In chapter 3, we explored how to schedule workflows in Airflow based on a time interval. The time intervals can be given as convenience strings (e.g., "@daily"), timedelta objects (e.g., timedelta(days=3)), or cron strings (e.g., "30 14 * * *"). These are all notations to instruct the workflow to trigger at a certain time or interval. Airflow will compute the next time to run the workflow given the interval and start the first task(s) in the workflow at the next date and time.
In this chapter, we explore other ways to trigger workflows. This is often desired following a certain action, in contrast to the time-based intervals, which start workflows at predefined times. Trigger actions are often the result of external events; think of a file being uploaded to a shared drive, a developer pushing their code to a repository, or the existence of a partition in a Hive table, any of which could be a reason to start running your workflow.