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https://issues.apache.org/jira/browse/FLINK-9808?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16663988#comment-16663988
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ASF GitHub Bot commented on FLINK-9808:
---------------------------------------

tzulitai commented on issue #6875: [FLINK-9808] [state backends] Migrate state 
when necessary in state backends
URL: https://github.com/apache/flink/pull/6875#issuecomment-433123440
 
 
   This PR is now subsumed by #6931.
   All comments have been addressed in the new PR.

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> 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
>            Priority: Critical
>              Labels: pull-request-available
>             Fix For: 1.7.0
>
>
> 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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