KAFKA-3153: KStream,Type and Serialization

[ymatsuda]

@guozhangwang

[guozhangwang]

@ymatsuda What is yet to be finished?

[guozhangwang]

Can we just use java.lang.reflect.ParameterizedType?

[ymatsuda]

No. ParameterizedType is an interface, and its method are not appropriate for our use.
We want a type representation that every type variable is resolved. It is better to have our own class to represent such a type.

[guozhangwang]

In the final state, we can consider:

1. Remove Default Serde from configs.
2. Add default serdes for primitives / and Java collections types.

[ymatsuda]

• pulled recent changes in from trunk.
• removed serdes from kstream.aggregateByKey()

@guozhangwang

[ymatsuda]

• added type inference for stateless transformation

[ymatsuda]

• added type inference to stateful transformation (transform() and transformValue())

[aviflax]

Just curious, why was this closed? Was it merged elsewhere? Or has this been canceled?

[ymatsuda]

It is canceled for now.
It turned out that it has an issue with lambda expressions of current Java8. Java8 compiler does not capture the type information of the return type. Thus, we cannot infer the type when a user use lambda expression.
And there are people who are concerned about this type inference approach although they didn't publicly comment their opinions here. Basically their concern is that type inference is a difficult concept for users to understand especially when it does not always work.

[aviflax]

@ymatsuda that makes sense, thanks for explaining!