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Thoughts on Ignition Gazebo Simulator and ROS2 bridge

First, excellent work from the Ignition Dev Team, and congrats on the new release of Citadel!

Reading through the feature comparison between this and Gazebo-classic version 11 and the status of their migration to Ignition Citadel, I noticed the new approach for ROS integration:

ROS integration with Ignition will be done primarily via a transport bridge instead of plugins, contained in the ros_ign package.
https://ignitionrobotics.org/docs/citadel/comparison#ros-integration

ros_ign_bridge provides a network bridge which enables the exchange of messages between ROS 2 and Ignition Transport. Its support is limited to only certain message types.
https://ignitionrobotics.org/docs/citadel/ros2_integration

As opposed to using the ROS client API directly within system level plugins for simulating sensors and actuators, as with gazebo_ros_pkgs/gazebo_plugins in Gazebo-classic, it looks like the integration approach has shifted: using a network bridging process to exchange bidirectional message traffic between ROS and Ignition Transport.

Correct me if I’m mistaken, but wouldn’t this approach in bridging Ignition Transport with ROS2 add a significant overhead in comparison to the classic gazebo ROS2 plugins? I imagine the additional memory copies and re-serialization between Google Protocol buffers and ZeroMQ to IDL and RMW will introduce additional latencies and QoS complications.

As I understand, the merits behind the new approach include:

  • ROS-agnostic gazebo-plugins
    • decoupling ROS and Ignition releases and runtimes
  • Simpler server-client network topology with Ignition Transport
    • Ignition server and ROS bridge can easily run from separate hosts

Indeed, networking bridging is a common design pattern for ROS integration in other robotic simulators:

However, in my recent findings with AutoWare.Auto, it seems these network based bridges eventually become quite the bottleneck for simulation pipelines, given the ever increasing scale and fidelity of environmental sensing brought to bear in robotics. AutoWare.Auto is intent on upgrading it’s integration with LGSVL, replacing the current ros2-lgsvl-bridge with native plugins using ros2_dotnet client library, enabling more efficient and low latency messaging between ROS2 and the unity game engine:

Though Ignition Gazebo is perhaps a more general robotic simulation framework then those cited above, I suspect the robotic platforms, environments, and applications users will seek to emulate in Ignition Gazebo will not be so much different or less demanding, necessitating similar native optimizations and efficient intra process communication. High resolution depth sensors, feedback control loops, multi camera or multi robot setups, these are common cases that demand high bandwidth and low latency connections; slowing down the real time factor to avoid missed deadlines or dropped messages would be suboptimal and impair scalability.

Thinking out loud: would it be possible to optionally compose ros_ign_bridge within the Ignition Gazebo server at runtime? Would sharing the same process help to bypass re-serlizationing of message structs, while maximizing the use of shared memory transport and QoS options from available RMWs? In the current implementation, I’m guessing the protobuf buffers are encoded before being sent through the zeromq layer?

A last thought would be about porting over classic Gazebo ROS plugins to Ignition that may have served to emulate hardware driver interfaces, such as services for controlling sensor scan patterns, camera exposure parameters, or motor control gains. If the ros_ign_bridge does not support ROS services or RPC or more exotic message types, e.g. radar, ultrasonic, etc, is the user encouraged to use the Ignition plugin API with rclcpp directly to emulate such interfaces? Or is the emulation of the interface logic to be bifurcated between a separate Ignition plugin and accompanied ros node?

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Hi @ruffsl, thank you for the questions.

That’s correct. As the migration guide says, the integration is done primarily through the bridge. This doesn’t mean that it isn’t possible or recommended to write ROS-enabled Ignition plugins though. It just means that we expect everyone to default to the bridge whenever possible, because that should be good enough for most cases. But we do expect users to write ROS-Ignition plugins if that fits their use case best.

That’s true. But the expectation is that the extra overhead for most of the use cases, which involve low-frequency data, should be negligible.

For example, I believe the benefit from not having to maintain two separate diff-drive plugins like we have to on Gazebo-classic (gazebo::DiffDrivePlugin / gazebo_plugins::GazeboRosDiffDrive) outweighs the overhead of converting twist commands. There’s a balance to be stricken between maintenability and runtime performance :slight_smile:

I agree that the bridge is not a good solution for these cases, although it currently works.

A while back we did some experiments with a package that publishes point clouds directly to ROS: ros_ign_point_cloud, but it stopped working a while ago and we never went back to fix it.

The idea is to eventually pick that up again. I’d also expect similar functionality to be added for images and any other large / high-frequency data. These aren’t planned for our short-term roadmap but we do take contributions :innocent:

I think that should still require the conversion from Ignition Transport to ROS, which you may still want to avoid.

The approach I’d try before that would be making the pure-Ignition plugins virtual and extending them with a thin ROS layer. This is the approach taken by some gazebo_plugins, such as gazebo_ros_harness.

One thing I’ve observed over the years though is that the “thin” ROS layer starts to receive features of its own and soon enough we’re maintaining 2 different plugins.

Anecdote time: It often happens that an external contributor opens a PR to gazebo_ros_pkgs adding a feature that should really go into Gazebo. The maintainers propose this to them, and they lose interest. Understandably so, since that’s a whole other codebase with a different workflow and contribution process. The hope as we move to Ignition is to make the experience better for both contributors and maintainers.

That’s to say that I think this could be a good approach for whenever the bridge really isn’t a good solution.

Definitely. A small example of that is the create node on the ros_ign_gazebo package, used to spawn entities. It uses rclcpp to access ROS params and ign-transport to call a service on the simulation side. This wouldn’t be possible using the bridge.

For an example using the ign-gazebo and ROS APIs together, refer to the ros_ign_point_cloud package mentioned above.

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