public void onHostHeartBeatLost(Host host) {
    if (AmbariServer.getController() == null) {
      return;
    }
    ensureInitialized();
    synchronized (availableHosts) {
      LOG.info("Hearbeat for host {} lost thus removing it from available hosts.", host.getHostName());
      availableHosts.remove(host);
    }
  }
Through this method {@see TopologyManager} gets notified when a connection to a host in the cluster is lost.
The passed host will be excluded from scheduling any tasks onto it as it can't be reached.
@param host
ambari-server/src/main/java/org/apache/ambari/server/topology/TopologyManager.java