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https://issues.apache.org/jira/browse/FLINK-6178?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15944743#comment-15944743
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Xiaogang Shi commented on FLINK-6178:
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The idea that allowing the upgrades to state serializers is excellent. But I
have some concerns about the "convertState" phase. Currently, Flink has no
knowledge of the serializers to use before users access the states (via the
methods provided in {{RuntimeContext}}). That means, we can only convert the
states when users are about to access them. The conversion may be very costly
and the processing of data streams will be paused for quite a long time.
Actually, i am very interested at the offline tool provided in the future. Now
many efforts are made in Flink runtime to allow the restoring from old
savepoints. They make the code very complicated and hard to follow. I prefer to
move them from the main program to the offline tool.
I think the offline tool also eases the burdens of users to implement
{{TypeSerializer}}s that allow the deserialization of the data in different
serialization formats. They only need to provide the new serializers to access
the states stored in the savepoints.
> Allow upgrades to state serializers
> -----------------------------------
>
> Key: FLINK-6178
> URL: https://issues.apache.org/jira/browse/FLINK-6178
> Project: Flink
> Issue Type: New Feature
> Components: State Backends, Checkpointing, Type Serialization System
> Reporter: Tzu-Li (Gordon) Tai
> Assignee: Tzu-Li (Gordon) Tai
>
> Currently, users are locked in with the serializer implementation used to
> write their state.
> This is suboptimal, as generally for users, it could easily be possible that
> they wish to change their serialization formats / state schemas and types in
> the future.
> This is an umbrella JIRA for the required tasks to make this possible.
> Here's an overview description of what to expect for the overall outcome of
> this JIRA (the specific details are outlined in their respective subtasks):
> Ideally, the main user-facing change this would result in is that users
> implementing their custom {{TypeSerializer}} s will also need to implement
> hook methods that identify whether or not there is a change to the serialized
> format or even a change to the serialized data type. It would be the user's
> responsibility that the {{deserialize}} method can bridge the change between
> the old / new formats.
> For Flink's built-in serializers that are automatically built using the
> user's configuration (most notably the more complex {{KryoSerializer}} and
> {{GenericArraySerializer}}), Flink should be able to automatically
> "reconfigure" them using the new configuration, so that the reconfigured
> versions can be used to de- / serialize previous state. This would require
> knowledge of the previous configuration of the serializer, therefore
> "serializer configuration metadata" will be added to savepoints.
> Note that for the first version of this, although additional infrastructure
> (e.g. serializer reconfigure hooks, serializer configuration metadata in
> savepoints) will be added to potentially allow Kryo version upgrade, this
> JIRA will not cover this. Kryo has breaking binary formats across major
> versions, and will most likely need some further changes. Therefore, for the
> {{KryoSerializer}}, "upgrading" it simply means changes in the registration
> of specific / default serializers, at least for now.
> Finally, we would need to add a "convertState" phase to the task lifecycle,
> that takes place after the "open" phase and before checkpointing starts / the
> task starts running. It can only happen after "open", because only then can
> we be certain if any reconfiguration of state serialization has occurred, and
> state needs to be converted. Ideally, the code for the "convertState" is
> designed so that it can be easily exposed as an offline tool in the future.
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