Take me to your leader – Using Juju leadership for cron tasks in a multiunit service

I’m working on adding some periodic maintenance tasks to a service deployed using Juju. It’s a standard 3-tier web application with a number of Django application server units for load balancing and distribution.

Clearly the maintenance tasks’ most natural place to run is in one of these units, since they have all of the application’s software installed and doing the maintenance is as simple as running a “management command” with the proper environment set up.

A nice property we have by using Juju is that these application server units are just clones of each other, this allows scaling up/down very easily because the units are treated the same. However, the periodic maintenance stuff introduces an interesting problem, because we want only one of the units to run the maintenance tasks (no need for them to run several times). The maintenance scripts can conceivably be run in all units, even simultaneously (they do proper locking to avoid stepping on each other). And this would perhaps be OK if we only had 2 service units, but what if, as is the case, we have many more? there is still a single database and hitting it 5-10 times with what is essentially a redundant process sounded like an unacceptable tradeoff for the simplicity of the “just run them on each unit” approach.

We could also implement some sort of duplicate collapsing, perhaps by using something like rabbitmq and celery/celery beat to schedule periodic tasks. I refused to consider this since it seemed like swatting flies with a cannon, given that the first solution coming to mind is a one-line cron job. Why reinvent the wheel?

The feature that ended up solving the problem, thanks to the fine folks in Freenet’s #juju channel, is leadership, a feature which debuted in recent versions of Juju. Essentially, each service has one unit designated as the “leader” and it can be targeted with specific commands, queried by other units (‘ask this to my service’s leader’) and more importantly, unambiguously identified: a unit can determine whether it is the leader, and Juju events are fired when leadership changes, so units can act accordingly. Note that leadership is fluid and can change, so the charm needs to account for these changes. For example, if the existing leader is destroyed or has a charm hook error, it will be “deposed” and a new leader is elected from among the surviving units. Luckily all the details of this are handled by Juju itself, and charms/units need only hook on the leadership events and act accordingly.

So it’s then as easy as having the cron jobs run only on the leader unit, and not on the followers.

The simplistic way of using leadership to ensure only the leader unit performs an action was something like this in the crontab:

This uses juju-run with the unit’s name (which is hardcoded in the crontab – this is a detail of how juju run is used which I don’t love, but it works) to run the is-leader command in the unit. This will print out “True” if the executing unit is the leader, and False otherwise. So this will condition execution on the current unit being the leader.

Discussing this with my knowledgeable colleagues, a problem was pointed out: juju-run is blocking and could potentially stall if other Juju tasks are being run. This is possibly not a big deal but also not ideal, because we know leadership information changes infrequently and we also have specific events that are fired when it does change.

So instead, they suggested updating the crontab file when leadership changes, and hardcoding leadership status in the file. This way units can decide whether to actually run the command based on locally-available information which removes the lock on Juju.

The solution looks like this, when implemented using Ansible integration in the charm. I just added two tasks: One registers a variable holding is-leader output when either the config or leadership changes:

The second one fires on the same events and just uses the registered variable to write the crontabs appropriately. Note that Ansible’s “cron” plugin takes care of ensuring “crupdate” behavior for these crontab entries. Just be mindful if you change the “name” because Ansible uses that as the key to decide whether to update or create anew:

A created crontab file (in /etc/cron.d/roadmr-maintenance) looks like this:

A few notes about this. The IS_LEADER variable looks redundant. We could have put it directly in the comparison or simply wrote the crontab file only in the leader unit, removing it on the other ones. We specifically wanted the crontab to exist in all units and just be conditional on leadership. IS_LEADER makes it super obvious, right there in the crontab, whether the command will run. While redundant, we felt it added clarity.

Save for the actual value of IS_LEADER, the crontab is present and identical in all units. This helps people who log directly into the unit to understand what may be going on in case of trouble. Traditionally people log into the first unit; but what if that happens to not be the leader? If we write the crontab only on the leader and remove from other units, it will not be obvious that there’s a task running somewhere.

Charm Ansible integration magically runs tasks by tags identifying the hook events they should fire on. So by just adding the three tags, these events will fire in the specified order on config-changed, leader-elected and leader-settings-changed events.

The two leader hooks are needed because leader-elected is only fired on the actual leader unit; all the others get leader-settings-changed instead.

Last but not least, on’t forget to also declare the new hooks in your hooks.py file, in the hooks declaration which now looks like this (see last two lines added):

Finally, I’d be remiss not to mention an existing bug in leadership event firing. Because of that, until leadership event functionality is fixed and 100% reliable, I wouldn’t use this technique for tasks which absolutely, positively need to be run without fail or the world will end. Here, I’m just using them for maintenance and it’s not a big deal if runs are missed for a few days. That said, if you need a 100% guarantee that your tasks will run, you’ll definitely want to implement something more robust and failproof than a simple crontab.