@Override
  public Set<Cluster> getClustersForHost(String hostname)
      throws AmbariException {
    Set<Cluster> clusters = getHostClustersMap().get(hostname);
    if(clusters == null){
      throw new HostNotFoundException(hostname);
    }
    if (LOG.isDebugEnabled()) {
      LOG.debug("Looking up clusters for hostname, hostname={}, mappedClusters={}", hostname, clusters.size());
    }
    return Collections.unmodifiableSet(clusters);

  }
Initializes all of the in-memory state collections that this class
unfortunately uses.
<p>
This method should be called only once, when the data is first needed.
<p/>
Because some of these stateful initializations may actually reference this
{@link Clusters} instance, we must do this after the object has been
instantiated and injected.
ambari-server/src/main/java/org/apache/ambari/server/state/cluster/ClustersImpl.java