public Map<String, List<String>> nodeToSourceTopics() {
        return Collections.unmodifiableMap(nodeToSourceTopics);
    }
Returns the map of topic groups keyed by the group id.
A topic group is a group of topics in the same task.

@return groups of topic names
streams/src/main/java/org/apache/kafka/streams/processor/internals/InternalTopologyBuilder.java