Tutorial: use ROS input and output

The ROS adapter connects model variables to ROS 2 topics using checked-in topic and message-type mappings. This is a live workflow: the Trustworthiness Checker waits for ROS messages and publishes results until its foreground process is stopped.

Prerequisites and overlay

Run the setup from the repository root. You need a sourced ROS 2 installation, colcon, the ros2 CLI, and the custom message package in ros_interfaces. Replace <ros-distro> with the ROS 2 distribution installed on the machine:

source /opt/ros/<ros-distro>/setup.bash
cd ros_interfaces
colcon build
source install/setup.bash
cd ..

Build the overlay once unless ros_interfaces/ changes. Source both setup files again in every terminal that runs cargo or ros2.

Add two values received through ROS

The DSRV program adds inputs x and y and exposes the result as z:

in x: Int
in y: Int
out z: Int
z = x + y

The repository stores this program as examples/simple_add.dsrv. Its input mapping binds x to /x and y to /y as std_msgs/msg/Int32; its output mapping binds z to /z with the same message type. From the repository root, start the checker in Terminal 1:

cargo run --features ros -- examples/simple_add.dsrv \
  --input-ros-file examples/ros/simple_add_mapping.json \
  --output-ros-file examples/ros/simple_add_output_mapping.json

In Terminal 2, from the repository root, source ROS and the overlay, then watch z:

source /opt/ros/<ros-distro>/setup.bash
source ros_interfaces/install/setup.bash
ros2 topic echo /z

In Terminal 3, source the same environments and publish one input pair:

source /opt/ros/<ros-distro>/setup.bash
source ros_interfaces/install/setup.bash
ros2 topic pub --once /x std_msgs/msg/Int32 '{data: 1}'
ros2 topic pub --once /y std_msgs/msg/Int32 '{data: 2}'

Terminal 2 should show an Int32 message containing:

data: 3
---

The first publication alone may not yield a visible z value because the other input is missing for that evaluation. The first z message proves that the checker has received enough input, evaluated the model, and published through its ROS output path. Topic discovery or a running checker process alone does not provide a separate Trustworthiness Checker readiness signal.

MSTLO message compatibility

The ordinary DSRV mapping above carries ordinary typed values such as Int32. MSTLO is different: select it with --language mstlo and use the generated MstloTimedValue message, as shown by examples/ros/mstlo_timed_value_mapping.json:

{
  "x": [
    "/signals/x",
    "MstloTimedValue"
  ]
}

MstloTimedValue is not a drop-in replacement for the ordinary DSRV/Value ROS backend. Conversely, an ordinary Int32 mapping is not an MSTLO timed-value mapping. Build and source the overlay before compiling the MSTLO path; do not pass an explicit --runtime with --language mstlo, because MSTLO selects its own runtime.

Stop and cleanup

The checker and ros2 topic echo are long-lived foreground processes. Press Ctrl-C in Terminal 1 and Terminal 2 when finished; the --once publishers in Terminal 3 exit after one message. This walkthrough creates no container or persistent topic data, and the built ROS overlay can be kept for the next run.