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ros2_qos_examples (C++ + Python)

Runnable demos for docs/ros2_qos.md. Source lives in the notes repo (robotic_notes/ros2/); the colcon workspace at ~/ros2_ws/src/ holds relative symlinks to it.

Layout

~/workspace/robotic_notes/ros2/ros2_qos_examples_cpp/   # source (this dir)
~/workspace/robotic_notes/ros2/ros2_qos_examples_py/
~/ros2_ws/src/ros2_qos_examples_cpp -> ../../workspace/robotic_notes/ros2/ros2_qos_examples_cpp
~/ros2_ws/src/ros2_qos_examples_py  -> ../../workspace/robotic_notes/ros2/ros2_qos_examples_py

The relative symlinks resolve to the same place inside the container as long as the notes repo is mounted at /root/workspace/robotic_notes.

Setup — one-time

Recreate the persistent dev container with the additional mount:

docker rm -f ros2 2>/dev/null

docker run -d --name ros2 \
  --network host --ipc host \
  -e DISPLAY=$DISPLAY \
  -v /tmp/.X11-unix:/tmp/.X11-unix:rw \
  -v $HOME/ros2_ws:/root/ros2_ws \
  -v $HOME/workspace/robotic_notes:/root/workspace/robotic_notes \
  -v $HOME/.ros:/root/.ros \
  --device /dev/dri:/dev/dri \
  osrf/ros:humble-desktop-full \
  sleep infinity

Build:

docker exec -it ros2 bash
# inside the container:
source /opt/ros/humble/setup.bash
cd /root/ros2_ws
rosdep install --from-paths src --ignore-src -y    # first time only
colcon build --packages-select ros2_qos_examples_cpp ros2_qos_examples_py --symlink-install
source install/setup.bash

For new shells, source both:

source /opt/ros/humble/setup.bash
source /root/ros2_ws/install/setup.bash

(Bake those two lines into ~/.bashrc inside the container — see docs/ros2.md §6 for the Dockerfile pattern.)

Run

Each demo wants two shells. The shorthand below assumes you've sourced both setup files in each new docker exec -it ros2 bash.

1. Slow consumer drops intermediate frames (the canonical pattern)

# Shell A — 60 Hz publisher with SensorDataQoS
ros2 run ros2_qos_examples_cpp camera_publisher

# Shell B — 50 ms/frame consumer; observe non-monotonic markers
ros2 run ros2_qos_examples_cpp latest_frame_subscriber

The marker= value (frame counter mod 256) jumps as intermediate frames are dropped at the queue. age= stays bounded — that's QoS doing its job.

2. Threaded latest-frame buffer (production pattern)

# Shell A
ros2 run ros2_qos_examples_cpp camera_publisher

# Shell B — ROS callback only swaps a pointer; worker thread does the work
ros2 run ros2_qos_examples_cpp latest_frame_buffer

Same drop behavior, but the executor thread is never blocked by processing time. Scales when you have multiple consumers or expensive per-frame work.

3. Custom QoS (RELIABLE + TRANSIENT_LOCAL)

# Shell A — publish at 1 Hz, history of 10 with transient-local durability
ros2 run ros2_qos_examples_cpp custom_qos_publisher

# Shell B — late join; receives the last 10 snapshots immediately
ros2 topic echo --qos-reliability reliable --qos-durability transient_local /config/snapshot

4. Python equivalents

ros2 run ros2_qos_examples_py camera_publisher
ros2 run ros2_qos_examples_py latest_frame_subscriber

Changing QoS

See docs/ros2_qos.md §8 for the three approaches (edit + rebuild, runtime parameters, QoS overrides) and the runtime inspection commands.

The QoS line in each demo is a single statement near the top of the constructor — easy to edit:

// camera_publisher.cpp
pub_ = create_publisher<sensor_msgs::msg::Image>(
  "camera/image", rclcpp::SensorDataQoS());   // ← change me

After editing:

colcon build --packages-select ros2_qos_examples_cpp --symlink-install
source install/setup.bash