moodle, Mautic and Akaunting crons, and WordPress's WP-Cron when you move it to the system timer.

Daemons#

Daemons keep programs running: queue workers, Node servers, bots. Each process is a systemd service that restarts when it exits; logs go to the journal.

Add daemon with a Name, the user to Run as, the Command, an optional Working directory and the number of Processes (for example four queue workers). Status and logs shows each process and its recent output; Restart, Stop and Edit manage it. Daemons of root or a sudo user need your passkey.

Laravel sites have a shortcut on their Deploy pane: the Scheduler (every minute) switch and the number of Queue workers. Node.js and Python apps get their web process as a daemon automatically.

Resource limits#

Cron jobs and daemons (Node.js and Python apps included) run together in one resource group with half the CPU and disk priority of the web server and databases, so a busy app or a runaway job cannot slow down your sites or the database. The group starts reclaiming memory above 80% of the server's RAM and never gets more than 90%. Each daemon is limited to 1,024 processes and threads.