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From "Jeremiah Lowin (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (AIRFLOW-47) ExternalTaskSensor causes scheduling dead lock
Date Tue, 10 May 2016 22:57:12 GMT

    [ https://issues.apache.org/jira/browse/AIRFLOW-47?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15279154#comment-15279154
] 

Jeremiah Lowin commented on AIRFLOW-47:
---------------------------------------

Actually, hang on -- I think we're all confusing our vocabulary here. [~criccomini] and I
are discussing an Airflow concept called {{deadlock}} in which a DAG marks itself as failed
when it believes none of its tasks can run. I'm not sure that's actually happening here, or
at least the provided logs don't show it. [~hilaviz] when you say the DAG is deadlocked do
you mean it's just hanging? Or do you mean it's actually reporting a deadlock and failing?


If I'm reading this correctly, the root issue is that an external task sensor isn't recognizing
a task as expected -- let's not jump to scheduler issues if we don't have to. The second issue,
about 16 executors, sounds like it would be solved by bumping that up to 25 executors? In
any case I think it would be solved if we can understand what's happening with the external
task sensor.

[~criccomini] I think the points about the deadlock situation still stand but I'm not convinced
that it's relevant here...

> ExternalTaskSensor causes scheduling dead lock
> ----------------------------------------------
>
>                 Key: AIRFLOW-47
>                 URL: https://issues.apache.org/jira/browse/AIRFLOW-47
>             Project: Apache Airflow
>          Issue Type: Bug
>          Components: operators, scheduler
>    Affects Versions: Airflow 1.7.0
>         Environment: CentOS 6.5
> Airflow 1.7.0 with SequentialExecuter 
>            Reporter: Hila Visan
>
> We are trying to use 'ExternalTaskSensor' to coordinate between a daily DAG and an hourly
DAG  (daily dags  depend on hourly).
> Relevant code: 
> *Daily DAG definition:*
> {code:title=2_daily_dag.py|borderStyle=solid}
> default_args = {
>     …
>     'start_date': datetime(2016, 4, 2),
>     …
> }
> dag = DAG(dag_id='2_daily_agg', default_args=default_args, schedule_interval="@daily")
> ext_dep = ExternalTaskSensor(
>     external_dag_id='1_hourly_agg',
>     external_task_id='print_hourly1',
>     task_id='evening_hours_sensor',
>     dag=dag)
> {code}
> *Hourly DAG definition:*
> {code:title=1_hourly_dag.py|borderStyle=solid}
> default_args = {
>     …
>     'start_date': datetime(2016, 4, 1),
>     …
> }
> dag = DAG(dag_id='1_hourly_agg', default_args=default_args, schedule_interval="@hourly")
> t1 = BashOperator(
>     task_id='print_hourly1',
>     bash_command='echo hourly job1',
>     dag=dag)
> {code}
> The hourly dag was executed twice for the following execution dates:
> 04-01T00:00:00	
> 04-01T01:00:00
> Then the daily dag was executed, and is still running....	 
> According to logs, daily dag is waiting for hourly dag to complete:
> {noformat}
> [2016-05-04 06:01:20,978] {models.py:1041} INFO - Executing<Task(ExternalTaskSensor):
evening_hours_sensor> on 2016-04-03 00:00:00
> [2016-05-04 06:01:20,984] {sensors.py:188} INFO - Poking for 1_hourly_agg.print_hourly1
on 2016-04-02 00:00:00 ... 
> [2016-05-04 06:02:21,053] {sensors.py:188} INFO - Poking for 1_hourly_agg.print_hourly1
on 2016-04-02 00:00:00 ... }}
> {noformat}
> How can I solve this dead-lock?
> In Addition- I didn't understand if it means that the daily dag depends only on the "last"
hourly dag of the same day (23-24pm)? 
> What happens if the hourly dag of other hour fails?
> Thanks a lot! 



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