map_builder_server.cc中注册了一大堆回调函数，例如：

server_builder.RegisterHandler<handlers::GetLocalToGlobalTransformHandler>();

// get_local_to_global_transform_handler.cc 文件
void GetLocalToGlobalTransformHandler::OnRequest(
    const proto::GetLocalToGlobalTransformRequest& request)
{
  auto response = absl::make_unique<proto::GetLocalToGlobalTransformResponse>();
  auto local_to_global =
      GetContext<MapBuilderContextInterface>()
          ->map_builder()
          .pose_graph()
          ->GetLocalToGlobalTransform(request.trajectory_id());
  *response->mutable_local_to_global() = transform::ToProto(local_to_global);
  Send(std::move(response));
}

GetLocalToGlobalTransform可用于获取local坐标系的原点

transform::Rigid3d PoseGraph2D::GetLocalToGlobalTransform(
    const int trajectory_id) const
{
  absl::MutexLock locker(&mutex_);
  return ComputeLocalToGlobalTransform(data_.global_submap_poses_2d,
                                       trajectory_id);
}

同样的还有 PoseGraph2D::GetAllSubmapPoses, PoseGraph2D::GetTrajectoryStates, PoseGraph2D::GetTrajectoryNodePoses, PoseGraph2D::GetLandmarkPoses(), PoseGraph2D::constraints()等等

cartographer\cartographer\cloud\internal\handlers目录有一堆文件，每个文件只定义了一个对应的回调函数，这个回调函数只在map_builder_server.cc中调用，最终是运行cartographer_grpc_server