Lab 3.1. ROS 2 Topics

Objectives

At the end of this self-learning lab, you should be able to:

Communication between nodes via ROS Topics: Basic concepts

To say that two programs (i.e. nodes) are communicating with each other, it means that the output of one program is the input of another.
In ROS, one of the ways for nodes to communicate with each other is via a topic.
A topic is a named data bus that contains data. A node can write data onto this topic, and another node can read data from such topic. The data transfer is facilitated via the topic.
Each data packet on the bus is called a message. Nodes can only read/write to a topic one message at a time.
Each message has its own format and structure (this will become apparent with an example later). This structure is called a msg. We will use "message type" and "msg" interchangeably.
To say that a node is to write (output) to a topic, with ROS terminology is to publish.
To say that a node is to read (input) from a topic, with ROS terminology is to subscribe.
Multiple nodes can write (publish) to the same topic, and multiple nodes can also read (subscribe) from the same topic.
The same node can publish to topics and subscribe to multiple topics simultaneously.

You can now follow and self-learn the ROS 2 official tutorials on topics, publisher and subscribers here.