Timetables¶

For DAGs with time-based schedules (as opposed to event-driven), the scheduling decisions are driven by its internal “timetable”. The timetable also determines the data interval and the logical date of each run created for the DAG.

DAGs scheduled with a cron expression or timedelta object are internally converted to always use a timetable.

If a cron expression or timedelta is sufficient for your use case, you don’t need to worry about writing a custom timetable because Airflow has default timetables that handle those cases. But for more complicated scheduling requirements, you may create your own timetable class and pass that to the DAG’s schedule argument.

Here are some examples of when custom timetable implementations are useful:

Data intervals with “holes” between. (Instead of continuous, as both the cron expression and timedelta schedules represent.)
Run tasks at different times each day. For example, an astronomer may find it useful to run a task at dawn to process data collected from the previous night-time period.
Schedules not following the Gregorian calendar. For example, create a run for each month in the Traditional Chinese Calendar. This is conceptually similar to the sunset case above, but for a different time scale.
Rolling windows, or overlapping data intervals. For example, one may want to have a run each day, but make each run cover the period of the previous seven days. It is possible to “hack” this with a cron expression, but a custom data interval would be a more natural representation.

As such, Airflow allows for custom timetables to be written in plugins and used by DAGs. An example demonstrating a custom timetable can be found in the Customizing DAG Scheduling with Timetables how-to guide.

Note

As a general rule, always access Variables, Connections etc or anything that would access the database as late as possible in your code. See Timetables for more best practices to follow.

Built-in Timetables¶

Airflow comes with several common timetables built in to cover the most common use cases. Additional timetables may be available in plugins.

CronTriggerTimetable¶

A timetable that accepts a cron expression, and triggers DAG runs according to it.

DeltaDataIntervalTimetable¶

Schedules data intervals with a time delta. Can be selected by providing a datetime.timedelta or dateutil.relativedelta.relativedelta to the schedule parameter of a DAG.

@dag(schedule=datetime.timedelta(minutes=30))
def example_dag():
    pass

CronDataIntervalTimetable¶

A timetable that accepts a cron expression, creates data intervals according to the interval between each cron trigger points, and triggers a DAG run at the end of each data interval.

EventsTimetable¶

Simply pass a list of datetimes for the DAG to run after. Useful for timing based on sporting events, planned communication campaigns, and other schedules that are arbitrary and irregular but predictable.

The list of events must be finite and of reasonable size as it must be loaded every time the DAG is parsed. Optionally, the restrict_to_events flag can be used to force manual runs of the DAG to use the time of the most recent (or very first) event for the data interval, otherwise manual runs will run with a data_interval_start and data_interval_end equal to the time at which the manual run was begun. You can also name the set of events using the description parameter, which will be displayed in the Airflow UI.

from airflow.timetables.events import EventsTimetable


@dag(
    timetable=EventsTimetable(
        event_dates=[
            pendulum.datetime(2022, 4, 5, 8, 27, tz="America/Chicago"),
            pendulum.datetime(2022, 4, 17, 8, 27, tz="America/Chicago"),
            pendulum.datetime(2022, 4, 22, 20, 50, tz="America/Chicago"),
        ],
        description="My Team's Baseball Games",
        restrict_to_events=False,
    ),
)
def example_dag():
    pass

Differences between the two cron timetables¶

There are two timetables CronTriggerTimetable and CronDataIntervalTimetable that accepts a cron expression. There are some differences between the two: - CronTriggerTimetable does not take care of Data Interval, while CronDataIntervalTimetable does. - The time when a DAG run is triggered by CronTriggerTimetable is more intuitive and more similar to what people expect cron to behave than that of CronDataIntervalTimetable (when catchup is False).

Whether taking care of Data Interval¶

CronTriggerTimetable does not care the idea of data interval. It means the value of data_interval_start, data_interval_end and legacy execution_date are the same - the time when a DAG run is triggered.

On the other hand, CronDataIntervalTimetable does care the idea of data interval. It means the value of data_interval_start and data_interval_end (and legacy execution_date) are different. They are the start and end of the interval respectively.

The time when a DAG run is triggered¶

There is no difference between the two when catchup is True. Catchup tells you how DAG runs are triggered when catchup is True.

When catchup is False, there is difference in how a new DAG run is triggered. CronTriggerTimetable triggers a new DAG run after the current time, while CronDataIntervalTimetable does before the current time (assuming the value of start_date is past time).

Here is an example showing how the first DAG run is triggered. Supposes there is a cron expression @daily or 0 0 * * *, which is aimed to run at 12AM every day. If you enable DAGs using the two timetables at 3PM on January 31st, CronTriggerTimetable will trigger a new DAG run at 12AM on February 1st. CronDataIntervalTimetable, on the other hand, will immediately trigger a new DAG run which is supposed to trigger at 12AM on January 31st if the DAG had been enabled beforehand.

This is another example showing the difference in the case of skipping DAG runs. Suppose there are two running DAGs using the two timetables with a cron expression @daily or 0 0 * * *. If you pause the DAGs at 3PM on January 31st and re-enable them at 3PM on February 2nd, CronTriggerTimetable skips the DAG runs which are supposed to trigger on February 1st and 2nd. The next DAG run will be triggered at 12AM on February 3rd. CronDataIntervalTimetable, on the other hand, skips the DAG runs which are supposed to trigger on February 1st only. A DAG run for February 2nd is immediately triggered after you re-enable the DAG.

By these examples, you see how CronTriggerTimetable triggers DAG runs is more intuitive and more similar to what people expect cron to behave than how CronDataIntervalTimetable does.