Minor Description
scala.KafkaStreams#process currently expects a ()=>Processor, which is semantically equivalent to a ProcessorSupplier, but it's the only such method to do so.
All the similar methods in the Scala DSL take a Supplier like their Java counterparts.
Note that on Scala 2.12+, SAM conversion allows callers to pass either a function or a supplier when the parameter is a ProcessorSupplier. (But if the parameter is a function, you must pass a function)
But on scala 2.11-, callers will have to pass a function if the parameter is a function and a supplier if the parameter is a supplier. This means that currently, 2.11 users are confronted with an api that demands they construct suppliers for all the methods except process, which demands a function.
Mitigating factor: we have some implicits available to convert a Function0 to a supplier, and we could add an implicit from ProcessorSupplier to Function0 to smooth over the API.
What to do about it?
We could just change the existing method to take a ProcessorSupplier instead.
- 2.12+ users would not notice a difference during compilation, as SAM conversion would kick in. However, if they just swap in the new jar without recompiling, I think they'd get a MethodDefNotFound error.
- 2.11- users would not be able to compile their existing code. They'd have to swap their function out for a ProcessorSupplier or pull the implicit conversion into scope.
- Note that we can delay this action until we drop 2.11 support, and we would break no one.
We could deprecate the existing method and add a new one taking a ProcessorSupplier.
- All scala users would be able to compile their existing code and also swap in the new version at runtime.
- Anyone explicitly passing a function would get a deprecation warning, though, regardless of SAM conversion or implicit conversion, since neither conversion won't kick in if there's actually a method overload expecting a function. This would drive everyone to explicitly create a supplier (unnecessarily)
We could leave the existing method without deprecating it and add a new one taking a ProcessorSupplier.
- All scala users would be able to compile their existing code and also swap in the new version at runtime.
- There would be no unfortunate deprecation warnings.
- The interface would list two process methods, which is untidy.
- Once we drop 2.11 support, we would just drop the function variant.