ROS 2

Notes on ROS 2 (Robot Operating System 2): the core concepts, the build and launch tooling, and the workflow for building and running robot software.

Author

Benedict Thekkel

What ROS 2 Is

ROS 2 is not an operating system. It is a set of libraries and conventions for writing robot software as many small processes that talk to each other over a message bus. A robot becomes a graph of nodes, each one doing a single job (read the lidar, plan a path, drive the wheels), connected by topics, services and actions.

The important differences from ROS 1: there is no roscore (discovery is peer-to-peer via DDS), the transport is DDS rather than a custom protocol, real-time and multi-robot cases are first-class, and the build tool is colcon with ament packages rather than catkin.

This site covers the parts worth writing down: the concepts that do not fit in a tutorial, the commands actually used day to day, and the failure modes that are hard to search for because the symptom is silence rather than an error.

Which Distribution

A ROS 2 distribution is pinned to one Ubuntu release, so the choice is mostly decided by what is on the robot.

Distribution	Ubuntu	Support
Humble Hawksbill	22.04	LTS, 5 years (to May 2027)
Jazzy Jalisco	24.04	LTS, 5 years (to May 2029)
Kilted Kaiju	24.04	non-LTS, about 18 months
Rolling Ridley	latest	rolling development, no stability promise

Everything here targets Jazzy Jalisco on Ubuntu 24.04, with Kilted and Rolling differences called out where they matter. Prefer an LTS: non-LTS releases get about 18 months, which is shorter than most robot projects. Mixing distributions on one network works in principle (the wire protocol is DDS) but message definitions drift between releases, so treat it as something to avoid rather than rely on.

Core Concepts

The layer everything else assumes: the process model, the three communication patterns, the type system, and how callbacks actually get run.

Notebook	What it covers
01_Core_Concepts/00_Nodes_and_Topics.ipynb	What a node is, topics as a streaming primitive, name resolution and namespaces, inspecting a live graph, and what changed from ROS 1
01_Core_Concepts/01_Services_and_Actions.ipynb	Choosing between the three patterns, service servers and clients, the callback deadlock, action servers with feedback and cancellation, goal states and preemption
01_Core_Concepts/02_Interfaces_and_Parameters.ipynb	The standard message packages, defining custom msg/srv/action, parameter declaration and validation callbacks, YAML parameter files and the node-name mismatch trap
01_Core_Concepts/03_QoS_Profiles.ipynb	Reliability, durability, history, the built-in profiles, and the incompatibility table for the case where two endpoints exist and no messages flow
01_Core_Concepts/04_Executors_Lifecycle_and_Composition.ipynb	Single vs multi-threaded executors, callback groups and the deadlock fix, lifecycle node states, composition and intra-process zero copy

Build System and Tooling

Getting from an empty machine to a built, launched and introspected workspace.

Notebook	What it covers
02_Build_and_Tooling/00_Workspaces_and_Packages.ipynb	Installing ROS 2 from apt, workspaces and overlay precedence, ament_python and ament_cmake manifests, colcon invocations worth knowing, rosdep
02_Build_and_Tooling/01_Launch.ipynb	Python launch files, substitutions and the two-stage evaluation, conditions, includes and namespace groups, remapping across a bringup, composable node containers
02_Build_and_Tooling/02_CLI_and_Introspection.ipynb	The `ros2` subcommand reference, RViz2 and rqt, `ros2 doctor`, rosbag2 recording and replay, the logging system and its throttles
02_Build_and_Tooling/03_Client_Libraries.ipynb	The rclpy / rclcpp / rcl / rmw / DDS layering and which layer owns which symptom, what each client library is good for, swapping the middleware

Spatial and Temporal Foundations

Where things are and when they were measured. The transform maths, the kinematics and the synchronisation policy in these four run for real, with numpy and scipy rather than a ROS install.

Notebook	What it covers
03_Spatial_and_Temporal/00_tf2.ipynb	The transform tree and the map/odom contract, broadcasters and listeners, transform composition and inversion worked through numerically, time-indexed lookups, and a table of tree failures by symptom
03_Spatial_and_Temporal/01_Robot_Description.ipynb	URDF links and joints, forward kinematics computed from a parsed URDF and checked against a closed form, robot_state_publisher and what publishes /joint_states, xacro macros, and where SDF fits
03_Spatial_and_Temporal/02_Conventions_and_Time.ipynb	REP-103 units and axes, ENU/NED conversion and the heading sign trap, REP-105 frame guarantees, wall against simulated time, integer nanosecond stamps, and what clock skew does to a lookup
03_Spatial_and_Temporal/03_Message_Synchronisation.ipynb	Why two sensor streams never align, ExactTime against ApproximateTime, the slop trade-off measured on synthetic streams, how to pick slop from robot speed, and diagnosing a silent synchroniser

Perception

Images and point clouds as ROS 2 carries them, and what it takes to put a model in the graph.

