Asteris

When Should I Use Chronos?

// Brian Hicks // Mesos

How can you schedule periodic tasks into your Mesos cluster? For example, if you have a large ETL job that needs to run every night, how do you guarantee consistent runs across the cluster while making sure that the work is scheduled fairly and with a minimum of duplicated effort? This is where Chronos shines.

Chronos (formerly an AirBnB project, but now maintained by Mesosphere) occupies the role that cron does on a single machine. It can execute shell scripts or Docker containers, and has a nice web UI to see the state of your jobs (pictured below, screenshot courtesy of the Chronos documentation site.)

Chronos Web UI

Chronos actually improves on cron's scheduling techniques by using ISO8601 repeating intervals instead of the cron syntax you may be used to. This can cause some confusion at first, but ultimately gives you much more flexibility in deciding when to schedule your tasks. As a simple example, let’s say you want to schedule a task to run at midnight at the beginning of every month. In cron syntax, that would be 0 0 1 * * /path/to/command where in ISO8601 syntax, that’s R/2015-01-01T00:00:00/P1M (and Chronos’ web UI can help you when constructing these.)

Using a system like Chronos also gives your scheduled jobs a measure of fault tolerance. In a situation where you’re just using cron, if the server goes down then your tasks will definitely not be run. With Chronos, multiple servers can still fail and the jobs will still be scheduled.

You can also specify a dependency graph of tasks. For example if you have an ETL job, each stage depends on the previous stage being finished. In this case, you would schedule the extract stage on a schedule. The transform stage would then depend on the extract stage, and the load stage on the transform stage. Chronos will keep all of this straight for you, and only schedule work when dependencies are met.

When should I not use Chronos?

Sweet!

So is Chronos a good fit for the work you want to do with your Mesos cluster? Now you know! Here’s a cheat sheet based off of the information in this post to make things easier:

Should I use Chronos?

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