feat(#286): add serializers for mutable collections: ArrayDeque, ArrayBuffer, HashMap, Queue, HashSet

[arnaud-daroussin]

Hi @novakov-alexey,

I'm completing issue #286 with Scala mutable collection serializers:

• scala.collection.mutable.ArrayDeque
• scala.collection.mutable.Buffer & scala.collection.mutable.ArrayBuffer
• scala.collection.mutable.Queue
• scala.collection.mutable.Map & scala.collection.mutable.HashMap
• scala.collection.mutable.Set & scala.collection.mutable.HashSet

Serializers are implementing de/ser for collection traits (Buffer, Map, Set, etc...) but there are 2 implicit functions for each serializers: one for the trait (e.i. Buffer) and one for the default implementation (e.i. ArrayBuffer). It allows to serialize any implementation of a collection trait, but there is a drawback at deserialization: the default implementation is instantiated.

That's why I've made the choice to implement an ArrayDeque first class serializer to keep the benefits of an ArrayDeque over a "plain" ArrayBuffer.

It may be possible to make things generic like it's done in CollectionSerializerSnapshot but it increases state size to serialize class names, etc. so I prefer to keep things simple for this first version. And maybe adding more simple implementations later is a better strategy.

[novakov-alexey]

Hi @arnaud-daroussin ,

Thanks for another great PR.

May I ask you what is the use case of having all these mutable collections in your Flink pipelines? Do they offer better performance than their immutable counterparts? Just curious.