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| 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 |
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| |
| |
| Timetables |
| ========== |
| |
| A DAG's scheduling strategy is determined by its internal "timetable". This |
| timetable can be created by specifying the DAG's ``schedule_interval`` argument, |
| as described in :doc:`DAG Run </dag-run>`, or by passing a ``timetable`` argument |
| directly. The timetable also dictates the data interval and the logical time of each |
| run created for the DAG. |
| |
| Cron expressions and timedeltas are still supported (using |
| ``CronDataIntervalTimetable`` and ``DeltaDataIntervalTimetable`` under the hood |
| respectively), however, there are situations where they cannot properly express |
| the schedule. Some examples are: |
| |
| * 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. |
| |
| .. _`Traditional Chinese Calendar`: https://en.wikipedia.org/wiki/Chinese_calendar |
| |
| 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 |
| :doc:`/howto/timetable` how-to guide. |
