@Transactional
  protected void persistTopologyHostRegistration(long hostRequestId, final HostImpl host) {
    persistedState.registerHostName(hostRequestId, host.getHostName());
    persistedState.registerInTopologyHostInfo(host);
  }
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