code: func (r *RollingUpdater) existingController(controller *corev1.ReplicationController) (*corev1.ReplicationController, error) {
  	// without rc name but generate name, there's no existing rc
  	if len(controller.Name) == 0 && len(controller.GenerateName) > 0 {
  		return nil, errors.NewNotFound(corev1.Resource("replicationcontrollers"), controller.Name)
  	}
  	// controller name is required to get rc back
  	return r.rcClient.ReplicationControllers(controller.Namespace).Get(controller.Name, metav1.GetOptions{})
  }
docs: // existingController verifies if the controller already exists