[jira] [Created] (FLINK-9376) Allow upgrading to incompatible state serializers (state schema evolution)

["Tzu-Li (Gordon) Tai (JIRA)" <ji...@apache.org>]

Tzu-Li (Gordon) Tai created FLINK-9376:
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             Summary: Allow upgrading to incompatible state serializers (state schema evolution)
                 Key: FLINK-9376
                 URL: https://issues.apache.org/jira/browse/FLINK-9376
             Project: Flink
          Issue Type: New Feature
          Components: State Backends, Checkpointing, Type Serialization System
            Reporter: Tzu-Li (Gordon) Tai
             Fix For: 1.6.0


Currently, users have access to upgrade state serializers on the restore run of a stateful job, as long as the upgraded new serializer remains backwards compatible with all previous written data in the savepoint (i.e. it can read all previous and current schema of serialized state objects).

What is still lacking is the ability to upgrade to incompatible serializers. Upon being registered an incompatible serializer for existing restored state, that state needs to go through the process of -
1. read serialized state with the previous serializer
2. passing each deserialized state object through a “migration map function”, and
3. writing back the state with the new serializer

This should be strictly limited to state registrations that occur before the actual processing begins (e.g. in the `open` or `initializeState` methods), so that we avoid performing these operations during processing.

Procedure 2. will allow even state type migrations, but that is out-of-scope of this JIRA.
This ticket focuses only on procedures 1. and 3., where we try to enable schema evolution without state type changes.

This is an umbrella JIRA ticket that overlooks this feature, including a few preliminary tasks that work towards enabling it.



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