[GitHub] [airflow] KKcorps commented on a change in pull request #6295: [AIRFLOW-XXX] GSoD: Adding Task re-run documentation

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KKcorps commented on a change in pull request #6295: [AIRFLOW-XXX] GSoD: Adding Task re-run documentation
URL: https://github.com/apache/airflow/pull/6295#discussion_r336570463
 
 

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 File path: docs/dag-run.rst
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+ .. Licensed to the Apache Software Foundation (ASF) under one
+    or more contributor license agreements.  See the NOTICE file
+    distributed with this work for additional information
+    regarding copyright ownership.  The ASF licenses this file
+    to you under the Apache License, Version 2.0 (the
+    "License"); you may not use this file except in compliance
+    with the License.  You may obtain a copy of the License at
+
+ ..   http://www.apache.org/licenses/LICENSE-2.0
+
+ .. Unless required by applicable law or agreed to in writing,
+    software distributed under the License is distributed on an
+    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+    KIND, either express or implied.  See the License for the
+    specific language governing permissions and limitations
+    under the License.
+
+DAG Runs
+=========
+A DAG Run is an object representing an instantiation of the DAG in time.
+
+Each DAG may or may not have a schedule, which informs how ``DAG Runs`` are
+created. ``schedule_interval`` is defined as a DAG arguments, and receives
+preferably a
+`cron expression <https://en.wikipedia.org/wiki/Cron#CRON_expression>`_ as
+a ``str``, or a ``datetime.timedelta`` object. Alternatively, you can also
+use one of these cron "preset":
+
++--------------+----------------------------------------------------------------+---------------+
+| preset       | meaning                                                        | cron          |
++==============+================================================================+===============+
+| ``None``     | Don't schedule, use for exclusively "externally triggered"     |               |
+|              | DAGs                                                           |               |
++--------------+----------------------------------------------------------------+---------------+
+| ``@once``    | Schedule once and only once                                    |               |
++--------------+----------------------------------------------------------------+---------------+
+| ``@hourly``  | Run once an hour at the beginning of the hour                  | ``0 * * * *`` |
++--------------+----------------------------------------------------------------+---------------+
+| ``@daily``   | Run once a day at midnight                                     | ``0 0 * * *`` |
++--------------+----------------------------------------------------------------+---------------+
+| ``@weekly``  | Run once a week at midnight on Sunday morning                  | ``0 0 * * 0`` |
++--------------+----------------------------------------------------------------+---------------+
+| ``@monthly`` | Run once a month at midnight of the first day of the month     | ``0 0 1 * *`` |
++--------------+----------------------------------------------------------------+---------------+
+| ``@yearly``  | Run once a year at midnight of January 1                       | ``0 0 1 1 *`` |
++--------------+----------------------------------------------------------------+---------------+
+
+Your DAG will be instantiated for each schedule along with a corresponding 
+``DAG Run`` entry in 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 passed in the dag 
+will also be 2020-01-01.
+
+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.
+
+Re-run DAG
+''''''''''
+There can be cases where you will want to execute your DAG again. One such case is when the scheduled
+DAG run fails. Another can be the scheduled DAG run wasn't executed due to low resources or the DAG being turned off.
+
+Catchup
+-------
+
+An Airflow DAG with a ``start_date``, possibly an ``end_date``, and a ``schedule_interval`` defines a 
+series of intervals which the scheduler turn into individual DAG Runs and execute. A key capability 
+of Airflow is that these DAG Runs are atomic and idempotent items. The scheduler, by default, will
+kick off a DAG Run for any interval that has not been run (or has been cleared). This concept is called Catchup.
 
 Review comment:
   Updated with changes.

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