apache kafka – KGroupedStream reduce use key in Reducer logic

You could use the aggregate method of the KGroupedStream interface:

KTable aggregate(Initializer initializer, Aggregator super K,? super V,VR> aggregator)

Aggregate the values of records in this stream by the grouped key. Records with null key or value are ignored. Aggregating is a generalization of combining via reduce(...) as it, for example, allows the result to have a different type than the input values.

If you want to output a result for a key only if the value matches, you create a Ktable, something like:

KTable resultTable = inputStream
    .groupByKey()
    .aggregate(
        () -> true,
        (key, value, agg) -> agg && key.equals(value),
        Materialized.with(Serdes.String(), Serdes.Boolean())
    );
 

You can then filter the values in the table, something like:

KStream finalResult = resultTable
    .toStream()
    .filter((key, allEqual) -> allEqual);