services.md

Writing a Simple Service and Client

Writing a Service Node

Here we'll create the service (add_two_ints_server) node which will receive two ints and return the sum.

Change directory into the beginner_tutorials package:

rosstd # have to do this once per terminal
roscd beginner_tutorials

Build the beginner_tutorials package (we added the AddTwoInts service, which needs to be built in order to be used):

natbuild beginner_tutorials

The Code

Create the src/add_two_ints_server.py file within the beginner_tutorials package (e.g. through Ctrl-N in VS Code) and paste the following inside it:

#!/usr/bin/env python3
from beginner_tutorials.srv import AddTwoInts, AddTwoIntsRequest, AddTwoIntsResponse
import rospy

def handle_add_two_ints(req):
    rospy.loginfo(f"Returning [{req.a} + {req.b} = {req.a + req.b}]")
    return AddTwoIntsResponse(req.a + req.b)

def add_two_ints_server():
    rospy.init_node('add_two_ints_server', anonymous=True)
    s = rospy.Service('add_two_ints', AddTwoInts, handle_add_two_ints)
    rospy.loginfo("Ready to add two ints.")
    rospy.spin()
 
if __name__ == "__main__":
    add_two_ints_server()

IMPORTANT: In Linux, files can't be executed/run as code unless they are specifically marked as executable. So, whenever we create a new Python node, we must mark it as executable. We do this by using the chmod command with the +x flag (add executable permissions) and giving it the path to our Python code:

rosstd
roscd beginner_tutorials
chmod +x src/add_two_ints_server.py

If you do not do this, you will see a helpful error telling you that ROS either couldn't find your script or it was not marked as executable, e.g:

[rosrun] Couldn't find executable named add_two_ints_server.py below /home/ubuntu/.2023RobotCode.readonly/zebROS_ws/src/beginner_tutorials
[rosrun] Found the following, but they're either not files,
[rosrun] or not executable:
[rosrun]   /home/ubuntu/.2023RobotCode.readonly/zebROS_ws/src/beginner_tutorials/src/add_two_ints_server.py

The Code Explained

Now, let's break the code down.

There's very little to writing a service using rospy. We declare our node using init_node() and then declare our service:

    s = rospy.Service('add_two_ints', AddTwoInts, handle_add_two_ints)

This declares a new service named add_two_ints with the AddTwoInts service type. All requests are passed to handle_add_two_ints function. handle_add_two_ints is called with instances of AddTwoIntsRequest and returns instances of AddTwoIntsResponse.

Just like with the subscriber example, rospy.spin() keeps your code from exiting until the service is shutdown.

Writing the Client Node

The Code

Create the src/add_two_ints_client.py file within the beginner_tutorials package and paste the following inside it:

#!/usr/bin/env python3
import sys
import rospy
from beginner_tutorials.srv import AddTwoInts, AddTwoIntsRequest, AddTwoIntsResponse
def add_two_ints_client(a, b):
    rospy.wait_for_service('add_two_ints')
    try:
        add_two_ints = rospy.ServiceProxy('add_two_ints', AddTwoInts)
        resp1 = add_two_ints(a, b)
        return resp1.sum
    except rospy.ServiceException as e:
        rospy.logerr(f"Service call failed: {e}")

def usage():
    return f"{sys.argv[0]} [a b]"

if __name__ == "__main__":
    if len(sys.argv) == 3:
        a = int(sys.argv[1])
        b = int(sys.argv[2])
    else:
        print(usage())
        sys.exit(1)
    rospy.init_node('add_two_ints_client', anonymous=True)
    rospy.loginfo(f"Requesting {a}+{b}")
    result = add_two_ints_client(a, b)
    print(f"{a} + {b} = {result}")

Don't forget to make the node executable:

rosstd
roscd beginner_tutorials
chmod +x src/add_two_ints_client.py

The Code Explained

Now, let's break the code down.

The client code for calling services is also simple. We first call:

    rospy.wait_for_service('add_two_ints')

This is a convenience method that blocks until the service named add_two_ints is available.

Next we create a handle for calling the service:

        add_two_ints = rospy.ServiceProxy('add_two_ints', AddTwoInts)

We can use this handle just like a normal function and call it:

        resp1 = add_two_ints(a, b)
        return resp1.sum

Because we've declared the type of the service to be AddTwoInts, it does the work of generating the AddTwoIntsRequest object for you (you're free to pass in your own instead). The return value is an AddTwoIntsResponse object. If the call fails, a rospy.ServiceException may be thrown, so you should setup the appropriate try/except block.

Try it out!

(helpful tip: type terminator to get a terminal that you can split horizontally and vertically into multiple terminals by right clicking)

In a new terminal inside Docker, run

rosstd
roscore -p 5802 &
rosrun beginner_tutorials add_two_ints_server.py

---

In a new terminal, run

rosstd
rosrun beginner_tutorials add_two_ints_client.py

• And you will see the usage information printed, similar to /home/ubuntu/2023RobotCode/zebROS_ws/src/beginner_tutorials/src/add_two_ints_client.py [a b]

In the same terminal, then run

rosrun beginner_tutorials add_two_ints_client.py 4 5

• And you will get

    Requesting 4+5
    4 + 5 = 9

• And the server will print out

    Returning [4 + 5 = 9]

• And the turtles will rejoice amid the great integer adding

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