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