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https://issues.apache.org/jira/browse/AIRFLOW-703?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15767367#comment-15767367
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ASF subversion and git services commented on AIRFLOW-703:
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Commit 96c787f390ad9852ae6c0c0fbb0510e36df185b1 in incubator-airflow's branch
refs/heads/master from Ben Lee Rodgers
[ https://git-wip-us.apache.org/repos/asf?p=incubator-airflow.git;h=96c787f ]
[AIRFLOW-703][AIRFLOW-1] Stop Xcom being cleared too early
XComs should only be cleared when it is certain
that the task will run. Previously, XComs were cleared
before it was determined if tasks were runnable, queable,
or just being marked success. Now XComs are cleared
immediately before the task actually starts.
Closes #1951 from blrnw3/fix/xcom_bug_AIRFLOW-703
> Xcom data cleared too soon
> --------------------------
>
> Key: AIRFLOW-703
> URL: https://issues.apache.org/jira/browse/AIRFLOW-703
> Project: Apache Airflow
> Issue Type: Bug
> Components: core, scheduler, xcom
> Affects Versions: Airflow 2.0, Airflow 1.7.1.3
> Environment: Tested using Dockerized Airflow setup with MySQL backend
> and Celery executor
> Reporter: Len Frodgers
> Labels: xcom
> Attachments: xcom_bug.py, xcom_bug_op1_logs.txt, xcom_bug_op2_logs.txt
>
>
> Xcom data is cleared at the start of the `run` method of the `TaskInstance`,
> regardless of whether the TI is subsequently executed (e.g. if the TI has
> previously succeeded, it won't execute). This means that if a TI for a DagRun
> is run twice in close succession, the latter will correctly not execute
> (since the former TI succeeded or is still running), but WILL clear any xcoms
> set by the former TI. Therefore, any downstream tasks depending on these
> xcoms will fail.
> I noticed this bug when I changed num_runs of the scheduler from None to 10.
> It didn't happen every time, but probably 50% or so.
> However, I can reproduce this reliably and repeatably with the following test
> dag:
> [attached]
> To make op1 execute twice, I use the UI to run it twice while op2 is doing
> the `time.sleep`.
> Logs from running this:
> [attached]
> The fix seems straightforward: don't clear xcom unless the TI will actually
> execute. Will happily create a PR.
> The suspect line is here:
> https://github.com/apache/incubator-airflow/blob/master/airflow/models.py#L1202
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