[GitHub] [airflow] sighingnow commented on pull request #17405: Add XCom.clear so it's hookable in custom XCom backend

[GitBox <gi...@apache.org>]

sighingnow commented on pull request #17405:
URL: https://github.com/apache/airflow/pull/17405#issuecomment-986210786


   > @sighingnow @MatrixManAtYrService Wondering if you guys could help me understand:
   > 
   > When is the [`clear`](https://airflow.apache.org/docs/apache-airflow/stable/_api/airflow/models/xcom/index.html#airflow.models.xcom.BaseXCom.clear) method called in the DAG process? How are we avoiding the case that we clear the data from the external database, but the tasks fails and has to rerun? If the task reruns, won't it need to query for the data again, but if we deleted it already, then it wont be there anymore.
   
   There's a command `airflow dags delete` to delete runs that belongs to dag.
   
   > 
   > Also what is the difference between [`delete`](https://airflow.apache.org/docs/apache-airflow/stable/_api/airflow/models/xcom/index.html#airflow.models.xcom.BaseXCom.delete) and `clear`. When is `delete` called?
   
   I think `delete` could be used to delete a specfic run, and `clear` deletes all runs of a dag.
   
   > but if we deleted it already, then it wont be there anymore.
   
   I think you could assume the previous step will be triggered to generate (query from the underlying database) the data.


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