Re: Transition Flink job from Java to Scala with state migration

[Daksh Talwar <da...@flipkart.com>]

Thanks for the pointers Andrey, they helped us zero in on the issue.
We found that it was a bug in our Scala code that caused Flink to use Kryo
serializer to read Avro-based state.
We've fixed it, and are able to evolve state-schema seamlessly now.

Cheers,
Daksh


On Tue, Aug 25, 2020 at 9:53 PM Andrey Zagrebin <az...@apache.org>
wrote:

> Hi Daksh,
>
> You need to find which type causes the problem: Long, MyCustomObject or
> maybe something else. You could share the logs with full exception stack
> trace.
> My guess is that your scala code uses another serializer for the failing
> type. See also docs to understand serialization in Flink [1]
> When you restore the state after changing the job, the new serializer,
> used for the state type, should be compatible with the serializer, used to
> store the state in the previous version of the job [2].
>
> I also cc Gordon who could have more ideas about the problem.
>
> Best,
> Andrey
>
> [1]
> https://ci.apache.org/projects/flink/flink-docs-release-1.10/dev/types_serialization.html
> <https://ci.apache.org/projects/flink/flink-docs-release-1.11/dev/types_serialization.html>
> [2]
> https://ci.apache.org/projects/flink/flink-docs-release-1.10/dev/stream/state/schema_evolution.html
> <https://ci.apache.org/projects/flink/flink-docs-release-1.11/dev/stream/state/schema_evolution.html>
>
> On Tue, Aug 25, 2020 at 4:43 PM Daksh Talwar <da...@flipkart.com>
> wrote:
>
>> Hello,
>>
>> We run a Stream API based Flink application on 1.10.0, coded in Java.
>> While moving this job to Scala (reasons unrelated to Flink/this
>> application), we are getting the following error when trying to instantiate
>> Scala job from a savepoint taken in the Java job:
>>
>> *org.apache.flink.util.StateMigrationException: The new state
>> typeSerializer for operator state must not be incompatible.*
>>
>>
>> We can see that this error occurs while instantiating a ValueState<T>
>> state in our KeyedProcessFunction.
>> The KeyedProcessFunction instantiates two ValueState objects -
>> ValueState<Long> and ValueState<MyCustomObject>. Our hunch is that it's the
>> latter facing issues while loading from Savepoint.
>>
>> We have ensured that the KeyedProcessFunction has the same id in both the
>> jobs; and the new job is run with `--allowNonRestoredState` flag to discard
>> state for other operators.
>>
>> Has anyone in the community attempted this before, and faced such issues
>> or succeeded?
>>
>> Cheers,
>> Daksh
>>
>>
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