public LogicalRequest getRequest(long requestId) {
    ensureInitialized();
    return allRequests.get(requestId);
  }
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