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Subject [GitHub] [airflow] ashb commented on a change in pull request #6295: [AIRFLOW-XXX] GSoD: Adding Task re-run documentation
Date Fri, 22 Nov 2019 21:10:59 GMT
ashb 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_r349795840
 
 

 ##########
 File path: docs/dag-run.rst
 ##########
 @@ -0,0 +1,197 @@
+ .. 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 argument, and receives
+preferably a
+`cron expression <https://en.wikipedia.org/wiki/Cron#CRON_expression>`_ as
+a ``str``, or a ``datetime.timedelta`` object. 
+
+.. tip::
+    You can use an online editor for CRON expressions such as `Crontab guru <https://crontab.guru/>`_
+
+Alternatively, you can also use one of these cron "presets":
+
++--------------+----------------------------------------------------------------+---------------+
+| 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 
 
 Review comment:
   ```suggestion
       started once the period it covers has ended.  The ``execution_date`` available in the
context
   ```

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