List<String> getUpInstances(String zkAddr) {
    return helixAdmins.get(zkAddr).getUpInstances();
  }
@param zkAddr the address of the zk service on which this operation is to be done.
@return the set of instances associated with the given zk service that are up.
ambry-clustermap/src/test/java/com/github/ambry/clustermap/MockHelixCluster.java