Publisher and Subscriber Nodes¶

Concept¶

A publisher sends messages on a named topic. A subscriber listens on a topic and receives those messages. Nodes communicate via topics without needing direct knowledge of each other — only the topic name and message type must match.

File Locations¶

Both Python files go inside:

~/ROBOT/src/robot_pkg/robot_pkg/

publisher.py¶

import rclpy
from rclpy.node import Node
from std_msgs.msg import String

def main():
    rclpy.init()
    node = Node('publisher_node')
    pub = node.create_publisher(String, 'chat', 10)

    def timer_callback():
        msg = String()
        msg.data = 'Hello from Publisher!'
        pub.publish(msg)
        node.get_logger().info('Sent: ' + msg.data)

    node.create_timer(1.0, timer_callback)  # publish every 1 second
    rclpy.spin(node)
    rclpy.shutdown()

main()

subscriber.py¶

import rclpy
from rclpy.node import Node
from std_msgs.msg import String

def main():
    rclpy.init()
    node = Node('subscriber_node')

    def callback(msg):
        node.get_logger().info('Received: ' + msg.data)

    node.create_subscription(String, 'chat', callback, 10)
    rclpy.spin(node)
    rclpy.shutdown()

main()

Register in setup.py¶

Open ~/ROBOT/src/robot_pkg/setup.py and update entry_points:

entry_points={
    'console_scripts': [
        'publisher  = robot_pkg.publisher:main',
        'subscriber = robot_pkg.subscriber:main',
    ],
},

Build and Run¶

Terminal 1 — Build

cd ~/ROBOT && colcon build && source install/setup.bash

Terminal 2 — Publisher

source ~/ROBOT/install/setup.bash
ros2 run robot_pkg publisher

Terminal 3 — Subscriber

source ~/ROBOT/install/setup.bash
ros2 run robot_pkg subscriber

Terminal 4 — Verify the topic

ros2 topic echo /chat

Expected Output¶

Subscriber terminal:

[INFO] [subscriber_node]: Received: Hello from Publisher!
[INFO] [subscriber_node]: Received: Hello from Publisher!
...