[jira] [Created] (KAFKA-3519) Refactor Transformer templates to return the same strong-typed value.

["Guozhang Wang (JIRA)" <ji...@apache.org>]

Guozhang Wang created KAFKA-3519:
------------------------------------

             Summary: Refactor Transformer templates to return the same strong-typed value.
                 Key: KAFKA-3519
                 URL: https://issues.apache.org/jira/browse/KAFKA-3519
             Project: Kafka
          Issue Type: Bug
          Components: kafka streams
            Reporter: Guozhang Wang
            Assignee: Guozhang Wang
             Fix For: 0.10.0.0


Currently the Transformer interface's template is:

{code}
R transform(K key, V value);
{code}

While all its usage requires a key-value pair, and hence constructing the object as:

{code}
new Transformer<K, V, KeyValue<K1, V1>>
{code}

Since we expect Transformer to be only used in the Streams DSL, which is supposed to return another key-value pair streams, it's better to define its template just as

{code}
public interface Transformer<K, V, K1, V1> {

    KeyValue<K1, V1> transform(K key, V value);
}
{code}

Also, we can allow punctuate() to also return a nullable KeyValue<K1, V1> pair, and let the implementation to forward to downstream processor only when the returned pair is not null.



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