collinmcnulty opened a new pull request, #41558:
URL: https://github.com/apache/airflow/pull/41558
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The first run behavior for cron based DAGs (and most DAGs really) is quite
unintuitive. It is very common for users to want to run a DAG for the first
time either immediately, with a logical date of the past run that would have
occurred most recently, or to wait until the next run time and then begin. The
former is only possible is you set catchup to False and the latter is not
possible without knowing ahead of time when that next run would be and
hardcoding the start time for the DAG (which is not feasible when doing
dev/text/prod environment promotion of the same DAG file).
It is also very common to set the start_date to a dynamic function that
calculates a date in the recent past, such as days_ago(1), which can bite you
if you're used to doing that (or have coded it into a DAG factory pattern) and
then you set up your first monthly DAG. The existing timetables behavior is to
silently miss the run, which makes it even worse.
This PR adds the ability to set start_date to `None` and get the intuitive
behavior of running the last run if that run would have happened in the very
recent past (10% of the distance between DAG runs) or to wait for the next run
otherwise. It also allows using the `run_immediately` parameter to specify
always running the last run, always waiting for the next run, or passing a
timedelta to exactly control what "recent past" means for this particular DAG.
Setting start_date to `None` is a much more accurate and clear indication of
author intent that `days_ago(x)`.
I hope that if this is widely adopted, we can stop seeing users miss DAG
runs or using dynamic start dates.
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