ROS 2 breaks complex systems down into many modular nodes. Topics are a vital element of the ROS graph that act as a bus for nodes to exchange messages.
A node may publish data to any number of topics and simultaneously have subscriptions to any number of topics.
Topics are one of the main ways in which data is moved between nodes and therefore between different parts of the system.
By now you should be comfortable starting up turtlesim.
Open a new terminal and run:
ros2 run turtlesim turtlesim_node
Open another terminal and run:
ros2 run turtlesim turtle_teleop_key
Recall from the tutorial ROS 2 Node that the names of these nodes are /turtlesim and /teleop_turtle by default.
Throughout this tutorial, we will use rqt_graph to visualize the changing nodes and topics, as well as the connections between them.
The turtlesim tutorial tells you how to install rqt and all its plugins, including rqt_graph.
To run rqt_graph, open a new terminal and enter the command:
rqt_graph
You can also open rqt_graph by opening rqt and selecting Plugins > Introspection > Node Graph.
