Creating a service server in ROS - Try It Yourself

Creating a service server

📖 Scenario: You are building a simple ROS service server that will respond to requests by adding two integers. This is useful in robotics when you want to perform calculations on demand.

🎯 Goal: Build a ROS service server node that waits for requests with two integers and returns their sum.

📋 What You'll Learn

Create a ROS node named add_two_ints_server

Define a service called AddTwoInts using the standard ROS service type

Implement a callback function named handle_add_two_ints that takes the request and returns the sum

Initialize the service server with the name add_two_ints

💡 Why This Matters

🌍 Real World

Robots often need to perform calculations or actions on demand. Service servers let robots respond to specific requests like adding numbers or moving to a location.

💼 Career

Understanding how to create and run ROS service servers is essential for robotics software engineers who build interactive robot systems.

1

Create the initial ROS node and import necessary modules

Write the first lines to import rospy and AddTwoInts from beginner_tutorials.srv. Then create a ROS node named add_two_ints_server using rospy.init_node.

ROS

# Import rospy and AddTwoInts service
# Initialize the node with name 'add_two_ints_server'
# Your code here

Hint

Use import rospy and from beginner_tutorials.srv import AddTwoInts. Then call rospy.init_node('add_two_ints_server').

2

Define the service callback function

Define a function called handle_add_two_ints that takes a single argument req. Inside the function, return AddTwoIntsResponse(req.a + req.b) to add the two integers from the request.

ROS

import rospy
from beginner_tutorials.srv import AddTwoInts, AddTwoIntsResponse

rospy.init_node('add_two_ints_server')

# Define the callback function handle_add_two_ints(req)
# Your code here

Hint

Define handle_add_two_ints(req) and return AddTwoIntsResponse(req.a + req.b).

3

Create the service server

Create a service server named service by calling rospy.Service with the service name 'add_two_ints', service type AddTwoInts, and callback function handle_add_two_ints.

ROS

import rospy
from beginner_tutorials.srv import AddTwoInts, AddTwoIntsResponse

rospy.init_node('add_two_ints_server')

def handle_add_two_ints(req):
    return AddTwoIntsResponse(req.a + req.b)

# Create the service server named 'service'
# Your code here

Hint

Use rospy.Service('add_two_ints', AddTwoInts, handle_add_two_ints) and assign it to service.

4

Keep the service server running

Add a call to rospy.spin() to keep the service server running and waiting for requests.

ROS

import rospy
from beginner_tutorials.srv import AddTwoInts, AddTwoIntsResponse

rospy.init_node('add_two_ints_server')

def handle_add_two_ints(req):
    return AddTwoIntsResponse(req.a + req.b)

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

# Keep the service server running
# Your code here

Hint

Call rospy.spin() to keep the node alive and responsive.

Practice

1. What is the main role of a service server in ROS?

easy

A. To publish messages continuously

B. To wait for requests and send back replies

C. To subscribe to topics and process data

D. To log information to the console

Solution

1. Step 1: Understand the service server role

   A service server in ROS waits for incoming requests from clients and sends back replies after processing.

2. Step 2: Differentiate from other ROS nodes

   Unlike publishers or subscribers, a service server handles request-response communication, not continuous data streams.

3. Final Answer:

   To wait for requests and send back replies -> Option B

4. Quick Check:

   Service server = request and reply [OK]

Hint: Service servers handle requests and replies, not continuous data [OK]

Common Mistakes:

• Confusing service servers with publishers
• Thinking service servers subscribe to topics
• Assuming service servers only log data

2. Which of the following is the correct way to create a service server in ROS using rospy?

easy

A. server = rospy.Client('service_name', ServiceType, handler_function)

B. server = rospy.Publisher('service_name', ServiceType, handler_function)

C. server = rospy.Subscriber('service_name', ServiceType, handler_function)

D. server = rospy.Service('service_name', ServiceType, handler_function)

Solution

1. Step 1: Identify the correct rospy function

   The function to create a service server is rospy.Service, which takes the service name, service type, and handler function.

2. Step 2: Exclude other ROS communication methods

   Publisher and Subscriber are for topics, Client is not a rospy function for servers.

