Cron is great for automation, but when tasks begin to rely on each other (task C can only run after both tasks A and B finish) cron does not do the trick.
Apache Airflow is open source software (from airbnb) designed to handle the relationship between tasks. I recently setup an airflow server which coordinates automated jobs on databricks (great software for coordinating spark clusters). Connecting databricks and airflow ended up being a little trickier than it should have been, so I am writing this blog post as a resource to anyone else who attempts to do the same in the future.
For the most part I followed this tutorial from A-R-G-O when setting up airflow. Databricks also has a decent tutorial on setting up airflow. The difficulty here is that the airflow software for talking to databricks clusters (DatabricksSubmitRunOperator) was not introduced into airflow until version 1.9 and the A-R-G-O tutorial uses airflow 1.8.
Airflow 1.9 uses Celery version >= 4.0 (I ended up using Celery version 4.1.1). Airflow 1.8 requires Celery < 4.0. In fact, the A-R-G-O tutorial notes that using Celery >= 4.0 will result in the error:
I can attest that this is true! If you use airflow 1.9 with Celery < 4.0, everything might appear to work, but airflow will randomly stop scheduling jobs after awhile (check the airflow-scheduler logs if you run into this). You need to use Celery >= 4.0! Preventing the Wrong destination error is easy, but the fix is hard to find (hence why I wrote this post).
After much ado, here’s the fix! If you follow the A-R-G-O tutorial, install airflow 1.9, celery >=4.0 AND set broker_url in airflow.cfg as follows:
Note that compared to the A-R-G-O tutorial, I am just adding “py” in front of amqp. Easy!