@Override
    @Deprecated
    public <K1, V1> KStream<K1, V1> flatTransform(final TransformerSupplier<? super K, ? super V, Iterable<KeyValue<K1, V1>>> transformerSupplier,
                                                  final String... stateStoreNames) {
        Objects.requireNonNull(transformerSupplier, "transformerSupplier can't be null");
        final String name = builder.newProcessorName(TRANSFORM_NAME);
        return flatTransform(transformerSupplier, Named.as(name), stateStoreNames);
    }
Repartition a stream. This is required on join operations occurring after
an operation that changes the key, i.e, selectKey, map(..), flatMap(..).
streams/src/main/java/org/apache/kafka/streams/kstream/internals/KStreamImpl.java