3. Final Answer:

   server = rospy.Service('service_name', ServiceType, handler_function) -> Option D

4. Quick Check:

   Service server creation = rospy.Service [OK]

Hint: Use rospy.Service to create service servers, not Publisher or Subscriber [OK]

Common Mistakes:

• Using rospy.Publisher instead of rospy.Service
• Confusing service server with subscriber
• Trying to use a non-existent rospy.Client

3. Given the following ROS service server code snippet, what will be the output when a client sends a request with data=5?

import rospy
from beginner_tutorials.srv import AddTwoInts, AddTwoIntsResponse

def handle_add_two_ints(req):
    return AddTwoIntsResponse(req.a + req.b)

rospy.init_node('add_two_ints_server')
service = rospy.Service('add_two_ints', AddTwoInts, handle_add_two_ints)
rospy.spin()

medium

A. The server returns the sum of req.a and req.b

B. The server returns the product of req.a and req.b

C. The server returns the difference of req.a and req.b

D. The server returns an error because of missing return statement

Solution

1. Step 1: Analyze the handler function

   The function handle_add_two_ints returns the sum of req.a and req.b wrapped in AddTwoIntsResponse.

2. Step 2: Understand the server behavior on request

   When a client sends a request with data=5 (assuming req.a=5 and req.b is provided), the server replies with their sum.

3. Final Answer:

   The server returns the sum of req.a and req.b -> Option A

4. Quick Check:

   Handler returns sum = The server returns the sum of req.a and req.b [OK]

Hint: Handler returns sum of inputs; check return statement carefully [OK]

Common Mistakes:

• Assuming multiplication instead of addition
• Forgetting to return a response object
• Confusing request fields names

4. Identify the error in this ROS service server code snippet:

import rospy
from beginner_tutorials.srv import AddTwoInts, AddTwoIntsResponse

def handle_add(req):
    sum = req.a + req.b

rospy.init_node('add_two_ints_server')
service = rospy.Service('add_two_ints', AddTwoInts, handle_add)
rospy.spin()

medium

A. The handler function does not return a response object

B. The service name is invalid

C. The node is not initialized

D. The service type is incorrect

Solution

1. Step 1: Check the handler function

   The handler function calculates sum but does not return AddTwoIntsResponse, which is required.

2. Step 2: Verify other parts of the code

   The service name, node initialization, and service type are correct and valid.

3. Final Answer:

   The handler function does not return a response object -> Option A

4. Quick Check:

   Handler must return response [OK]

Hint: Always return a response object from the handler function [OK]

Common Mistakes:

• Forgetting to return AddTwoIntsResponse
• Misnaming the handler function
• Not calling rospy.init_node

5. You want to create a ROS service server that returns the maximum of two integers sent by the client. Which handler function correctly implements this behavior?

hard

A. def handle_max(req):
return MaxIntsResponse(max(req.a, req.b))

B. def handle_max(req):
return MaxIntsResponse(req.a + req.b)

C. def handle_max(req):
if req.a > req.b:
return MaxIntsResponse(req.a)
else:
return MaxIntsResponse(req.b)

D. def handle_max(req):
return MaxIntsResponse(min(req.a, req.b))

Solution

1. Step 1: Understand the requirement

   The server must return the maximum of two integers sent by the client.

2. Step 2: Evaluate each handler function

   def handle_max(req): return MaxIntsResponse(max(req.a, req.b)) uses max() but assumes MaxIntsResponse accepts a single argument correctly; def handle_max(req): if req.a > req.b: return MaxIntsResponse(req.a) else: return MaxIntsResponse(req.b) explicitly returns the max value using if-else and returns response properly; def handle_max(req): return MaxIntsResponse(req.a + req.b) returns sum, which is incorrect; def handle_max(req): return MaxIntsResponse(min(req.a, req.b)) returns minimum, which is wrong.

3. Final Answer:

   def handle_max(req): if req.a > req.b: return MaxIntsResponse(req.a) else: return MaxIntsResponse(req.b) -> Option C

4. Quick Check:

   Correct max logic with proper return = def handle_max(req): if req.a > req.b: return MaxIntsResponse(req.a) else: return MaxIntsResponse(req.b) [OK]

Hint: Use if-else to return max value wrapped in response [OK]

Common Mistakes:

• Returning sum instead of max
• Returning min instead of max
• Incorrectly using max() without proper response