Map<String, Map<String, Set<String>>> getPartitionToResourceMapByDC() {
    return Collections.unmodifiableMap(partitionToResourceNameByDc);
  }
Exposed for testing
@return a map of partition to its corresponding resource grouped by data center
ambry-clustermap/src/main/java/com/github/ambry/clustermap/HelixClusterManager.java