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  • KIP-149: Enabling key access in ValueTransformer, ValueMapper, and ValueJoiner
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Current state"accepted"

Discussion threadHERE

JIRAKAFKA-4218, KAFKA-4726, KAFKA-3745

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

The PR can be found here.


(taken from JIRA descriptions) 


  • Key access to ValueTransformer:  While transforming values via KStream.transformValues and ValueTransformer, the key associated with the value may be needed, even if it is not changed. For instance, it may be used to access stores.

    As of now, the key is not available within these methods and interfaces, leading to the use of KStream.transform and Transformer, and the unnecessary creation of new KeyValue objects.

  • Key access to ValueMapperValueMapper should have read-only access to the key for the value it is mapping. Sometimes the value transformation will depend on the key.

    It is possible to do this with a full blown KeyValueMapper but that loses the promise that you won't change the key – so you might introduce a re-keying phase that is totally unnecessary.

  • Key access to ValueJoiner interface: In working with Kafka Stream joining, it's sometimes the case that a join key is not actually present in the values of the joins themselves (if, for example, a previous transform generated an ephemeral join key.) In such cases, the actual key of the join is not available in the ValueJoiner implementation to be used to construct the final joined value. This can be worked around by explicitly threading the join key into the value if needed, but it seems like extending the interface to pass the join key along as well would be helpful

Additionally we consider adding key access to Initializer and Reducer interfaces.

Public Changes

  • KStream interface:


    • KGroupedStream interface:



Proposed Changes


  • Handling lambdas

For  ValueMapper, ValueJoiner and their "withKey"  interfaces we support lambdas. For ValueTransformer interface we don't need lambdas by the core definition of the class. 

To support lambdas, we separate withKey interface from original ones, meaning we don't inherit or extend from one to another. 


  • ValueMapperWithKey



  • ValueJoinerWithKey


  • ValueTransformerWithKeySupplier



  • Handling withKey interfaces while building the topology


In general, we change the constructors of all related backend Processors to be withKey types as we can easily convert regular (withoutKey) interfaces to withKey interfaces.


  • ValueMapperWithKey


  • ValueJoinerWithKey



Test Plan

The unit tests are changed accordingly to support the changes in core classes.


Rejected Alternatives


  • Lambdas are not supported

This document is proposed with ValueMapper example but it can be applied to other interfaces as well. Rich functions are proposed:
public interface RichFunction {
void init(final ProcessorContext context);

void close();
public abstract class AbstractRichFunction implements RichFunction {
  public void init(final ProcessorContext context) {}

  public void close() {}


public abstract class RichValueJoiner<K, V1, V2, VR> extends AbstractRichFunction implements ValueJoiner<V1, V2, VR>  {
  public final VR apply(final V1 value1, final V2 value2) {
return apply(null, value1, value2);

public abstract VR apply(final K key, final V1 value1, final V2 value2);


Inside processor, we check if the instance (for example ValueMapper instance) is rich (for example RichValueMapper):

KStreamFlatMapValues(ValueMapper<? super V, ? extends Iterable<? extends V1>> mapper) {
this.mapper = mapper;
  isRichFunction = mapper instanceof RichValueMapper ? true : false;
public void process(K key, V value) {
Iterable<? extends V1> newValues;
if (isRichFunction) {
newValues = ((RichValueMapper<? super K, ? super V, ? extends Iterable<? extends V1>>) mapper).apply(key, value);
  } else {
newValues = mapper.apply(value);
for (V1 v : newValues) {
context().forward(key, v);


  • Not backward-compatible


We propose adding key information for ValueJoiner,  ValueTransformer, and ValueMapper classes and their apply(...) methods.

As a result, we perform the following public changes (and their overloaded versions)

KStream<VR> KStream<K, VR> mapValues(ValueMapper<? super V, ? extends VR> mapper); <VR> KStream<K, VR> mapValues(ValueMapper<? super K, ? super V, ? extends VR> mapper);
KStream<VR> KStream<K, VR> transformValues(final ValueTransformerSupplier<? super V, ? extends VR> valueTransformerSupplier, final String... stateStoreNames);<VR> KStream<K, VR> transformValues(final ValueTransformerSupplier<? super K, ? super V, ? extends VR> valueTransformerSupplier,final String... stateStoreNames);

<VO, VR> KStream<K, VR> join(final KStream<K, VO> otherStream,

 final ValueJoiner<? super V, ? super VO, ? extends VR> joiner,

final JoinWindows windows);

<VO, VR> KStream<K, VR> join(final KStream<K, VO> otherStream,

final ValueJoiner<? super K, ? super V, ? super VO, ? extends VR> joiner,

final JoinWindows windows);


<VR> KTable<K, VR> mapValues(final ValueMapper<? super V, ? extends VR> mapper);

<VR> KTable<K, VR> mapValues(final ValueMapper<? super K, ? super V, ? extends VR> mapper);

<VO, VR> KTable<K, VR> join(final KTable<K, VO> other, final ValueJoiner<? super V, ? super VO, ? extends VR> joiner);

<VO, VR> KTable<K, VR> join(final KTable<K, VO> other, final ValueJoiner<? super K, ? super V, ? super VO, ? extends VR> joiner);



  • Lacking performance because deep-copy and need for RichFunctions


  1. We extend the target interfaces ValueJoiner,  ValueTransformer, and ValueMapper as ValueJoinerWithKey,  ValueTransformerWithKey, and ValueMapperWithKey. In extended abstract classes we have an access to keys.
  2. In Processor we check the actual instance of object:
    this.valueTransformer = valueTransformer;
    if (valueTransformer instanceof ValueTransformerWithKey) {
    isTransformerWithKey = true;
    } else {
    isTransformerWithKey = false;



    public void process(K key, V value) {
    if (isTransformerWithKey) {
    K keyCopy = (K) Utils.deepCopy(key);
      context.forward(key, ((ValueTransformerWithKey<K, V, R>) valueTransformer).transform(keyCopy, value));
      } else {
    context.forward(key, valueTransformer.transform(value));

  3. As we can see from the above code snippet, we can guard the key change in Processors by deeply copying the object before calling the apply() method.

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