Mahmoud Abas, Full-Stack Software Engineer & AI

ROS 2 Practical - ROS 2 Topics, Publisher, and Subscriber Nodes

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ROS 2 Practical - Lesson 2

ROS 2 Topics, Publisher, and Subscriber Nodes

The pub/sub model in practice

Topics and the Pub/Sub Model

Nodes communicate via topics using the publish/subscribe model
This decouples nodes: publishers and subscribers don't know about each other directly

Publisher

sends messages

publishes to

Topic

delivers to

Subscriber

receives messages

Worked Example

Classic Example: Talker and Listener

Talker publishes messages to /chatter
Listener subscribes to /chatter and prints what it receives

bash

ros2 run demo_nodes_py talker
ros2 run demo_nodes_py listener

Run each node in its own terminal.

Inspect: list active topics, then echo the messages

bash

ros2 topic list
ros2 topic echo /chatter

Turtlesim: Visual Topic Demo

turtlesim gives a graphical way to see pub/sub in action

bash

ros2 run turtlesim turtlesim_node
ros2 run turtlesim turtle_teleop_key

turtle_teleop_key

publishes velocity commands

/turtle1/cmd_vel

turtlesim_node

moves the turtle

Inspecting Topic Info and Message Types

bash

ros2 topic list
ros2 topic echo /turtle1/cmd_vel
ros2 topic info /turtle1/cmd_vel
ros2 interface show geometry_msgs/msg/Twist

ros2 topic info shows the message type plus publisher/subscriber counts

Twist: linear velocity (x, y, z) plus angular velocity (x, y, z), all float64

Publisher Node: Draw a Circle

Dependencies needed in package.xml: rclpy, geometry_msgs, turtlesim
Publishes Twist messages on a timer to move the turtle in a circle

python

class DrawCircleNode(Node):
    def __init__(self):
        super().__init__("draw_circle")
        self.cmd_vel_pub = self.create_publisher(Twist, "/turtle1/cmd_vel", 10)
        self.timer = self.create_timer(0.5, self.send_velocity_command)

    def send_velocity_command(self):
        msg = Twist()
        msg.linear.x = 2.0
        msg.angular.z = 1.0
        self.cmd_vel_pub.publish(msg)

create_publisher + create_timer = periodic publishing.

Subscriber Node: Turtle Pose

turtlesim/msg/Pose: used by /turtle1/pose: x, y, theta, linear_velocity, angular_velocity

Dependencies needed in package.xml: rclpy, turtlesim

python

class PoseSubscriberNode(Node):
    def __init__(self):
        super().__init__("pose_subscriber")
        self.subscription = self.create_subscription(
            Pose, "/turtle1/pose", self.pose_callback, 10)

    def pose_callback(self, msg):
        self.get_logger().info(
            f"x: {msg.x}, y: {msg.y}, theta: {msg.theta}")

create_subscription + a callback = reacting to incoming messages.

Building and Running the Nodes

bash

colcon build --symlink-install
source install/setup.bash

ros2 run my_first_py_pkg draw_circle
ros2 run my_first_py_pkg pose_subscriber   # separate terminal

Visualizing the Graph

rqt_graph shows nodes and topic connections visually

bash

rqt_graph
ros2 topic list
ros2 topic echo /topic

Key Takeaways

Pub/sub via topics decouples publishers and subscribers
ros2 topic info + ros2 interface show reveal a topic's message type and structure
create_publisher + create_timer = periodic publishing
create_subscription + a callback = reacting to incoming messages
Always register new nodes as executables in setup.py before running them
rqt_graph and ros2 topic echo are essential for debugging pub/sub

Open this lesson

Mahmoud AbasROS 2 Topics, Publisher, and Subscriber Nodes

class DrawCircleNode(Node): def __init__(self): super().__init__("draw_circle") self.cmd_vel_pub = self.create_publisher(Twist, "/turtle1/cmd_vel", 10) self.timer = self.create_timer(0.5, self.send_velocity_command) def send_velocity_command(self): msg = Twist() msg.linear.x = 2.0 msg.angular.z = 1.0 self.cmd_vel_pub.publish(msg)

class PoseSubscriberNode(Node): def __init__(self): super().__init__("pose_subscriber") self.subscription = self.create_subscription( Pose, "/turtle1/pose", self.pose_callback, 10) def pose_callback(self, msg): self.get_logger().info( f"x: {msg.x}, y: {msg.y}, theta: {msg.theta}")