ROS2 Architecture

Covers the three current ROS2 distributions; patterns are consistent across all three with version-specific notes called out where they diverge. C++ (rclcpp) and Python (rclpy) treated as equal first-class targets. Development environment via Pixi + RoboStack — the modern, cross-platform alternative to sudo apt install ros-*. See STACK.md for distro details and pinned tool versions.

Python-side rclpy nodes inherit the typing, dataclass, asyncio, and testing rules from python-architect — this skill adds the ROS2-specific layer on top.

0. Distribution selection

• Migrate from Kilted before Dec 2026 — direct target is Lyrical (next LTS) for a 5-year runway.
• Jazzy → Lyrical migration is straightforward; most code ports with minimal changes.
• ros2-architect's patterns apply to all three — the differences are in specific package APIs and tooling. Where this matters, the section calls it out.

1. Workspace layout

A ROS2 workspace is the root of one or more packages built together by colcon. Pixi sits at the workspace root and gives every contributor the same ROS distro, the same compiler, the same Python — on Linux, macOS, or Windows. Full tree in RECIPES § Workspace layout.

• src/ holds packages, never code at the workspace root.
• One package, one responsibility — drivers, navigation, perception, custom messages, bringup. Don't put everything in one my_robot package.
• _msgs packages hold ONLY interfaces (msg/srv/action). Rebuilt rarely, consumed by everyone.
• _bringup package holds launch files and config for assembling the full robot stack. No source code.

2. Pixi for the development environment

Pixi solves the historical ROS2 pain: needing a specific Ubuntu version, conflicting Python installs, broken updates from apt. RoboStack channel on conda-forge distributes ROS distros as conda packages, so Pixi can install ROS2 anywhere conda runs — Linux, macOS, Windows. Skeleton: RECIPES.md § pixi.toml — workspace manifest.

• pixi.lock is committed. Reproducibility is the whole point.
• platforms = [...] declares every OS/arch your team uses. CI uses the same lockfile.
• No sudo apt install on contributor machines. Pixi's .pixi/ directory is isolated; the system stays clean.
• CI uses Pixi too. pixi run build in GitHub Actions = the same env every developer uses.
• colcon build --symlink-install in dev — Python changes hot-reload without rebuilding. Drop the flag for release builds.

3. Package structure — ament_cmake vs ament_python

The build type is declared in package.xml. Use ament_cmake for C++ (and for interface-only _msgs packages); ament_python for pure-Python packages. Skeletons: RECIPES.md § package.xml — ament_cmake and § ament_python.

• format="3" for package.xml. Older formats still work but lack features.
• Every dep declared. Implicit dependencies (std_msgs showing up because something else pulled it) is a latent bug.
• depend vs build_depend / exec_depend: prefer the unified <depend> tag when the dependency is needed at both build and runtime. Split only when they differ.
• Tests declared as test_depend. Linters (ament_lint_*) are part of every package's test suite.

4. Nodes — lifecycle, executors, single-responsibility

A node is the unit of computation. One process can host one or many nodes (rclcpp::executors::MultiThreadedExecutor / rclpy.executors).

• One node, one job. A node that publishes IMU data, listens for joystick input, and runs path planning has three responsibilities. Three nodes.
• Lifecycle nodes for anything with non-trivial init: unconfigured → inactive → active → finalized. Allows the launch system to bring up nodes in the right order, retry init, and gracefully shut down. Use for any node that holds an open connection (camera, motor controller, network socket).
• Standard Node for stateless or trivially-init nodes.
• Single-threaded executor by default; switch to multi-threaded only when callbacks genuinely contend (e.g. a long-running service callback shouldn't block fast topic callbacks). Multi-threaded executors require thread-safe callback groups — easy to get wrong.
• Node names: lowercase snake_case, namespaced by component (/imu_driver, /path_planner).
• No business logic in main.cpp / main.py. Construct the node, spin, shut down. Logic lives in node classes.

5. Topics & QoS — explicit profiles

Every publisher / subscriber declares an explicit QoS profile. Default behavior bites. Profile reference + C++/Python snippets in RECIPES § QoS.

• Topic names use plural / hierarchical paths: /sensors/imu/data, /control/cmd_vel. Namespace by subsystem.
• Publisher and subscriber QoS must match. Mismatched QoS means subscribers silently fail to connect (visible only in ros2 topic info -v).
• Sensor streams = best-effort + KEEP_LAST(N). High-rate, drop the old.
• Low-rate state = reliable + KEEP_LAST(10). /robot/battery, /system/status.
• Reliable + KEEP_ALL on a high-rate topic is a memory leak.

6. Services and actions

• Services for short queries: parameter get/set, mode change, calibration trigger.
• Actions for everything navigation-like, manipulation-like, planning-like.
• Never call a service synchronously from inside a topic callback. That's an executor deadlock. Use async_send_request (C++) / call_async (rclpy) and handle the future.

7. Parameters — declared, typed, validated

• Declare every parameter on node init with declare_parameter(name, default, descriptor). Undeclared parameters fail to read.
• Provide a ParameterDescriptor with type and description. Visible in ros2 param describe.
• Validate on change via add_on_set_parameters_callback — reject bad values; never silently accept and crash later.
• YAML config files in the _bringup package, loaded by launch via Node(parameters=[config_path]). Per-robot, per-environment configs live there.
• Use ranges and choices in the descriptor: IntegerRange, FloatingPointRange, additional_constraints — the parameter system enforces them.

8. Launch files — Python DSL

Launch files describe how to bring up a system: which nodes to run, with which parameters, in which order, on which conditions. Skeleton: RECIPES.md § Launch file — Python DSL.

• Always Python launch (*.launch.py) — XML launch is legacy.
• Launch arguments for environment differences (use_sim, robot_model). One launch file, many configurations.
• Compose smaller launch files with `IncludeLaunchDescription