Nodes, Topics, Services

ROS 2 (Robot Operating System 2) provides the communication framework for robotic applications. Understanding its core concepts of nodes, topics, and services is essential for building distributed robotic systems.

Nodes

A node is an executable that uses ROS 2 to communicate with other nodes. Nodes are the fundamental building blocks of a ROS 2 system, each performing a specific task such as sensor data processing, control algorithms, or user interfaces.

Topics

Topics enable asynchronous communication between nodes through a publish-subscribe model. Publishers send messages to topics, and subscribers receive messages from topics. This decoupled communication allows for flexible system architectures.

Services

Services provide synchronous request-response communication between nodes. A client sends a request to a service, and the service processes the request and returns a response. This is useful for operations that require immediate feedback.

Python rclpy Examples

Here are examples of basic ROS 2 concepts using Python's rclpy library:

Simple Publisher Node

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

class MinimalPublisher(Node):
    def __init__(self):
        super().__init__('minimal_publisher')
        self.publisher_ = self.create_publisher(String, 'topic', 10)
        timer_period = 0.5  # seconds
        self.timer = self.create_timer(timer_period, self.timer_callback)
        self.i = 0

    def timer_callback(self):
        msg = String()
        msg.data = 'Hello World: %d' % self.i
        self.publisher_.publish(msg)
        self.get_logger().info('Publishing: "%s"' % msg.data)
        self.i += 1

def main(args=None):
    rclpy.init(args=args)
    minimal_publisher = MinimalPublisher()
    rclpy.spin(minimal_publisher)
    minimal_publisher.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Simple Subscriber Node

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

class MinimalSubscriber(Node):
    def __init__(self):
        super().__init__('minimal_subscriber')
        self.subscription = self.create_subscription(
            String,
            'topic',
            self.listener_callback,
            10)
        self.subscription  # prevent unused variable warning

    def listener_callback(self, msg):
        self.get_logger().info('I heard: "%s"' % msg.data)

def main(args=None):
    rclpy.init(args=args)
    minimal_subscriber = MinimalSubscriber()
    rclpy.spin(minimal_subscriber)
    minimal_subscriber.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Service Server

from add_two_ints_srv.srv import AddTwoInts
import rclpy
from rclpy.node import Node

class MinimalService(Node):
    def __init__(self):
        super().__init__('minimal_service')
        self.srv = self.create_service(AddTwoInts, 'add_two_ints', self.add_two_ints_callback)

    def add_two_ints_callback(self, request, response):
        response.sum = request.a + request.b
        self.get_logger().info('Incoming request\na: %d b: %d' % (request.a, request.b))
        return response

def main(args=None):
    rclpy.init(args=args)
    minimal_service = MinimalService()
    rclpy.spin(minimal_service)
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Service Client

import sys
from add_two_ints_srv.srv import AddTwoInts
import rclpy
from rclpy.node import Node

class MinimalClientAsync(Node):
    def __init__(self):
        super().__init__('minimal_client_async')
        self.cli = self.create_client(AddTwoInts, 'add_two_ints')
        while not self.cli.wait_for_service(timeout_sec=1.0):
            self.get_logger().info('service not available, waiting again...')
        self.req = AddTwoInts.Request()

    def send_request(self, a, b):
        self.req.a = a
        self.req.b = b
        self.future = self.cli.call_async(self.req)
        rclpy.spin_until_future_complete(self, self.future)
        return self.future.result()

def main(args=None):
    rclpy.init(args=args)
    minimal_client = MinimalClientAsync()
    response = minimal_client.send_request(int(sys.argv[1]), int(sys.argv[2]))
    minimal_client.get_logger().info(
        'Result of add_two_ints: for %d + %d = %d' %
        (int(sys.argv[1]), int(sys.argv[2]), response.sum))
    minimal_client.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()