cron

Drupal requires http://example.com/cron.php being called regularly. You can set this up in Plesk within each site. Plesk will create a corresponding entry in /var/spool/cron/tabs/site_user. The downside of this is that each cron run will produce an entry in the /var/log/messages system log. If you have a couple of sites and each one runs the cron job every hour, then you'll easily end up with hundreds of useless system log entries every day!