Current stateDiscarded (duplicated by KIP-160)

Discussion thread:

JIRA: here

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


As for now, KStream#print leads to a predefined output where key and value are printed with comma separation:

"[" + this.streamName + "]: " + keyToPrint + " , " + valueToPrint

We therefore suggest to extend print in a way that it takes KeyValueMapper as a parameter, to let the user modify the output according to its needs. If a mapper is present, the hard-coded key-value output can be replaced with:

"[" + this.streamName + "]: " + mapper.apply(keyToPrint, valueToPrint)

Public Interfaces

The affected interface is KStream, which needs to be extended with another overloaded version of print:

void print(final Serde<K> keySerde,
           final Serde<V> valSerde,
           final String streamName,
           final KeyValueMapper<K, V, String> mapper);

Proposed Changes

Apart from the extension of the interface, the previous implementation of print will be call a new overloaded version thereof. In case no KeyValuemapper is set, null serves as the indicator to use the default output:


public void print(Serde<K> keySerde, Serde<V> valSerde, String streamName) {
    print(keySerde, valSerde, streamName, null);

public void print(final Serde<K> keySerde, final Serde<V> valSerde, String streamName, final KeyValueMapper<K, V, String> mapper){


Compatibility, Deprecation, and Migration Plan

The extension of print will not introduce compatibility issues. Further more, we can maintain the current output by keeping the current output format as a default (if mapper was not set):

if(mapper == null) {
    printStream.println("[" + streamName + "]: " + keyToPrint + " , " + valueToPrint);
} else {
    printStream.println("[" + streamName + "]: " + mapper.apply(keyToPrint, valueToPrint));



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