Set<String> getDownInstances() {
    Set<String> downInstances = new HashSet<>();
    for (MockHelixAdmin helixAdmin : helixAdmins.values()) {
      downInstances.addAll(helixAdmin.getDownInstances());
    }
    return downInstances;
  }
@return set of all instances that are up in this cluster.
ambry-clustermap/src/test/java/com/github/ambry/clustermap/MockHelixCluster.java