Set<String> getAllPartitions() {
    Set<String> partitions = null;
    for (MockHelixAdmin helixAdmin : helixAdmins.values()) {
      if (partitions == null) {
        partitions = new HashSet<>(helixAdmin.getPartitions());
      } else {
        partitions.retainAll(helixAdmin.getPartitions());
      }
    }
    return partitions;
  }
@return the set of all partitions in this cluster.
ambry-clustermap/src/test/java/com/github/ambry/clustermap/MockHelixCluster.java