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From "Tzu-Li (Gordon) Tai (JIRA)" <j...@apache.org>
Subject [jira] [Created] (FLINK-6178) Allow upgrades to state serializers
Date Fri, 24 Mar 2017 07:59:49 GMT
Tzu-Li (Gordon) Tai created FLINK-6178:

             Summary: Allow upgrades to state serializers
                 Key: FLINK-6178
                 URL: https://issues.apache.org/jira/browse/FLINK-6178
             Project: Flink
          Issue Type: Improvement
          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. We can also consider exposing this hook / identification
only through {{StateDescriptor}} s that are configured with custom serializers.

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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