Topics are a key element of the ROS graph, acting as a bus for nodes to exchange messages. They allow data to be moved between different parts of the system. The rqt_graph tool can be used to visualize nodes and topics, while ros2 topic list, ros2 topic echo, and ros2 topic info can be used to view and interact with topics. The ros2 interface show command can be used to learn about message types, and ros2 topic pub can be used to publish data directly to a topic. The ros2 topic hz command can be used to view the rate at which data is published on a topic.
ROS 2 breaks complex systems down into many modular nodes. Topics are a vital element of the ROS graph that acts 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.
The rqt_graph is used to visualize the changing nodes and topics, as well as the connections between them.
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💡 Ensure that rqt_graph is installed using sudo apt install ~nros-humble-rqt*
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💡 Make sure /turtlesim & /teleop_turtle both are active, if not run ros2 run turtlesim turtlesim_node & ros2 run turtlesim turtle_teleop_key in separate terminal respectively
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To run rqt, open a new terminal and enter the command:
rqt_graph
You should see the above nodes and topic, as well as two actions around the periphery of the graph (will ignore those for now). If you hover your mouse over the topic in the center, you’ll see the color highlighting like the image above.
The graph is depicting how the /turtlesim node and the teleop_turtle node are communicating with each other over a topic. The /teleop_turtle node is publishing data (the keystrokes you enter to move the turtle around) to the /turtle1/cmd_vel topic, and the /turtlesim node is subscribed to that topic to receive data.
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💡 Make sure /turtlesim & /teleop_turtle both are active, if not run ros2 run turtlesim turtlesim_node & ros2 run turtlesim turtle_teleop_key in separate terminal respectively
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Running the ros2 topic list command in a new terminal will return a list of all the topics currently active in the system.