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https://issues.apache.org/jira/browse/AIRFLOW-47?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15279165#comment-15279165
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Chris Riccomini commented on AIRFLOW-47:
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My impression of the problem being discussed here is if you have 10 operators
that are all using an external task sensor waiting on another task. If you have
a parallelism of 10, and those tasks start, then the DAG that the sensors are
waiting for can NEVER start.
> 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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