private void queueHostTasks(ClusterTopology topology, HostOfferResponse response, String hostName) {
    LOG.info("TopologyManager.processAcceptedHostOffer: queueing tasks for host = {}", hostName);
    ExecutorService executorService = getOrCreateTopologyTaskExecutor(topology.getClusterId());
    response.executeTasks(executorService, hostName, topology, ambariContext);
  }
Gets a map of components keyed by host which have operations in the
{@link HostRoleStatus#PENDING} state. This could either be because hosts
have not registered or becuase the operations are actually waiting to be
queued.

@return a mapping of host with pending components.
ambari-server/src/main/java/org/apache/ambari/server/topology/TopologyManager.java