kaxil commented on a change in pull request #17552:
URL: https://github.com/apache/airflow/pull/17552#discussion_r708733553
##########
File path: docs/apache-airflow/dag-run.rst
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@@ -54,17 +54,36 @@ Cron Presets
Your DAG will be instantiated for each schedule along with a corresponding
DAG Run entry in the database backend.
-.. note::
- If you run a DAG on a schedule_interval of one day, the run stamped
2020-01-01
- will be triggered soon after 2020-01-01T23:59. In other words, the job
instance is
- started once the period it covers has ended. The ``execution_date``
available in the context
- will also be 2020-01-01.
+.. _data-interval:
- 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 ``schedule_interval``,
- sequentially. If your start_date is 2020-01-01 and schedule_interval is
@daily, the first run
- will be created on 2020-01-02 i.e., after your start date has passed.
+Data Interval
+-------------
+
+Each DAG run in Airflow has an assigned "data interval" that represents the
time
+range it operates in. For a DAG scheduled with ``@daily``, for example, each of
+its data interval would start at midnight of each day and end at midnight of
the
+next day.
+
+A DAG run is usually scheduled *after* its associated data interval has ended,
+to ensure the run is able to collect all the data within the time period. In
+other words, a run covering the data period of 2020-01-01 generally does not
+start to run until 2020-01-01 has ended, i.e. after 2020-01-02 00:00:00.
+
+All dates in Airflow are tied to the data interval concept in some way. The
+"logical date" (also called ``execution_date`` in Airflow versions prior to
2.2)
+of a DAG run, for example, denotes the start of the data interval, not when the
+DAG is actually executed.
+
+Similarly, since the ``start_date`` argument for the DAG and its tasks points
to
+the same logical date, it marks the start of *the DAG's fist data interval*,
not
+when tasks in the DAG will start running. In other words, a DAG run will only
be
+scheduled one interval after ``start_date``.
+
+.. tip::
+
+ If ``schedule_interval`` is not enough to express your DAG's schedule,
+ logical date, or data interval, see :doc:`Customizing imetables
</howto/timetable>`.
Review comment:
```suggestion
logical date, or data interval, see :doc:`Customizing timetables
</howto/timetable>`.
```
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