Basic architecture of ROS

ROS (Robot Operating System) is open-source middleware that provides communication, tools and libraries for robot software. Its architecture has three levels.

1. Computation graph level (runtime)

Element	Role
Master	Name service that lets nodes find each other
Node	A process that performs one task
Topic	Named channel for publish/subscribe messages
Message	Typed data structure sent over a topic
Service	Synchronous request/response between nodes
Action	Long-running goal with feedback and cancellation
Parameter server	Shared store of configuration values
Bag	Recording and playback of message data

2. Filesystem level

Packages (the basic unit, with package.xml and CMakeLists.txt), metapackages, message and service definitions (.msg, .srv), launch files and the Catkin workspace.

3. Community level

Distributions, repositories, the ROS wiki and forums, which allow software sharing.

Use in modern robotics

Modular reusable nodes, hardware abstraction, simulation (Gazebo), visualisation (RViz) and ready libraries for navigation and manipulation.

Note

The Master-based design is ROS 1. ROS 2 replaces it with DDS-based discovery and has no central master.

Exam tip

Draw the Master, two nodes and a topic, and list the three levels.