@Override
  public Set<FaultDomain> getAllFaultDomains() {
    return new HashSet<>(hostToRackMap.values());
  }
This method returns all the last cached rack values in a cluster, for all hosts that are healthy, up and running.
@return a set of {@link FaultDomain}s
samza-yarn3/src/main/java/org/apache/samza/job/yarn/YarnFaultDomainManager.java