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6. ROS 2 Nodes

1. Node Introduction​

Regardless of the communication method used, all ROS 2 communication relies on nodes.
Each node typically represents a single functional module. For example:

  • A radar driver node publishes radar scan data.
  • A camera driver node publishes image messages.

A complete robot system may run many nodes simultaneously.
A single executable file (C++ or Python) can contain one or more nodes.

2. Node Creation Process​

  1. Create a program file\
  2. Import necessary ROS 2 libraries\
  3. Implement node logic\
  4. Configure package metadata\
  5. Compile and run the node

3. Hello World Node Example (Python)​

3.1 Creating the Python Package​

Replace workspace with your actual workspace path:

cd workspace/src
ros2 pkg create pkg_helloworld_py --build-type ament_python --dependencies rclpy --node-name helloworld

3.2 Writing the Node Code​

A template file helloworld.py will be generated.
Replace its contents with the following code:

import rclpy                  # ROS 2 Python client library
from rclpy.node import Node   # Base Node class
import time

"""
Create a HelloWorld node and log 'Hello World' periodically.
"""

class HelloWorldNode(Node):
    def __init__(self, name):
        super().__init__(name)     # Initialize parent Node
        while rclpy.ok():          # Keep running while ROS 2 is active
            self.get_logger().info("Hello World")
            time.sleep(0.5)        # Loop delay

def main(args=None):
    rclpy.init(args=args)          # Initialize ROS 2 system
    node = HelloWorldNode("helloworld")
    rclpy.spin(node)               # Keep node alive
    node.destroy_node()            # Cleanup
    rclpy.shutdown()               # Shutdown ROS 2

Python Node File Structure

Adding Entry Points in setup.py​

Open the package's setup.py and add:

entry_points={
    'console_scripts': [
        'helloworld = pkg_helloworld_py.helloworld:main'
    ],
},

Setup.py Entry Point

3.3 Compiling the Package​

Compile only the new package:

colcon build --packages-select pkg_helloworld_py

Refresh the workspace environment:

source install/setup.bash

3.4 Running the Node​

ros2 run pkg_helloworld_py helloworld

If successful, you will see recurring output:

Hello World Output