@Override
  public Cluster getClusterById(long id) throws AmbariException {
    ConcurrentHashMap<Long, Cluster> clustersById = getClustersById();
    Cluster cluster = clustersById.get(id);
    if (null == cluster) {
      throw new ClusterNotFoundException("clusterID=" + id);
    }

    return clustersById.get(id);
  }
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