@Override
  public Cluster getCluster(String clusterName)
      throws AmbariException {
    Cluster cluster = null;
    if (clusterName != null) {
      cluster = getClustersByName().get(clusterName);
    }
    if (null == cluster) {
      throw new ClusterNotFoundException(clusterName);
    }
    RetryHelper.addAffectedCluster(cluster);
    return cluster;
  }
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