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ROS 2 Nodes, Topics & Services

The Building Blocks of a Robot​

A ROS 2 application is made up of nodes — independent processes that each perform one task. Think of each node as a specialist employee: one reads the camera, another plans the path, another controls the motors.

These nodes communicate through three mechanisms:

1. Topics (Publish/Subscribe)​

A topic is a named data stream. Any node can publish data to it; any other node can subscribe to receive it.

Camera Node ──publishes──> /image_raw ──subscribes──> Perception Node
import rclpy
from rclpy.node import Node
from sensor_msgs.msg import Image

class CameraSubscriber(Node):
    def __init__(self):
        super().__init__('camera_subscriber')
        self.subscription = self.create_subscription(
            Image,
            '/image_raw',
            self.listener_callback,
            10  # QoS queue size
        )

    def listener_callback(self, msg):
        self.get_logger().info(f'Received image: {msg.height}x{msg.width}')

def main():
    rclpy.init()
    node = CameraSubscriber()
    rclpy.spin(node)
    rclpy.shutdown()

2. Services (Request/Response)​

A service is a two-way call: one node sends a request, another processes it and sends back a response.

Navigation Node ──request──> /compute_path ──response──> Path Planner Node

Services are synchronous — use them for one-time queries like "what is the current battery level?"

3. Actions (Long-Running Tasks)​

Actions are like services but for tasks that take time and need feedback (e.g., "walk to position X").

They have three parts:

  • Goal: what you want done
  • Feedback: progress updates
  • Result: final outcome
Main Node ──goal──> /navigate_to_pose ──feedback──> (position updates) ──result──> success/fail

Key ROS 2 CLI Commands​

# List all active nodes
ros2 node list

# List all active topics
ros2 topic list

# See what a topic is publishing
ros2 topic echo /chatter

# Get info about a topic
ros2 topic info /cmd_vel

# Call a service manually
ros2 service call /set_bool std_srvs/srv/SetBool "data: true"

DDS — The Communication Layer​

ROS 2 uses DDS (Data Distribution Service) as its underlying transport. Unlike ROS 1 (which used a central "ROS Master"), DDS is decentralized — nodes discover each other automatically on the network.

This means:

  • No single point of failure
  • Works across multiple robots on the same network
  • Supports real-time guarantees

Next: rclpy — Writing Python Nodes