ROS 2 Services tutorials

In ROS 2, Services provide a way for nodes to communicate in a request-response manner, similar to a function call in programming. One node sends a request, and another node responds. This is useful when you need a one-time communication, such as requesting the current state of the robot's battery.

Services are another method of communication for nodes in the ROS graph. Services are based on a call-and-response model versus the publisher-subscriber model of topics. While topics allow nodes to subscribe to data streams and get continual updates, services only provide data when they are specifically called by a client.

Example: Creating a Service Server and Client

Let's create a simple service to get the square of a number.

1. Service Definition: First, we need to define a service type. ROS 2 comes with several built-in service types, but we'll use a basic one here: example_interfaces/srv/AddTwoInts.

2. Create a Service Server:

   • Open a new file simple_service_server.py and add the following code:

     import rclpy
     from rclpy.node import Node
     from example_interfaces.srv import AddTwoInts
     
     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)
     
         def add_two_ints_callback(self, request, response):
             response.sum = request.a + request.b
             self.get_logger().info(f'Received request: a={request.a}, b={request.b}')
             return response
     
     def main(args=None):
         rclpy.init(args=args)
         node = AddTwoIntsServer()
         rclpy.spin(node)
         rclpy.shutdown()
     
     if __name__ == '__main__':
         main()

3. Create a Service Client:

   • Open another file simple_service_client.py and add the following code:

     import rclpy
     from rclpy.node import Node
     from example_interfaces.srv import AddTwoInts
     
     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('Waiting for service...')
             self.req = AddTwoInts.Request()
     
         def send_request(self, a, b):
             self.req.a = a
             self.req.b = b
             future = self.cli.call_async(self.req)
             return future
     
     def main(args=None):
         rclpy.init(args=args)
         client = AddTwoIntsClient()
         future = client.send_request(5, 10)
         rclpy.spin_until_future_complete(client, future)
         response = future.result()
         client.get_logger().info(f'Service result: {response.sum}')
         rclpy.shutdown()
     
     if __name__ == '__main__':
         main()

4. Run the Server and Client:

   • First, in one terminal, run the service server:

     python3 simple_service_server.py

   • In another terminal, run the service client:

     python3 simple_service_client.py

The client will send two integers (5 and 10), and the server will respond with the sum (15).