Cleanup: remove compatibility with older versions

[arnaud-daroussin]

Hi @novakov-alexey,

Here is a small PR to cleanup code around some snapshots:

• I removed the very complex logic I introduced in PR Flink1: Handle state evolution without breaking state compatibility #251 to keep the compatibility with older versions, it was released in version 1.2.7 almost one year ago. I think it's time to do this cleanup!
• I found a way to get rid of Java snapshots and move them to Scala:
  • ScalaCaseClassSerializerSnapshot to same file as CaseClassSerializer
  • ScalaEitherSerializerSnapshot to same file as EitherSerializer
  • ScalaOptionSerializerSnapshot to same file as OptionSerializer
  • The problem was Flink's CompositeTypeSerializerSnapshot that requires to call 2 different constructors:
    • one with serializer instance to create the snapshot when serializing,
    • the other without arguments to instantiate by reflection the snapshot when deserializing.
  • But in Scala we can't call 2 different parent constructors, that's why they were left in Java I guess.
  • But I found a way to workaround this Scala limitation: we don't need to call parent constructor with serializer instance, we can reproduce its behavior instead.
  • I moved these snapshots to the same files as their serializers but I can't rename them without breaking the compatibility because their fully qualified class names are written in checkpoints. Or I can move them in their own files if you prefer?
• I also started a transition to use a CompositeTypeSerializerSnapshot in CoproductSerializerSnapshot. Indeed, usage of a composite applies perfectly well for serializing sealed trait as almost the only thing to serialize is the array of subtype serializers and it can be delegate to the composite.
  • But I had to introduce an "adapter" or a "delegate" to keep the compatibility during the transition to a CompositeTypeSerializerSnapshot because its readSnapshot() method is final so I can't override it to handle both versions to maintain the compatibility.
  • When we want to end the transition (next year? 😂), the code will be very simple like other snapshots using composite.

All these cleanups are preparing the ground for a big feature coming soon, stay tuned 😉

[novakov-alexey]

hi @arnaud-daroussin ,

thanks! Would it be a release with minor or patch version change?

[arnaud-daroussin]

You can release it in a minor version if you want but you can also merge it and wait for other PRs, there is no hurry.