Ros2 Engineering Skills

ROS 2 Engineering Skills

A progressive-disclosure skill for ROS 2 development — from first workspace to production fleet deployment. Each section below gives you the essential decision framework; detailed patterns, code templates, and anti-patterns live in the references/ directory. Read the relevant reference file before writing code.

How to use this skill

1. Identify what the user is building (see Decision Router below).
2. Read the matching references/*.md file for detailed guidance.
3. Apply the Core Engineering Principles in every piece of code you generate.
4. When multiple domains intersect (e.g. Nav2 + ros2_control), read both files.

Decision router

When a task spans multiple domains, read all relevant files and reconcile conflicting recommendations by favoring safety, then determinism, then simplicity.

Cross-cutting concern — Security: Security is not isolated to references/security.md. Every domain should consider its security implications: hardware interfaces need safe shutdown on auth failure, DDS topics may need encryption, deployment images need supply chain verification, and fleet communication must use TLS. When reviewing code in any domain, check whether the data path crosses a trust boundary.

Core engineering principles

These apply to every ROS 2 artifact you produce, regardless of domain.

1. Distro awareness

Always ask which ROS 2 distribution the user targets. Key differences:

When the user does not specify, default to the latest LTS (Jazzy). Pin the exact distro in Dockerfile, CI, and documentation so builds are reproducible.

2. C++ vs Python decision

Choose the language based on the node's role, not personal preference.

Use rclcpp (C++) when:

• The node sits in a control loop running ≥100 Hz
• Deterministic memory allocation matters (real-time path)
• The node is a hardware driver or controller plugin
• Intra-process zero-copy communication is required

Use rclpy (Python) when:

• The node is orchestration, monitoring, or parameter management
• Rapid prototyping with frequent iteration
• Heavy use of ML frameworks (PyTorch, TensorFlow) that are Python-native
• The node does not sit in a latency-critical path

Mixed stacks are normal. A typical robot has C++ drivers/controllers and Python orchestration/monitoring. Note: component_container (composition) only loads C++ components via pluginlib. Python nodes run as separate processes, but can share a launch file and communicate via zero-overhead intra-host DDS.

Intra-process communication works for any nodes sharing a process — not only composable components. Any nodes instantiated in the same process with use_intra_process_comms(true) can use zero-copy transfer.

3. Package structure conventions

Every package should follow this layout. Consistency across a workspace reduces onboarding time and makes CI scripts portable.

my_package/
├── CMakeLists.txt          # or setup.py for pure Python
├── package.xml             # format 3, with <depend> tags
├── config/
│   └── params.yaml         # default parameters
├── launch/
│   └── bringup.launch.py   # Python launch file
├── include/my_package/     # C++ public headers (if library)
├── src/                    # C++ source files
├── my_package/             # Python modules (if ament_python or mixed)
├── test/                   # gtest, pytest, launch_testing
├── urdf/                   # URDF/xacro (if applicable)
├── msg/ srv/ action/       # custom interfaces (dedicated _interfaces package preferred)
└── README.md

Separate interface definitions into a *_interfaces package so downstream packages can depend on interfaces without pulling in implementation.

4. Parameter discipline

• Declare every parameter with a type, description, range, and default in the node constructor — never use undeclared parameters.
• Use ParameterDescriptor with FloatingPointRange or IntegerRange for numeric bounds. The parameter server rejects out-of-range values at set time.
• Group related parameters under a namespace prefix: controller.kp, controller.ki, controller.kd.
• Load defaults from a config/params.yaml; allow launch-time overrides.
• For dynamic reconfiguration, register a set_parameters_callback and validate new values atomically before accepting.

5. Error handling philosophy

• Nodes must not silently swallow errors. Log at the appropriate severity, then take a safe action (stop motion, request help, transition to error state).
• Prefer lifecycle node error transitions over ad-hoc boolean flags.
• When calling a service, always handle the "service not available" and "future timed out" cases explicitly.
• For hardware drivers, distinguish transient errors (retry with backoff) from fatal errors (transition to FINALIZED and alert the operator).

6. Quality of Service defaults

Start from these profiles and adjust per use case:

QoS mismatches are the #1 cause of "I published but nobody receives." Always check compatibility with ros2 topic info -v when debugging.

DEADLINE and LIFESPAN are critical for safety-critical systems. DEADLINE fires an event when no message arrives within the specified period (detect stale data). LIFESPAN discards messages older than the specified duration before delivery (prevent acting on stale data). See references/communication.md section 9 for full API and examples.

7. Naming conventions

8. Thread safety and callbacks

• A MutuallyExclusiveCallbackGroup serializes its callbacks — safe for shared state without locks, but limits throughput.
• A ReentrantCallbackGroup allows parallel execution — you must protect shared state with std::mutex (C++) or threading.Lock (Python).
• Calling a service from a callback: The service client must be in a separate MutuallyExclusiveCallbackGroup from the calling callback. Otherwise the executor deadlocks — the callback waits for the response while the executor cannot deliver it. Always use async_send_request with a response callback; never use spin_until_future_complete inside an executor callback.
• Never do blocking work (file I/O, long computation, sleep) inside a timer or subscription callback on the default executor. Offload to a dedicated thread or use a MultiThreadedExecutor with a reentrant group.
• In rclcpp, prefer std::shared_ptr<const MessageT> in subscription callbacks to avoid unnecessary copies and enable zero-copy intra-process.

