ROS2 Service - Client Server Project

In the previous blog, we saw how to create a publisher subscriber project.

This project makes remakes it into a service based client server project.

For developing a service, a service package needs to be created using ament_cmake build tool. 

Go to the src folder in the root of the workspace and create a package called ‘sum_nums_srv’ using ament_cmake. 

$ cd ~/ros2_ws/src 
$ ros2 pkg create --build-type ament_cmake sum_nums_srv 

If we look at the directory structure of the new package ‘sum_nums_srv’, we can see that there is an include and src folder which we don't need. We will add a srv folder to place our service structure file called ‘SumNumsSrv.srv’. 

$ cd sum_nums_srv 
$ rm -rf include src 
$ mkdir srv 
$ cd srv 
$ touch SumNumsSrv.srv 

NOTE: A service structure file is treated as an interface file in ROS2 and hence needs to start with a Capital letter. 

Now our package ‘sum_nums_srv’ folder should look like the following structure: 

sum_nums_srv/ 
├── CMakeLists.txt 
├── package.xml 
└── srv 
    └── SumNumsSrv.srv 

Populate the srv file with the following service request and response structure and save it: 

int8 n 
--- 
int64 sum 

This srv file informs ROS2 that this service will request an integer n, and responds with an integer called sum. 

To convert the service interfaces into language-specific code (like C++ and Python) we should add the following lines to CMakeLists.txt, including any dependencies if any: 

find_package(rosidl_default_generators REQUIRED) 
 
rosidl_generate_interfaces(${PROJECT_NAME} 
  "srv/SumNumsSrv.srv" 
) 

Add the following lines within the <package> element of package.xml: 

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

Since the interfaces rely on rosidl_default_generators for generating language-specific code, we need to declare a build tool dependency on it. rosidl_default_runtime is a runtime or execution-stage dependency, needed for using the interfaces later. The rosidl_interface_packages is the name of the dependency group that our package, ‘sum_nums_srv’, should be associated with, declared using the <member_of_group> tag. 

Build the ‘sum_nums_srv’ package (selecting only the new package). 

$ source install/setup.bash 
$ ros2 interface show sum_nums_srv/srv/SumNumsSrv 

Test the new service. 

$ source install/setup.bash 
$ ros2 interface show sum_nums_srv/srv/SumNumsSrv 

This will display the service structure. 

Now go back to our ‘sum_nums’ package and use this newly created service package. 

Create two files called ‘sum_nums_service_server.py’ and ‘sum_nums_service_client.py’ in the same folder as our publisher subscriber files are. 

$ cd src/sum_nums/sum_nums 
$ touch sum_nums_service_server.py sum_nums_service_client.py 

Populate the sum_nums_service_server.py file with the service server code: 

from sum_nums_srv.srv import SumNumsSrv 
import rclpy 
from rclpy.node import Node 
 
class SumNumsClient(Node): 
    """ 
    Node to send requests to compute the sum of first n integer numbers. 
    """ 
    def __init__(self): 
        """ 
        Initializes the SumNumsClient class. 
        """ 
        super().__init__('sum_nums_client_node') 
        self.cli = self.create_client(SumNumsSrv, 'sum_n_nums') 
        while not self.cli.wait_for_service(timeout_sec=1.0): 
            self.get_logger().info(' SumNumsSrv service not available, waiting...') 
        self.req = SumNumsSrv.Request() 
 
    def send_request(self, n): 
        """ 
        Sends a request to compute the sum of first n integer numbers. 
 
        Parameters: 
            n: The number of integer numbers to sum. 
 
        Returns: 
            The response containing the computed sum. 
        """ 
        self.req.n = n 
        self.future = self.cli.call_async(self.req) 
        rclpy.spin_until_future_complete(self, self.future) 
        return self.future.result() 
 
def main(args=None): 
    """ 
    Main function to initialize the node and send a request to compute the sum of first n integer numbers. 
    """ 
    rclpy.init(args=args) 
 
    sum_nums_client_node = SumNumsClient() 
 
    num_input = int(input("Enter a positive integer number: ")) 
    if num_input < 0: 
        print("Input number is not positive") 
    else: 
        print('Entered:', num_input) 
        response = sum_nums_client_node.send_request(num_input)     
        print("Sum of first", num_input, "integer numbers is:", response.sum) 
 
    # Destroy the node explicitly 
    sum_nums_client_node.destroy_node() 
    rclpy.shutdown() 
 
if __name__ == '__main__': 
    main() 

Populate the sum_nums_service_client.py file with the service client code: 

from sum_nums_srv.srv import SumNumsSrv 
import rclpy 
from rclpy.node import Node 
 
class SumNumsClient(Node): 
    """ 
    Node to send requests to compute the sum of first n integer numbers. 
    """ 
    def __init__(self): 
        """ 
        Initializes the SumNumsClient class. 
        """ 
        super().__init__('sum_nums_client_node') 
        self.cli = self.create_client(SumNumsSrv, 'sum_n_nums') 
        while not self.cli.wait_for_service(timeout_sec=1.0): 
            self.get_logger().info(' SumNumsSrv service not available, waiting...') 
        self.req = SumNumsSrv.Request() 
 
    def send_request(self, n): 
        """ 
        Sends a request to compute the sum of first n integer numbers. 
 
        Parameters: 
            n: The number of integer numbers to sum. 
 
        Returns: 
            The response containing the computed sum. 
        """ 
        self.req.n = n 
        self.future = self.cli.call_async(self.req) 
        rclpy.spin_until_future_complete(self, self.future) 
        return self.future.result() 
 
def main(args=None): 
    """ 
    Main function to initialize the node and send a request to compute the sum of first n integer numbers. 
    """ 
    rclpy.init(args=args) 
 
    sum_nums_client_node = SumNumsClient() 
 
    num_input = int(input("Enter a positive integer number: ")) 
    if num_input < 0: 
        print("Input number is not positive") 
    else: 
        print('Entered:', num_input) 
        response = sum_nums_client_node.send_request(num_input)     
        print("Sum of first", num_input, "integer numbers is:", response.sum) 
 
    # Destroy the node explicitly 
    sum_nums_client_node.destroy_node() 
    rclpy.shutdown() 
 
if __name__ == '__main__': 
    main() 

Add the entry points to setup.py inside console_scripts section: 

'sum_nums_server = sum_nums.sum_nums_service_server:main', 
'sum_nums_client = sum_nums.sum_nums_service_client:main', 

Since our ‘sum_nums’ package depends on ‘sum_nums_srv’ package for the service structure, add it as a dependency inside the package.xml file: 

<exec_depend>sum_nums_srv</exec_depend> 

Build the ‘sum_nums’ package again. 

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

Execute the service node in the same terminal window. 

$ ros2 run sum_nums sum_nums_server 

Execute the client node in a new terminal window. 

$ source install/setup.bash 
$ ros2 run sum_nums sum_nums_client 

Enter a positive integer number and press Enter. 

The sum will be displayed by the sum_nums_client node.