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

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

MatrixManAtYrService commented on pull request #17405:
URL: https://github.com/apache/airflow/pull/17405#issuecomment-929890018


   I see that `XCom.clear` has parameters that let you know what you're clearing (`dag_id`, `task_id`, `execution_date`).  When I set out to implement a custom backend, I totally expected these--so I'm glad they're here ❤️ .
   
   What confuses me is that [the recommended approach for custom xcom backends ](https://airflow.apache.org/docs/apache-airflow/stable/concepts/xcoms.html#custom-backends) is to override `serialize_value` and `deserialize_value`, and those **don't** have parameters of that sort.
   
   So when I'm "serializing" a value, I don't know which task it's for.  I have to do something like generate a UUID or a hash of the data or something to come up with a key that won't collide with other values (this key is then used to recover the stored value while "deserializing").   So if I later want to clear that value, I can't distinguish it from its neighbors which I shouldn't clear.
   
   Am I missing something?  Or does inviting users to implement `clear` put them in a challenging place re: how they implement the other two functions?


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