Let’s now create a Service-Server so we can practice the concept learnt.

Our aim is to create a service server that will take two numbers (as a request) and return the sum (as a response). A service is defined by two things, a name which is a common interface between nodes, so they can easily find the service and also a service type. Right now, we will use an existing service type that we can see using the following command.

ros2 interface show example_interfaces/srv/AddTwoInts

The first two lines here A and B are both integers and int64 is the primitive data-type for that integer. This is the request. The request contains two fields, A and B.

Then the three-dash separator that separates Request variables from Response variables.

We are getting only one integer, that is, sum as the response.

In other words, as a client you will send a and b and as a server you will receive A and B, process the request as you want, and return a sum.

For now, let us use this service definition (and in the upcoming tutorials we will learn how to create our own service definition).

Python Server Service Node

cd ~/ros2_ws/src/my_py_pkg/my_py_pkg/
touch add_two_ints_server.py
chmod +x add_two_ints_server.py

Now, we will write the constructor of our Node, using the template provided previously.

#!/usr/bin/env python3
import rclpy
from rclpy.node import Node

class AddTwoIntsServerNode(Node):
    def __init__(self):
        super().__init__("add_two_ints_server")

def main(args=None):
    rclpy.init(args=args)
    node = AddTwoIntsServerNode()
    rclpy.spin(node)
    rclpy.shutdown()

if __name__ == "__main__":
    main()

Additionally, we will also need to import the service-type we require from example_interfaces/srv

Then, create a server variable using create_service function.

#!/usr/bin/env python3
import rclpy
from rclpy.node import Node

from example_interfaces.srv import AddTwoInts

class AddTwoIntsServerNode(Node):
    def __init__(self):
        super().__init__("add_two_ints_server")
        self.server_ = self.create_service()

def main(args=None):
    rclpy.init(args=args)
    node = AddTwoIntsServerNode()
    rclpy.spin(node)
    rclpy.shutdown()

if __name__ == "__main__":
    main()

We, need to give three arguments inside create_service() function.

1. srv_type: the service type. Here, AddTwoInts
2. sev_name: the name of the serivce. Here, add_two_ints
3. callback: is required here because when we send a request, we need a function to process the request and return the response.