Managing cron jobs with Pacemaker

It's not uncommon in Pacemaker clusters to run specific cron jobs only on a node that currently runs a particular resource. The ocf:heartbeat:symlink resource agent can be exceptionally helpful in this situation. Here's how to use it.

Suppose you've got a cron job for Postfix whose definition normally lives in /etc/cron.d/postfix. All your Postfix related data is in a mountpoint /srv/postfix (that filesystem could live on iSCSI, or DRBD, or it could be a GlusterFS mount – that's irrelevant for the purposes of this discussion). And as such, you've moved your cron definition to /srv/postfix/cron.

Now you want that cron job to execute only on the node that also is currently the active Postfix host. That's not hard at all:

primitive p_postfix ocf:heartbeat:postfix \
  params config_dir="/etc/postfix" \
  op monitor interval="10"
primitive p_symlink ocf:heartbeat:symlink \
  params target="/srv/postfix/cron" \
    link="/etc/cron.d/postfix" \
    backup_suffix=".disabled" \
  op monitor interval="10"
primitive p_cron lsb:cron \
  op monitor interval=10
order o_symlink_before_cron inf: p_symlink p_cron
colocation c_cron_on_symlink inf: p_cron p_symlink
colocation c_symlink_on_postfix inf: p_symlink p_postfix

What this will do for you is this:

• Check whether a file named postfix already exists in /etc/cron.d
• If it does, rename it to postfix.disabled (remember, cron ignores job definitions with dots in the filename)
• (Re-)Create the postfix job definition as a symlink to /srv/postfix/cron
• Restart cron when it's done.

The c_symlink_on_postfix colocation ensures that all of this happens on the node where the p_postfix resource is also active.