John Roesler created KAFKA-8410:
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Summary: Strengthen the types of Processors, at least in the DSL,
maybe in the PAPI as well
Key: KAFKA-8410
URL: https://issues.apache.org/jira/browse/KAFKA-8410
Project: Kafka
Issue Type: Improvement
Components: streams
Reporter: John Roesler
Presently, it's very difficult to have confidence when adding to or modifying
processors in the DSL. There's a lot of raw types, duck-typing, and casting
that contribute to this problem.
The root, though, is that the generic types on `Processor<K,V>` refer only to
the _input_ key and value types. No information is captured or verified about
what the _output_ types of a processor are. For example, this leads to
widespread confusion in the code base about whether a processor produces `V`s
or `Change<V>`s. The type system actually makes matters worse, since we use
casts to make the processors conform to declared types that are in fact wrong,
but are never checked due to erasure.
We can start to make some headway on this tech debt by adding some types to the
ProcessorContext that bound the `<K,V>` that may be passed to
`context.forward`. Then, we can build on this by fully specifying the input and
output types of the Processors, which in turn would let us eliminate the
majority of unchecked casts in the DSL operators.
I'm not sure whether adding these generic types to the existing
ProcessorContext and Processor interfaces, which would also affect the PAPI has
any utility, or whether we should make this purely an internal change by
introducing GenericProcessorContext and GenericProcessor peer interfaces for
the DSL to use.
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