9. Lifecycle-first design

Default to lifecycle (managed) nodes for anything that owns resources: hardware drivers, sensor pipelines, planners, controllers.

                 ┌──────────────┐
  create() ──►  │  Unconfigured │
                 └──────┬───────┘
            on_configure │
                 ┌──────▼───────┐
                 │   Inactive    │
                 └──────┬───────┘
            on_activate  │
                 ┌──────▼───────┐
                 │    Active     │
                 └──────┬───────┘
           on_deactivate │
                 ┌──────▼───────┐
                 │   Inactive    │
                 └──────┬───────┘
            on_cleanup   │
                 ┌──────▼───────┐
                 │  Unconfigured │
                 └──────┬───────┘
           on_shutdown   │
                 ┌──────▼───────┐
                 │   Finalized   │
                 └───────────────┘

This gives the system manager (launch file, orchestrator, or operator) explicit control over when resources are allocated, when the node starts processing, and how it shuts down. It also makes error recovery predictable.

10. Build and CI hygiene

• Use colcon build --cmake-args -DCMAKE_BUILD_TYPE=RelWithDebInfo for development; Release for deployment.
• Enable -Wall -Wextra -Wpedantic and treat warnings as errors in CI.
• Run colcon test with --event-handlers console_cohesion+ so test output groups by package.
• Pin rosdep keys in rosdep.yaml for reproducible dependency resolution.
• Cache /opt/ros/, .ccache/, and build//install/ in CI to cut build times by 60–80%.

Common anti-patterns

Distro-specific migration notes

When upgrading between distributions, check these breaking changes first:

Foxy → Humble:

• Complete API overhaul. Foxy packages require significant rework.
• ros2_control was not bundled in Foxy — must be built separately.
• Lifecycle node API stabilized in Humble.
• Action server/client API changed significantly.

Humble → Jazzy:

• ros2_control API changed from 2.x to 4.x — export_state_interfaces() and export_command_interfaces() are now auto-generated by the framework. Manual overrides use on_export_state_interfaces(). See references/hardware-interface.md.
• Handle get_value() deprecated → use get_optional<T>() on LoanedStateInterface / LoanedCommandInterface (controller side). Hardware interfaces use set_state() / get_state() / set_command() / get_command() helpers with fully qualified names.
• All joints in <ros2_control> tag must exist in the URDF.
• Controller parameter loading changed — use --param-file with spawner.
• Default bag format changed from sqlite3 to MCAP. Use storage_id='mcap'.
• Default middleware changed internal config paths. Regenerate DDS profiles.
• nav2_params.yaml schema changes — recoveries_server renamed to behavior_server.
• ROS_AUTOMATIC_DISCOVERY_RANGE replaces ROS_LOCALHOST_ONLY (values: LOCALHOST, SUBNET, OFF, SYSTEM_DEFAULT).
• launch_ros actions have new parameter handling — test launch files explicitly.

Jazzy → Kilted (non-LTS):

• Zenoh promoted to Tier 1 middleware — rmw_zenoh is production-ready. Install: sudo apt install ros-kilted-rmw-zenoh-cpp, set RMW_IMPLEMENTATION=rmw_zenoh_cpp. Supports router/peer/client modes.
• EventsExecutor graduated from experimental — available in rclcpp::executors (no experimental namespace). Also ported to rclpy.
• ament_target_dependencies() deprecated — use target_link_libraries() with modern CMake targets (e.g. rclcpp::rclcpp, std_msgs::std_msgs__rosidl_typesupport_cpp).
• Multi-bag replay support in ros2 bag play.
• Gazebo Ionic is the paired simulator (Harmonic was Jazzy; Ionic is the Kilted pairing).

ROS 1 → ROS 2:

• See references/migration-ros1.md for a step-by-step strategy.

Quick reference — ros2 CLI

# Workspace
colcon build --symlink-install --packages-select my_pkg
colcon test --packages-select my_pkg
colcon graph --dot                       # dependency graph (DOT format)
source install/setup.bash

# Introspection
ros2 node list
ros2 topic list -t
ros2 topic info /topic_name -v          # shows QoS details
ros2 topic hz /topic_name
ros2 topic bw /topic_name
ros2 service list -t
ros2 action list -t
ros2 param list /node_name
ros2 param describe /node_name param
ros2 interface show std_msgs/msg/String

# ros2_control
ros2 control list_controllers
ros2 control list_hardware_interfaces
ros2 control list_hardware_components

# Debugging
ros2 doctor --report                    # alias: ros2 wtf
ros2 run tf2_tools view_frames
ros2 bag record -a -o my_bag
ros2 bag info my_bag
ros2 bag play my_bag --clock

# Lifecycle
ros2 lifecycle list /node_name
ros2 lifecycle set /node_name configure
ros2 lifecycle set /node_name activate