3. Nodes, topics, messages

These three words are all you need to read a ROS 2 system.

Term	What it is	Everyday equivalent
node	One small program doing one job	One employee
topic	A named channel carrying data	A labelled conveyor belt
message	The shape of the data sent	An agreed form

A node that sends into a topic is a publisher; one that receives is a subscriber.

Important

A publisher does not know who is listening, and does not care whether anyone is. It publishes and is done. Zero subscribers or ten, it behaves identically.

That is what makes the system flexible — and it is also why failures can be silent. Publish with nobody listening and nothing complains.

Try it with turtlesim

turtlesim ships with ROS 2 and is the fastest way to see this working.

Open three terminals (source /opt/ros/humble/setup.bash in each).

Terminal 1 — the turtle window

ros2 run turtlesim turtlesim_node

Terminal 2 — drive it with the arrow keys

ros2 run turtlesim turtle_teleop_key

Press the arrows and the turtle moves (this terminal must have focus).

Terminal 3 — watch the data in flight

ros2 topic echo /turtle1/cmd_vel

Press an arrow again and you will see:

linear:
  x: 2.0
  y: 0.0
  z: 0.0
angular:
  x: 0.0
  y: 0.0
  z: 0.0

Tip

This is the single most valuable thing ROS 2 gives you. You just watched the data flowing between two programs without modifying either one and without stopping them.

When a real robot will not move, this is the first command to run — it tells you immediately whether the command is even being sent.

The shape of a message

That data is a geometry_msgs/msg/Twist. See its full definition:

ros2 interface show geometry_msgs/msg/Twist

Vector3  linear
Vector3  angular

linear.x is forward speed in m/s; angular.z is turning rate in rad/s.

Note

Twist is exactly what a real AGV is commanded with, over a topic called /cmd_vel. This turtle and a 200 kg robot take the identical message.

That is genuinely useful — code that drives the turtle drives the real robot with almost no changes.

See who talks to whom

ros2 node list

/turtlesim
/teleop_turtle

ros2 topic list

/parameter_events
/rosout
/turtle1/cmd_vel
/turtle1/color_sensor
/turtle1/pose

Inspect one topic — who publishes, who subscribes, how many:

ros2 topic info /turtle1/cmd_vel

Type: geometry_msgs/msg/Twist
Publisher count: 1
Subscription count: 1

Important

Publisher count: 0 is a strong clue — nothing is sending into this topic at all, even though the name appears in ros2 topic list.

Common when the node that should publish was never started, or crashed on startup.

The classic trap — mismatched names

The most common beginner failure is a topic name that does not match.

One node publishes to /cmd_vel, another listens on /cmd_vel_nav. Both run perfectly. No error, no warning, and the data never meets.

Check with ros2 topic list and look for names that should not be there. Seeing both /cmd_vel and /cmd_vel_nav when there should be one means a typo.

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