public KTableKTableJoinNodeBuilder<K, V1, V2, VR> withKeySerde(final Serde<K> keySerde) {
            this.keySerde = keySerde;
            return this;
        }
The supplier which provides processor with KTable-KTable join merge functionality.
streams/src/main/java/org/apache/kafka/streams/kstream/internals/graph/KTableKTableJoinNode.java