code: ClusterHeartbeat createCluster(String clusterName)
    {
      ClusterHeartbeat cluster = _clusterMap.get(clusterName);
      
      if (cluster == null) {
        cluster = new ClusterHeartbeat(clusterName, this);
        
        _clusterMap.putIfAbsent(clusterName, cluster);
        
        cluster = _clusterMap.get(clusterName);
      }
      
      return cluster;
    }
docs: Returns the cluster with the given name, creating it if necessary.