Notebook	What it covers
05_Perception/00_Images_and_Calibration.ipynb	image_transport and the compressed transports, cv_bridge encodings and the depth-unit trap, what CameraInfo holds, pinhole projection and rectification computed with numpy and OpenCV, and a symptom table for wrong geometry
05_Perception/01_Point_Clouds_and_Lidar.ipynb	PointCloud2 as a byte buffer decoded with numpy structured dtypes, why field offsets are not optional, pointcloud_to_laserscan reimplemented and plotted, what the 2D reduction throws away, and organised clouds from depth images
05_Perception/02_Inference_Node_Integration.ipynb	The QoS profile that stops a slow model queueing, keeping inference off the executor, choosing between ONNX Runtime, TensorRT and OpenVINO, which vision messages to publish, and budgeting on a small machine

Simulation and Hardware

Where ROS 2 meets a simulator and where it meets a motor. The ros2_control boundary is what lets one set of controllers drive both.

Notebook	What it covers
04_Simulation_and_Hardware/00_Gazebo_and_Bridges.ipynb	Which Gazebo is which (Classic is end of life), `gz` basics, ros_gz_bridge direction syntax and config, bridging the clock first, spawning from one URDF, sensor plugins and the missing-system trap, authoring worlds
04_Simulation_and_Hardware/01_Other_Simulators_and_Sim_to_Real.ipynb	Webots and Isaac Sim against Gazebo with their real costs, a table for choosing, what transfers from simulation unchanged and what has to be earned on hardware, and the practices that narrow the gap
04_Simulation_and_Hardware/02_ros2_control.ipynb	The control loop and its layers, declaring hardware in the URDF, writing a hardware interface and the real-time rules for read/write, the controller manager lifecycle, the standard controllers, and mock hardware as the fastest test
04_Simulation_and_Hardware/03_Drivers_and_micro_ROS.ipynb	What a driver owes the graph, serial and udev rules, SocketCAN and bus-off, micro-ROS and its agent and when not to use it, and what real-time actually requires

Middleware and DDS

The layer underneath everything, and the one that owns the case of two machines that cannot see each other.

Notebook	What it covers
07_Middleware_DDS/00_Discovery_and_RMW.ipynb	Discovery without a master, the port arithmetic a domain ID implies, discovery-range settings, the RMW implementations and the rule that they must match, interface pinning on a multi-homed machine, the discovery server, and a symptom-to-layer table
07_Middleware_DDS/01_Multi_Machine_and_Zenoh.ipynb	The three settings that must agree across machines, multicast over Wi-Fi and static peers, the bandwidth arithmetic and the measured cost of per-subscriber copies, VPNs and their caveats, and the Zenoh RMW

Navigation and Manipulation

The two big application stacks. The costmap and behaviour-tree notebooks build working miniatures of both mechanisms rather than describing them.

Notebook	What it covers
06_Navigation_and_Manipulation/00_SLAM_and_Localization.ipynb	The map/odom contract and who may publish it, slam_toolbox modes and the save-versus-serialize trap, AMCL tuning and why it needs an initial pose, robot_localization EKF configuration, and RGB-D SLAM lessons from real hardware
06_Navigation_and_Manipulation/01_Nav2_Bringup.ipynb	What the stack is made of and the request path for one goal, the lifecycle manager and its bond timeout, how the parameter file is keyed, sending goals, and a bring-up symptom table
06_Navigation_and_Manipulation/02_Costmaps_Planners_and_Controllers.ipynb	Costmap layers and why the local one lives in odom, the inflation cost model built and plotted, A* through a doorway, the radius that decides passability against the scaling factor that only shapes preference, and pure pursuit executed and measured
06_Navigation_and_Manipulation/03_Behaviour_Trees_and_Recovery.ipynb	Why a tree rather than a state machine, Sequence/Fallback/decorator semantics implemented and ticked, Nav2’s default tree and its three custom node types, the recovery behaviours and their real risks, waypoint following
06_Navigation_and_Manipulation/04_MoveIt2_Overview.ipynb	The move_group architecture, the planning scene and collision objects, attached objects and the octomap, the SRDF and its machine-generated collision list, what the Setup Assistant produces, and where a plan fails to reach hardware
06_Navigation_and_Manipulation/05_Kinematics_and_Motion_Planning.ipynb	Why IK is hard, the solvers and when to swap KDL, OMPL against Pilz and what sampling costs you, servo and its singularity thresholds, trajectory execution tolerances, and pick and place as an explicit sequence

Not Covered Yet

The tree has two more sections planned, tracked in issue #16 on the superproject. In the order they are expected to land:

08_Testing_Deployment_Ops/ - pytest and gtest, launch_testing, the ament linters, CI, containers and ARM cross-compilation, systemd and robot_upstart, diagnostics, SROS2, and fleet tooling.
09_Ecosystem_and_Process/ - the distribution and release cycle in full, REPs and design documents, rosdep internals, package release with bloom, and ROS 1 to ROS 2 migration.

The written sections already point forward to these, as backticked paths such as 07_Middleware_DDS/00_Discovery_and_RMW.ipynb rather than as links, so no page on this site links to a page that does not exist. Each becomes a live link as its section lands, as the Spatial and Perception references did.

Nothing here is private, so there are no p_ notebooks.