Vijay Krishna Ramesh created AIRFLOW-931:
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Summary: LocalExecutor fails to run queued task with race condition
Key: AIRFLOW-931
URL: https://issues.apache.org/jira/browse/AIRFLOW-931
Project: Apache Airflow
Issue Type: Bug
Affects Versions: Airflow 1.8, 1.8.0rc4
Reporter: Vijay Krishna Ramesh
https://gist.github.com/vijaykramesh/707262c83429ab2a3f5ee701879813e3 provides
a small example that consistently hits this problem with LocalExecutor.
Basically when the dag run kicks off (with concurrency > 1) and a LocalExecutor
with parallelism > 2 the scheduler marks more than concurrency tasks as queued
(https://github.com/apache/incubator-airflow/blob/master/airflow/jobs.py#L1095)
There is a second check before actually running the task
(https://github.com/apache/incubator-airflow/blob/master/airflow/models.py#L1291)
that leaves the task in the QUEUED state but then the scheduler never picks it
back up. This causes the DAG to get stuck (as the queued tasks never run)
until the scheduler is restarted (at which point the enqueued tasks are
considered orphaned, the status is set to NONE, and then they are picked up by
the scheduler again and run.
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