[jira] [Commented] (FLINK-23143) Support state migration

["Hangxiang Yu (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/FLINK-23143?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17524708#comment-17524708 ] 

Hangxiang Yu commented on FLINK-23143:
--------------------------------------

Hi, [~roman].
IIUC, we need to create and maintain a new state in _ChangelogKeyedStateBackend_ in your solution ?
It may not work well in the case of switching changelog from enabled to disabled ?

Another solution is to make changelog part use correct _RegisteredKeyValueStateBackendMetaInfo/RegisteredPriorityQueueStateBackendMetaInfo_ when recovery.

(We create _RegisteredKeyValueStateBackendMetaInfo/RegisteredPriorityQueueStateBackendMetaInfo_ with _StateMetaInfoSnapshot_ in

_ChangelogBackendLogApplier#restoreKvMetaData_ and _ChangelogBackendLogApplier#restorePqMetaData_ but not use them)

But two new methods may be introduced in {_}KeyedStateBackend{_}.e.g.{_}create(RegisteredPriorityQueueStateBackendMetaInfo metaInfo).{_}

I have tried to do a simple POC and it works. But it may not be a good idea to introduce some new methods in _KeyedStateBackend._
WDYT?

> Support state migration
> -----------------------
>
>                 Key: FLINK-23143
>                 URL: https://issues.apache.org/jira/browse/FLINK-23143
>             Project: Flink
>          Issue Type: Sub-task
>          Components: Runtime / State Backends
>            Reporter: Roman Khachatryan
>            Priority: Minor
>             Fix For: 1.16.0
>
>
> ChangelogKeyedStateBackend.getOrCreateKeyedState is currently used during recovery; on 1st user access, it doesn't update metadata nor migrate state (as opposed to other backends).
>  
> The proposed solution is to
>  # wrap serializers (and maybe other objects) in getOrCreateKeyedState
>  # store wrapping objects in a new map keyed by state name
>  # pass wrapped objects to delegatedBackend.createInternalState
>  # on 1st user access, lookup wrapper and upgrade its wrapped serializer
> This should be done for both KV/PQ states.
>  
> See also [https://github.com/apache/flink/pull/15420#discussion_r656934791]
>  
> cc: [~yunta]



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