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https://issues.apache.org/jira/browse/AIRFLOW-405?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17069135#comment-17069135
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Daniel Imberman commented on AIRFLOW-405:
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This issue has been moved to https://github.com/apache/airflow/issues/7938
> MySQL backend connection management bug (related to PythonOperator)
> -------------------------------------------------------------------
>
> Key: AIRFLOW-405
> URL: https://issues.apache.org/jira/browse/AIRFLOW-405
> Project: Apache Airflow
> Issue Type: Bug
> Components: operators
> Affects Versions: 1.7.1.2
> Reporter: Jiasi Zeng
> Priority: Major
>
> # Environment setup
> We are running Airflow 1.7.1.2 with MySQL 5.6. The `wait_timeout` of MySQL is
> set at 300 seconds, which means that idle connections will go away after 300
> seconds of inactivity. To reflect this, we set SQLAlchemy's `pool_recycle` in
> `airflow.cfg` to 290 seconds, which should force Airflow/SQLAlchemy to
> recycle/discard connections after 290 seconds. Thus Airflow shouldn't try to
> use an already-dead connection.
> # Symptom
> When running a PythonOperator that takes more than 300 seconds to execute,
> the task would finish executing the Python callable, but ends up with error:
> https://gist.github.com/garthcn/cd7bcdec12748406506f2b0710655c8b
> It seems that after the Python callable finishes executing, Airflow tries to
> push its return value to XCom. However, the SQL connection has gone away
> while Airflow/SQLAlchemy think it's still there.
> # Hypothesis
> I did some investigation and think that it might be caused by not calling
> `session.commit()` or `session.close()` for the DB operations before the XCom
> push. As far as I know, in SQLAlchemy, if you don't close a connection and
> let it be in `checked-out` state, it won't be recycled by connection pool,
> and thus SQLAlchemy will try to use it again after >300 seconds (which is the
> wait_timeout for MySQL in our case). This will result in a "MySQL connection
> has gone away" issue.
> It seems that Airflow codebase uses @provide_context decorator to help with
> session open/close, and my hunch is that some functions are not using it or
> misusing it.
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