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8. ROS 2 Service Communication

1. Introduction to Service Communication​

Service communication is a request-response communication model. The client sends a request to the server, and the server responds with data.

This follows the Client/Server (CS) communication model.

Service Communication Model

In this model: - The client requests data. - The server processes the request and returns a response.

A common real-world example is a web browser requesting data from a website server.

2. Create a New Package​

Navigate to the src directory of your ROS 2 workspace and run:

ros2 pkg create pkg_service --build-type ament_python --dependencies rclpy --node-name server_demo

After execution, the pkg_service package and server_demo node will be created.

Package Structure

3. Server Implementation​

3.1 Creating the Server​

Edit server_demo.py and add the following code:

# Import related libraries
import rclpy
from rclpy.node import Node
from example_interfaces.srv import AddTwoInts

class Service_Server(Node):
    def __init__(self, name):
        super().__init__(name)
        self.srv = self.create_service(AddTwoInts, '/add_two_ints', self.Add2Ints_callback)

    def Add2Ints_callback(self, request, response):
        response.sum = request.a + request.b
        print("response.sum =", response.sum)
        return response

def main():
    rclpy.init()
    server_demo = Service_Server("publisher_node")
    rclpy.spin(server_demo)
    server_demo.destroy_node()
    rclpy.shutdown()

3.2 Viewing the Service Interface​

To view the structure of the service message:

ros2 interface show example_interfaces/srv/AddTwoInts

Service Interface

3.3 Editing the Configuration File​

Open setup.py and add:

'server_demo = pkg_service.server_demo:main',

Setup Config

3.4 Compiling the Package​

colcon build --packages-select pkg_service

Build Result

3.5 Running the Server​

ros2 run pkg_service server_demo

Server Running

List services:

ros2 service list

Service List

Call the service:

ros2 service call /add_two_ints example_interfaces/srv/AddTwoInts "{a: 1, b: 4}"

Service Response

4. Client Implementation​

4.1 Creating the Client​

Create client_demo.py in the same directory as server_demo.py.

Client File

Add the following code:

# Import related libraries
import rclpy
from rclpy.node import Node
from example_interfaces.srv import AddTwoInts

class Service_Client(Node):
    def __init__(self, name):
        super().__init__(name)
        self.client = self.create_client(AddTwoInts, '/add_two_ints')

        while not self.client.wait_for_service(timeout_sec=1.0):
            print("Waiting for service...")

        self.req = AddTwoInts.Request()
        self.send_request()

    def send_request(self):
        self.req.a = 3
        self.req.b = 6
        self.future = self.client.call_async(self.req)
        self.future.add_done_callback(self.response_callback)

    def response_callback(self, future):
        try:
            response = future.result()
            print(f"Service Response: {response.sum}")
        except Exception as e:
            print(f"Service call failed: {e}")

def main():
    rclpy.init()
    client_demo = Service_Client("client_node")
    rclpy.spin(client_demo)
    client_demo.destroy_node()
    rclpy.shutdown()

This completes ROS 2 service communication using Python.