[GitHub] [airflow] milton0825 opened a new pull request #5498: [AIRFLOW-4509] SubDagOperator using scheduler instead of backfill

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milton0825 opened a new pull request #5498: [AIRFLOW-4509] SubDagOperator using scheduler instead of backfill
URL: https://github.com/apache/airflow/pull/5498
 
 
   Make sure you have checked _all_ steps below.
   
   ### Jira
   
   - [X] My PR addresses the following [Airflow Jira](https://issues.apache.org/jira/browse/AIRFLOW/) issues and references them in the PR title. For example, "\[AIRFLOW-XXX\] My Airflow PR"
     - https://issues.apache.org/jira/browse/AIRFLOW-4509
   
   ### Description
   
   - [X] Here are some details about my PR, including screenshots of any UI changes:
   
   Change `SubDagOperator` to use Airflow scheduler to schedule
   tasks in subdags instead of backfill.
   
   In the past, `SubDagOperator` relies on backfill scheduler
   to schedule tasks in the subdags. Tasks in parent `DAG`
   are scheduled via Airflow scheduler while tasks in
   a subdag are scheduled via backfill, which complicates
   the scheduling logic and adds difficulties to maintain
   the two scheduling code path.
   
   This PR simplifies how tasks in subdags are scheduled.
   `SubDagOperator` is responsible for creating a DagRun for subdag
   and wait until all the tasks in the subdag finish. Airflow
   scheduler picks up the DagRun created by SubDagOperator,
   create and schedule the tasks accordingly.
   
   Although `SubDagOperator` can occupy a pool/concurrency slot,
   user can specify the `mode=reschedule` so that the slot will be
   released periodically to avoid potential deadlock.
   
   ### Tests
   
   - [X] My PR adds the following unit tests __OR__ does not need testing for this extremely good reason:
   
   ### Commits
   
   - [X] My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "[How to write a good git commit message](http://chris.beams.io/posts/git-commit/)":
     1. Subject is separated from body by a blank line
     1. Subject is limited to 50 characters (not including Jira issue reference)
     1. Subject does not end with a period
     1. Subject uses the imperative mood ("add", not "adding")
     1. Body wraps at 72 characters
     1. Body explains "what" and "why", not "how"
   
   ### Documentation
   
   - [X] In case of new functionality, my PR adds documentation that describes how to use it.
     - All the public functions and the classes in the PR contain docstrings that explain what it does
     - If you implement backwards incompatible changes, please leave a note in the [Updating.md](https://github.com/apache/airflow/blob/master/UPDATING.md) so we can assign it to a appropriate release
   
   ### Code Quality
   
   - [X] Passes `flake8`
   

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