[jira] [Commented] (AIRFLOW-5391) Clearing a task skipped by BranchPythonOperator will cause the task to execute

["ASF GitHub Bot (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/AIRFLOW-5391?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17142034#comment-17142034 ] 

ASF GitHub Bot commented on AIRFLOW-5391:
-----------------------------------------

kaxil commented on a change in pull request #8992:
URL: https://github.com/apache/airflow/pull/8992#discussion_r443552728



##########
File path: tests/ti_deps/deps/test_trigger_rule_dep.py
##########
@@ -19,13 +19,13 @@
 
 import unittest
 from datetime import datetime
+from unittest.mock import Mock
 
 from airflow.models import BaseOperator, TaskInstance
 from airflow.utils.trigger_rule import TriggerRule
 from airflow.ti_deps.deps.trigger_rule_dep import TriggerRuleDep
 from airflow.utils.db import create_session
 from airflow.utils.state import State

Review comment:
       ```suggestion
   
   from airflow.models import BaseOperator, TaskInstance
   from airflow.utils.trigger_rule import TriggerRule
   from airflow.ti_deps.deps.trigger_rule_dep import TriggerRuleDep
   from airflow.utils.db import create_session
   from airflow.utils.state import State
   from tests.compat import Mock
   ```




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> Clearing a task skipped by BranchPythonOperator will cause the task to execute
> ------------------------------------------------------------------------------
>
>                 Key: AIRFLOW-5391
>                 URL: https://issues.apache.org/jira/browse/AIRFLOW-5391
>             Project: Apache Airflow
>          Issue Type: Bug
>          Components: operators
>    Affects Versions: 1.10.4
>            Reporter: Qian Yu
>            Assignee: Qian Yu
>            Priority: Major
>             Fix For: 2.0.0
>
>
> I tried this on 1.10.3 and 1.10.4, both have this issue: 
> E.g. in this example from the doc, branch_a executed, branch_false was skipped because of branching condition. However if someone Clear branch_false, it'll cause branch_false to execute. !https://airflow.apache.org/_images/branch_good.png!
> This behaviour is understandable given how BranchPythonOperator is implemented. BranchPythonOperator does not store its decision anywhere. It skips its own downstream tasks in the branch at runtime. So there's currently no way for branch_false to know it should be skipped without rerunning the branching task.
> This is obviously counter-intuitive from the user's perspective. In this example, users would not expect branch_false to execute when they clear it because the branching task should have skipped it.
> There are a few ways to improve this:
> Option 1): Make downstream tasks skipped by BranchPythonOperator not clearable without also clearing the upstream BranchPythonOperator. In this example, if someone clears branch_false without clearing branching, the Clear action should just fail with an error telling the user he needs to clear the branching task as well.
> Option 2): Make BranchPythonOperator store the result of its skip condition somewhere. Make downstream tasks check for this stored decision and skip themselves if they should have been skipped by the condition. This probably means the decision of BranchPythonOperator needs to be stored in the db.
>  
> [kevcampb|https://blog.diffractive.io/author/kevcampb/] attempted a workaround and on this blog. And he acknowledged his workaround is not perfect and a better permanent fix is needed:
> [https://blog.diffractive.io/2018/08/07/replacement-shortcircuitoperator-for-airflow/]
>  



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