Len Frodgers created AIRFLOW-703:
------------------------------------
Summary: 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
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 not execute (since the former
TI succeeded or is still running), but WILL clear any xcoms set by the former
TI. Therefore, and 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:
```
def func1(ti, **kwargs):
ti.xcom_push("k1", "xcom_custom")
return "xcom_default"
def func2(ti, **kwargs):
time.sleep(10)
xcom_custom = ti.xcom_pull("op1", key="k1")
xcom_default = ti.xcom_pull("op1")
logging.info("Default: %s, Custom: %s", xcom_default, xcom_custom)
assert None not in (xcom_custom, xcom_default)
bug_dag = DAG(
'xcom_bug',
max_active_runs=1,
schedule_interval=timedelta(minutes=3),
start_date=datetime(2016, 12, 16, 14, 57)
)
op1 = PythonOperator(python_callable=func1, provide_context=True,
task_id="op1", dag=bug_dag)
op2 = PythonOperator(python_callable=func2, provide_context=True,
task_id="op2", dag=bug_dag)
op1.set_downstream(op2)
```
To make op1 execute twice, I use the UI to run it twice while op2 is doing the
`time.sleep`.
Logs from running this:
[will attach]
The fix seems straightforward: don't clear xcom unless the TI will actually
execute. Will happily create a PR.
--
This message was sent by Atlassian JIRA
(v6.3.4#6332)
