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https://issues.apache.org/jira/browse/AIRFLOW-1419?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16688037#comment-16688037
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Iuliia Volkova commented on AIRFLOW-1419:
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[~conradlee], without dummy task, you don't have a branch, you just have the
task confluence_op what depend on branch_op, it means confluence_op is a
branch by itself, it not depend on some branch, it's branch. We don't have in
Airflow edges what you can say - o, this edge is Branch without a task.
In your case on your picture, confluence_op - is a branch of branch operator,
what never be returned by branch operator and it also depends on the result of
another branch.
> Trigger Rule not respected downstream of BranchPythonOperator
> -------------------------------------------------------------
>
> Key: AIRFLOW-1419
> URL: https://issues.apache.org/jira/browse/AIRFLOW-1419
> Project: Apache Airflow
> Issue Type: Bug
> Affects Versions: 1.8.2
> Reporter: Conrad Lee
> Priority: Major
>
> Lets consider the following DAG:
> {noformat}
> ____________________
> / \
> branch_op confluence_op
> \______work_op________/
> {noformat}
> This is implemented in the following code:
> {code:java}
> import airflow
> from airflow.operators.python_operator import BranchPythonOperator
> from airflow.operators.dummy_operator import DummyOperator
> from airflow.utils.trigger_rule import TriggerRule
> from airflow.models import DAG
> args = {
> 'owner': 'airflow',
> 'start_date': airflow.utils.dates.days_ago(2)
> }
> dag = DAG(
> dag_id='branch_skip_problem',
> default_args=args,
> schedule_interval="@daily")
> branch_op = BranchPythonOperator(
> task_id='branch_op',
> python_callable=lambda: 'work_op',
> dag=dag)
> work_op = DummyOperator(task_id='work_op', dag=dag)
> confluence_op = DummyOperator(task_id='confluence_op', dag=dag,
> trigger_rule=TriggerRule.ALL_DONE)
> branch_op.set_downstream(confluence_op)
> branch_op.set_downstream(work_op)
> work_op.set_downstream(confluence_op)
> {code}
> Note that branch_op is a BranchPythonOperator, work_op and confluence_op are
> DummyOperators, and that confluence_op has its trigger_rule set to ALL_DONE.
> In dag runs where brancher_op chooses to activate work_op as its child,
> confluence_op never runs. This doesn't seem right, because confluence_op has
> two parents and a trigger_rule set that it'll run as soon as all of its
> parents are done (whether or not they are skipped).
> I know this example seems contrived and that in practice there are better
> ways of conditionally executing work_op. However, this is the minimal code to
> illustrate the problem. You can imagine that this problem might actually
> creep up in practice where originally there was a good reason to use the
> BranchPythonOperator, and then time passes and someone modifies one of the
> branches so that it doesn't really contain any children anymore, thus
> resembling the example.
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