public synchronized ProcessorTopology buildSubtopology(final int topicGroupId) {
        final Set<String> nodeGroup = nodeGroups().get(topicGroupId);
        return build(nodeGroup);
    }
@param topicGroupId group of topics corresponding to a single subtopology
@return subset of the full topology
streams/src/main/java/org/apache/kafka/streams/processor/internals/InternalTopologyBuilder.java