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[jira] [Created] (FLINK-9808) Implement state conversion procedure in state backends

Tzu-Li (Gordon) Tai created FLINK-9808:

             Summary: Implement state conversion procedure in state backends
                 Key: FLINK-9808
                 URL: https://issues.apache.org/jira/browse/FLINK-9808
             Project: Flink
          Issue Type: Sub-task
          Components: State Backends, Checkpointing
            Reporter: Tzu-Li (Gordon) Tai
            Assignee: Aljoscha Krettek
             Fix For: 1.6.0

With FLINK-9377 in place and that config snapshots serve as the single source of truth for recreating restore serializers, the next step would be to utilize this when performing a full-pass state conversion (i.e., read with old / restore serializer, write with new serializer).

For Flink's heap-based backends, it can be seen that state conversion inherently happens, since all state is always deserialized after restore with the restore serializer, and written with the new serializer on snapshots.

For the RocksDB state backend, since state is lazily deserialized, state conversion needs to happen for per-registered state on their first access if the registered new serializer has a different serialization schema than the previous serializer.

This task should consist of three parts:

1. Allow {{CompatibilityResult}} to correctly distinguish between whether the new serializer's schema is a) compatible with the serializer as it is, b) compatible after the serializer has been reconfigured, or c) incompatible.

2. Introduce state conversion procedures in the RocksDB state backend. This should occur on the first state access.

3. Make sure that all other backends no longer do redundant serializer compatibility checks. That is not required because those backends always perform full-pass state conversions.

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