code: private OperationFuture<Group> createServers(GroupHierarchyConfig config, String rootGroupId) {
          List<CreateServerConfig> configs = config.getServerConfigs();
  
          List<JobFuture> futures = configs.stream()
              .map(serverConfig -> serverService().create(serverConfig).jobFuture())
              .collect(toList());
  
          return new OperationFuture<>(Group.refById(rootGroupId), new ParallelJobsFuture(futures));
      }
docs: Create servers based on {@link CreateServerConfig}
  @param config      group hierarchy config
  @param rootGroupId root group id
  @return OperationFuture wrapper for root group