Services

Services in ROS 2 provide a synchronous request/reply communication model. Unlike topics, which are designed for continuous, asynchronous data streaming, services are used when you need to send a request to a node and receive a single, immediate response. This is analogous to a function call in a traditional programming paradigm.

Request/Reply Model

• Service Server: A node that "advertises" a service and waits for incoming requests. When a request is received, the server executes a defined callback function to process the request and generate a response.
• Service Client: A node that "calls" a service on a server, sending a request message and blocking (or awaiting asynchronously) until it receives a response message.
• Service Name: A unique string identifier for the service (e.g., /get_robot_pose, /set_gripper_state).
• Service Definition: Defined by .srv files, which explicitly define the structure of both the request and response messages. A --- separates the request fields from the response fields.

Creating a Service (Python Example)

Let's create a simple service that adds two integers.

Define the Service (AddTwoInts.srv)

First, create a srv directory inside ~/ros2_ws/src/my_python_pkg (if it doesn't exist) and then AddTwoInts.srv:

int64 a
int64 b
---
int64 sum

Update package.xml

You need to declare that your package exports a service. Add these lines to ~/ros2_ws/src/my_python_pkg/package.xml:

  <buildtool_depend>ament_cmake</buildtool_depend>
  <build_depend>rosidl_default_generators</build_depend>
  <exec_depend>rosidl_default_runtime</exec_depend>
  <member_of_group>rosidl_interface_packages</member_of_group>

Also, ensure you have std_msgs or any other message dependencies in your package.xml. For custom services, you need rosidl_default_generators for building and rosidl_default_runtime for execution.

Update setup.py

You also need to tell setup.py to process the .srv files. Add the following to ~/ros2_ws/src/my_python_pkg/setup.py:

from setuptools import find_packages, setup
import os
from glob import glob

package_name = 'my_python_pkg'

setup(
    name=package_name,
    version='0.0.0',
    packages=find_packages(exclude=['test']),
    data_files=[
        ('share/' + package_name, ['package.xml']),
        ('share/' + package_name + '/srv', glob('srv/*.srv')), # Add this line
        ('share/ament_index/resource_index/packages',
            ['resource/' + package_name]),
        (os.path.join('share', package_name, 'launch'), glob(os.path.join('launch', '*launch.[pxy][yeml]*'))),
    ],
    install_requires=['setuptools'],
    zip_safe=True,
    maintainer='your_name',
    maintainer_email='your_email@example.com',
    description='TODO: Package description',
    license='TODO: License declaration',
    tests_require=['pytest'],
    entry_points={
        'console_scripts': [
            'talker = my_python_pkg.talker_node:main',
            'talker_publisher = my_python_pkg.talker_publisher:main',
            'listener_subscriber = my_python_pkg.listener_subscriber:main',
            'add_two_ints_server = my_python_pkg.add_two_ints_server:main',
            'add_two_ints_client = my_python_pkg.add_two_ints_client:main',
        ],
    },
)

Service Server (add_two_ints_server.py)

Create ~/ros2_ws/src/my_python_pkg/my_python_pkg/add_two_ints_server.py:

import rclpy
from rclpy.node import Node
from my_python_pkg.srv import AddTwoInts # Import your service type

class AddTwoIntsServer(Node):
    def __init__(self):
        super().__init__('add_two_ints_server')
        self.srv = self.create_service(AddTwoInts, 'add_two_ints', self.add_two_ints_callback)
        self.get_logger().info('Add two ints service ready.')

    def add_two_ints_callback(self, request, response):
        response.sum = request.a + request.b
        self.get_logger().info(f'Incoming request: a={request.a}, b={request.b}')
        self.get_logger().info(f'Sending response: sum={response.sum}')
        return response

def main(args=None):
    rclpy.init(args=args)
    add_two_ints_server = AddTwoIntsServer()
    rclpy.spin(add_two_ints_server)
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Service Client (add_two_ints_client.py)

Create ~/ros2_ws/src/my_python_pkg/my_python_pkg/add_two_ints_client.py:

import sys
import rclpy
from rclpy.node import Node
from my_python_pkg.srv import AddTwoInts # Import your service type

class AddTwoIntsClient(Node):
    def __init__(self):
        super().__init__('add_two_ints_client')
        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)
    add_two_ints_client = AddTwoIntsClient()
    if len(sys.argv) != 3:
        add_two_ints_client.get_logger().info('Usage: ros2 run my_python_pkg add_two_ints_client <arg1> <arg2>')
        sys.exit(1)
    
    a = int(sys.argv[1])
    b = int(sys.argv[2])
    response = add_two_ints_client.send_request(a, b)
    if response:
        add_two_ints_client.get_logger().info(f'Result of add_two_ints: {a} + {b} = {response.sum}')
    else:
        add_two_ints_client.get_logger().info('Service call failed.')
    
    add_two_ints_client.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Building and Running

1. Build the workspace (including new service definition):

   cd ~/ros2_ws
   colcon build --packages-select my_python_pkg
   source install/setup.bash

2. Run the service server in one terminal:

   ros2 run my_python_pkg add_two_ints_server

3. Run the service client in another terminal:

   ros2 run my_python_pkg add_two_ints_client 5 7

   The client should send the request, and the server should respond with 12.

ros2 service Commands

• ros2 service list: Lists all active services.
• ros2 service type <service_name>: Displays the service type.
• ros2 service find <service_type>: Finds all services of a given type.
• ros2 service call <service_name> <service_type> <args>: Calls a service from the command line.

Services are essential for specific, on-demand interactions between nodes, complementing the continuous data flow provided by topics.