The Airflow scheduler monitors all tasks and DAGs, then triggers the task instances once their dependencies are complete. Behind the scenes, the scheduler spins up a subprocess, which monitors and stays in sync with all DAGs in the specified DAG directory. Once per minute, by default, the scheduler collects DAG parsing results and checks whether any active tasks can be triggered.
The Airflow scheduler is designed to run as a persistent service in an
Airflow production environment. To kick it off, all you need to do is
airflow scheduler command. It uses the configuration specified in
The scheduler uses the configured Executor to run tasks that are ready.
To start a scheduler, simply run the command:
Your DAGs will start executing once the scheduler is running successfully.
The first DAG Run is created based on the minimum
start_date for the tasks in your DAG.
Subsequent DAG Runs are created by the scheduler process, based on your DAG’s
The scheduler won’t trigger your tasks until the period it covers has ended e.g., A job with
schedule_interval set as
@daily runs after the day
has ended. This technique makes sure that whatever data is required for that period is fully available before the dag is executed.
In the UI, it appears as if Airflow is running your tasks a day late
If you run a DAG on a
schedule_interval of one day, the run with
2019-11-21 triggers soon after
Let’s Repeat That, the scheduler runs your job one
schedule_interval AFTER the start date, at the END of the period.
You should refer to DAG Runs for details on scheduling a DAG.
Triggering DAG with Future Date¶
If you want to use ‘external trigger’ to run future-dated execution dates, set
allow_trigger_in_future = True in
scheduler section in
This only has effect if your DAG has no
If you keep default
allow_trigger_in_future = False and try ‘external trigger’ to run future-dated execution dates,
the scheduler won’t execute it now but the scheduler will execute it in the future once the current date rolls over to the execution date.