Skip to main content

Chapter 2: ROS 2 Architecture

Duration: Week 3 Hardware Tier: Tier 1-2 Lessons: 4

Chapter Overview​

ROS 2 is built on a simple but powerful idea: break complex robot systems into small, independent pieces (nodes) that communicate through well-defined channels (topics, services, actions).

In this chapter, you'll learn the architectural patterns that make ROS 2 the backbone of modern robotics - from warehouse robots to Mars rovers.

Learning Objectives​

By the end of this chapter, you will be able to:

  • Explain the ROS 2 distributed architecture and DDS middleware
  • Create and manage ROS 2 nodes as independent executables
  • Implement publisher/subscriber patterns using topics
  • Build request/response interactions using services
  • Design long-running tasks using actions with feedback

Prerequisites​

  • Chapter 1: Introduction to Physical AI
  • Python basics (functions, classes)
  • Terminal/command line familiarity

Lessons​

#LessonDurationTierDescription
2.1ROS 2 Core Concepts45 min1-2Architecture overview, DDS, and the computation graph
2.2Nodes - The Building Blocks60 min2Creating, running, and managing ROS 2 nodes
2.3Topics and Message Passing60 min2Publisher/subscriber pattern and message types
2.4Services and Actions60 min2Request/response and long-running operations

Key Concepts​

The Computation Graph​

┌─────────┐     /camera/image     ┌─────────────┐
│ Camera  │ ──────────────────────▶│   Vision    │
│  Node   │                       │  Processor  │
└─────────┘                       └──────┬──────┘
                                         │ /detected_objects
                                         ▼
┌─────────┐     /cmd_vel          ┌─────────────┐
│  Motor  │ ◀─────────────────────│  Navigation │
│ Driver  │                       │    Node     │
└─────────┘                       └─────────────┘

Communication Patterns​

PatternUse CaseExample
TopicContinuous data streamsSensor readings, video feeds
ServiceQuick request/responseGet robot state, set parameter
ActionLong-running with feedbackNavigate to goal, pick up object

Hardware Note​

Tier 1 Alternative

For Tier 1 (cloud) users: Use The Construct or Foxglove Studio for browser-based ROS 2 development.

Tier 2 Setup

For local development: Ubuntu 22.04 with ROS 2 Humble. Installation guide in Lesson 2.1.

Chapter Assessment​

After completing this chapter:

  • Can draw a ROS 2 computation graph for a simple robot
  • Created a publisher and subscriber node
  • Implemented a service server and client
  • Understand when to use topics vs services vs actions