HostGroupStrategy getHostGroupStrategy() {
    try {
      return injector.getInstance(serverConfig.getAddServiceHostGroupStrategyClass());
    }
    catch (ClassNotFoundException | ClassCastException |ConfigurationException | ProvisionException ex) {
      throw new IllegalStateException("Cannot load host group strategy", ex);
    }
  }
Extracts a [component -> hosts] map from the [service -> component -> hosts] map. Services
with empty component map will be ignored
@param serviceComponentHostMap the input map
@return the extracted map
ambari-server/src/main/java/org/apache/ambari/server/topology/addservice/StackAdvisorAdapter.java