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Nodes

As introduced earlier, nodes are the fundamental executable units in a ROS 2 system. Each node is designed to perform a specific task, promoting modularity and reusability of code.

The Purpose of Nodes​

Nodes encapsulate logic that performs a particular function, such as:

  • Sensor Drivers: Reading data from a camera, LIDAR, or IMU.
  • Actuator Controllers: Sending commands to motors, grippers, or robotic arms.
  • Data Processing: Filtering sensor data, performing object detection, or mapping an environment.
  • Decision Making: Implementing navigation algorithms, task planners, or AI models.
  • User Interfaces: Providing visualization tools or control panels.

Node Lifecycle​

ROS 2 nodes can have a lifecycle, especially managed nodes, which allows for more robust state management. A typical lifecycle includes states like unconfigured, inactive, active, and finalized, with transitions between them (e.g., configure, activate, deactivate, cleanup). This provides a structured way to initialize, run, and shut down nodes, particularly useful for mission-critical applications.

Creating a Node (Python Example)​

Let's look at a simple Python node that prints "Hello ROS 2!" periodically.

First, create a new Python package (if you haven't already):

cd ~/ros2_ws/src
ros2 pkg create --build-type ament_python my_python_pkg

Now, inside ~/ros2_ws/src/my_python_pkg/my_python_pkg, create a file named talker_node.py:

import rclpy
from rclpy.node import Node

class TalkerNode(Node):
    def __init__(self):
        super().__init__('talker_node')
        self.timer = self.create_timer(1.0, self.timer_callback)
        self.get_logger().info('Hello ROS 2 from TalkerNode!')

    def timer_callback(self):
        self.get_logger().info('Still talking...')

def main(args=None):
    rclpy.init(args=args)
    talker_node = TalkerNode()
    rclpy.spin(talker_node) # Keeps the node alive until Ctrl+C is pressed
    talker_node.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Make the script executable:

chmod +x ~/ros2_ws/src/my_python_pkg/my_python_pkg/talker_node.py

Then, you need to tell ROS 2 how to find and run this executable. Open ~/ros2_ws/src/my_python_pkg/setup.py and add the following inside the entry_points dictionary:

    entry_points={
        'console_scripts': [
            'talker = my_python_pkg.talker_node:main',
        ],
    },

Building and Running a Node​

  1. Build the workspace: 
    cd ~/ros2_ws
    colcon build --packages-select my_python_pkg
  2. Source the setup files: 
    source install/setup.bash
  3. Run the node: 
    ros2 run my_python_pkg talker
    You should see "Hello ROS 2 from TalkerNode!" followed by "Still talking..." every second.

ros2 run and ros2 node Commands​

  • ros2 run <package_name> <executable_name>: Used to start an executable defined within a ROS 2 package.
  • ros2 node list: Lists all active nodes in the ROS 2 graph.
  • ros2 node info <node_name>: Provides detailed information about a specific node, including its published topics, subscribed topics, services, actions, and parameters.

Nodes are the backbone of any ROS 2 application, allowing for the decomposition of complex robotic systems into manageable, interacting components.