ROS 2 Glossary
A consolidated reference of key terms used throughout Chapter 1. Refer back to this glossary whenever you encounter unfamiliar terminology.
Core Concepts
Node
A single-purpose, modular process that performs a specific task. Nodes communicate with each other using topics, services, and actions. Think of nodes as the "neurons" in the robot's nervous system.
Example: A camera node captures images; a vision node processes them.
Topic
A named channel for asynchronous, publish-subscribe communication. Publishers send messages to topics; subscribers receive them. Topics are ideal for streaming data like sensor readings.
Example: /camera/image, /imu/data, /cmd_vel
Message
A data structure that defines the format of information sent over topics. ROS 2 provides standard messages (e.g., std_msgs/String) and you can create custom messages.
Example: geometry_msgs/Twist contains linear and angular velocity.
Service
A synchronous request-response communication pattern. A client sends a request, waits for the server to process it, and receives a response. Best for one-time operations.
Example: /add_two_ints - client sends two numbers, server returns the sum.
Action
An asynchronous communication pattern for long-running tasks. Actions provide goals, feedback during execution, and results upon completion. Actions can be canceled mid-execution.
Example: Navigate to a waypoint, receiving position updates along the way.
Publisher
A node role that sends messages to a topic. A node can have multiple publishers for different topics.
Subscriber
A node role that receives messages from a topic. A node can subscribe to multiple topics simultaneously.
Client
A node role that sends requests to a service or action server.
Server
A node role that receives requests, processes them, and returns responses (service) or results with feedback (action).
ROS 2 Architecture
DDS (Data Distribution Service)
The underlying communication protocol used by ROS 2. DDS provides reliable, real-time data distribution. You rarely interact with DDS directly, but it enables ROS 2's advanced features.
QoS (Quality of Service)
Configuration settings that control message delivery reliability, history, and durability. QoS profiles include:
- Reliable: Guarantees delivery (may retry)
- Best Effort: Fast delivery, may lose messages
- Transient Local: Late subscribers receive last message
Executor
A ROS 2 component that manages callback execution. Executors determine how and when node callbacks (timers, subscriptions, services) are processed.
Callback
A function that executes in response to an event (message received, timer fired, service called). Callbacks are the primary way nodes respond to the world.
Callback Group
A way to organize callbacks for concurrent or sequential execution. Types include:
- Mutually Exclusive: Only one callback runs at a time
- Reentrant: Multiple callbacks can run simultaneously
Python-ROS 2 (rclpy)
rclpy
The Python client library for ROS 2. rclpy provides classes and functions to create nodes, publishers, subscribers, services, and actions in Python.
rclpy.init()
Initializes the ROS 2 Python client library. Must be called before creating any nodes.
rclpy.spin()
Blocks and processes callbacks for a node. Keeps the node running until interrupted.
rclpy.shutdown()
Cleanly shuts down the ROS 2 Python client library.
Node (Python class)
The base class for all ROS 2 Python nodes. Your custom nodes inherit from rclpy.node.Node.
Robot Description
URDF (Unified Robot Description Format)
An XML format for describing a robot's physical structure. URDF defines links (body parts), joints (connections), visual appearance, and collision geometry.
Link
A rigid body in a URDF model. Links have visual geometry (what you see) and collision geometry (for physics simulation). Examples: base_link, torso, head, arm_link.
Joint
A connection between two links in URDF. Joint types include:
- Fixed: No movement
- Revolute: Rotation with limits
- Continuous: Unlimited rotation
- Prismatic: Linear sliding
XACRO
A macro language for URDF that reduces repetition. XACRO files generate URDF files. Used for complex robots with repeated structures (e.g., left/right arms).
TF2 (Transform Library)
A ROS 2 library for tracking coordinate frames over time. TF2 maintains the relationships between robot parts and the world.
Frame
A coordinate system in 3D space. Robots have many frames: base_link, camera_frame, world, etc. TF2 transforms points between frames.
Sensors
IMU (Inertial Measurement Unit)
A sensor that measures acceleration and angular velocity. Used for orientation, balance, and motion detection. Contains accelerometers and gyroscopes.
LIDAR (Light Detection and Ranging)
A sensor that measures distances using laser pulses. Produces point clouds or 2D scans. Used for mapping, obstacle detection, and navigation.
Depth Camera
A camera that captures both color (RGB) and depth information. Examples: Intel RealSense, Microsoft Azure Kinect. Used for 3D perception and object recognition.
Force/Torque Sensor
Measures forces and torques applied to a surface. Used in robot hands and feet for grip control and balance.
Encoder
Measures rotation of a motor or wheel. Provides position and velocity feedback for motor control.
Simulation
Gazebo
A physics simulation environment for robots. Gazebo simulates sensors, actuators, and world physics. ROS 2 integrates tightly with Gazebo.
RViz2
A 3D visualization tool for ROS 2. Displays sensor data, robot models, paths, and markers. Essential for debugging and monitoring.
SDF (Simulation Description Format)
A format for describing simulated worlds and objects in Gazebo. More expressive than URDF for simulation-specific features.
Development Tools
colcon
The build tool for ROS 2 workspaces. colcon build compiles packages; colcon test runs tests.
Package
A unit of ROS 2 software. Packages contain nodes, libraries, configuration, and launch files. Packages declare dependencies and can be shared.
Workspace
A directory containing one or more ROS 2 packages. You source a workspace's setup file to use its packages.
Launch File
A Python or XML file that starts multiple nodes with configuration. Launch files orchestrate complex robot systems.
Bag (rosbag2)
A file format for recording and replaying ROS 2 messages. Bags are essential for debugging, testing, and data collection.
Common Abbreviations
| Abbreviation | Full Name |
|---|---|
| ROS | Robot Operating System |
| DDS | Data Distribution Service |
| QoS | Quality of Service |
| URDF | Unified Robot Description Format |
| TF | Transform (TF2 = Transform 2) |
| IMU | Inertial Measurement Unit |
| LIDAR | Light Detection and Ranging |
| RGB | Red Green Blue (color) |
| SLAM | Simultaneous Localization and Mapping |
| MPC | Model Predictive Control |