Nodes, Topics, Services & Actions

ROS 2 gives you four communication primitives. Choosing the right one is most of the skill.

Nodes

A node is a single-purpose process. One node reads the camera; another estimates balance; another drives the legs. A node is created in Python by subclassing rclpy.node.Node:

import rclpy
from rclpy.node import Node

class HeartbeatNode(Node):
    def __init__(self):
        super().__init__('heartbeat')
        self.get_logger().info('Heartbeat node is alive')

def main():
    rclpy.init()
    node = HeartbeatNode()
    rclpy.spin(node)        # process callbacks until Ctrl-C
    node.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Nodes discover one another automatically over DDS — there is no central master as in ROS 1.

Topics — streaming, many-to-many, fire-and-forget

A topic is a named bus carrying a stream of messages of one type. Publishers write; subscribers read; neither knows how many of the other exist. Use topics for continuous data: sensor readings, velocity commands, joint states.

from geometry_msgs.msg import Twist

class Driver(Node):
    def __init__(self):
        super().__init__('driver')
        self.pub = self.create_publisher(Twist, '/cmd_vel', 10)
        self.create_timer(0.1, self.tick)        # 10 Hz

    def tick(self):
        msg = Twist()
        msg.linear.x = 0.2      # drive forward 0.2 m/s
        msg.angular.z = 0.0
        self.pub.publish(msg)

A subscriber to the same topic:

class Odom(Node):
    def __init__(self):
        super().__init__('odom_printer')
        self.create_subscription(Twist, '/cmd_vel', self.on_cmd, 10)

    def on_cmd(self, msg: Twist):
        self.get_logger().info(f'commanded v={msg.linear.x} m/s')

Quality of Service (QoS)

Because ROS 2 runs on DDS, every topic has a QoS profile. The two policies you will tune most:

• Reliability: RELIABLE (retransmit until delivered, e.g. commands) vs. BEST_EFFORT (drop if late, e.g. high-rate camera frames).
• Durability: TRANSIENT_LOCAL lets late-joining subscribers receive the last message (useful for a map or a robot description that's published once).

A subscriber's QoS must be compatible with the publisher's or no messages flow — the #1 cause of "my topic is silent."

Services — request/response, one-to-one

A service is a synchronous call: send a request, block for a response. Use it for quick queries or commands with a result: "reset odometry", "is the gripper open?".

from example_interfaces.srv import AddTwoInts

class Adder(Node):
    def __init__(self):
        super().__init__('adder')
        self.create_service(AddTwoInts, 'add', self.handle)

    def handle(self, request, response):
        response.sum = request.a + request.b
        return response

Services should return quickly. Don't use a service to "walk to the kitchen" — that takes minutes and can fail halfway. That is what actions are for.

Actions — long-running goals with feedback

An action is the right tool for goals that take time, stream progress, and can be canceled: navigate to a pose, follow a trajectory, pick up an object. An action bundles three things: a goal, periodic feedback, and a final result.

from rclpy.action import ActionClient
from nav2_msgs.action import NavigateToPose

class Commander(Node):
    def __init__(self):
        super().__init__('commander')
        self.client = ActionClient(self, NavigateToPose, 'navigate_to_pose')

    def go(self, goal_pose):
        self.client.wait_for_server()
        goal = NavigateToPose.Goal()
        goal.pose = goal_pose
        # feedback streams progress; the future resolves when done
        return self.client.send_goal_async(goal, feedback_callback=self.on_feedback)

    def on_feedback(self, fb):
        self.get_logger().info(f'distance remaining: {fb.feedback.distance_remaining:.2f} m')

Choosing the right primitive

You need to…	Use
Stream sensor data or velocity commands	Topic
Ask a quick question / trigger an instant action	Service
Command a goal that takes time and may be canceled	Action
Share a value many nodes read at startup	Parameter

:::tip Inspect everything from the CLI ros2 topic list, ros2 topic echo /cmd_vel, ros2 service list, ros2 action list, and ros2 node info /driver are your debugging lifeline. If two nodes won't talk, check the topic name and the QoS first. :::

Next: Bridging Python AI Agents to ROS with rclpy →.