Creating a service server in ROS - Practice Exercises

Challenge - 5 Problems

❓ component_behavior

intermediate

Service Server Response Behavior

Consider a ROS service server that adds two integers sent by a client. What will be the response if the client sends 3 and 5?

ROS

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()

A The server responds with the difference -2.

B The server responds with the product 15.

C The server responds with the sum 8.

D The server does not respond and times out.

💡 Hint

Look at the return statement inside the handler function.

The handler function returns the sum of the two integers, so the server responds with 8 when given 3 and 5.

📝 Syntax

intermediate

Correct Service Server Initialization Syntax

Which option correctly initializes a ROS service server named 'multiply' that uses the 'MultiplyTwoInts' service type and calls 'handle_multiply' as the handler?

ROS

import rospy
from beginner_tutorials.srv import MultiplyTwoInts

def handle_multiply(req):
    pass

rospy.init_node('multiply_server')

A service = rospy.Service('multiply', handle_multiply, MultiplyTwoInts)

B service = rospy.Service('multiply', MultiplyTwoInts, handle_multiply)

C service = rospy.Service(handle_multiply, 'multiply', MultiplyTwoInts)

D service = rospy.Service('multiply', MultiplyTwoInts())

💡 Hint

The order is: service name, service type, handler function.

The rospy.Service constructor requires the service name as a string, the service type class, and the handler function in that order.

🔧 Debug

advanced

Identify the Error in Service Handler

What error will occur when running this service server code?

ROS

import rospy
from beginner_tutorials.srv import AddTwoInts, AddTwoIntsResponse

def handle_add(req):
    return req.a + req.b

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

A TypeError because the handler returns int instead of AddTwoIntsResponse.

B SyntaxError due to missing colon in function definition.

C NameError because AddTwoIntsResponse is not imported.

D No error; the server works correctly.

💡 Hint

The handler must return a response object, not a raw value.

The handler returns an int, but ROS expects an AddTwoIntsResponse object. This causes a TypeError at runtime.

❓ state_output

advanced

Service Server State After Multiple Requests

A service server keeps a count of how many times it was called. What will be the value of count after three client requests?

ROS

import rospy
from beginner_tutorials.srv import Empty, EmptyResponse

count = 0

def handle_count(req):
    global count
    count += 1
    return EmptyResponse()

rospy.init_node('count_server')
service = rospy.Service('count_service', Empty, handle_count)
rospy.spin()

A count will be 3 after three requests.

B count will be 0 because global variables reset each call.

C count will be 1 regardless of requests.

D count will be 4 after three requests.

💡 Hint

Global variables keep their value between calls.

The global variable count increments by 1 each time the handler runs, so after three calls it is 3.

🧠 Conceptual

expert

Service Server Behavior on Shutdown

What happens to a ROS service server when rospy.signal_shutdown() is called?

A The service server restarts automatically.

B The service server continues running but ignores new requests.

C The service server crashes immediately without cleanup.

D The service server stops accepting requests and the node shuts down cleanly.

💡 Hint

Think about what signal_shutdown does to the node.

Calling rospy.signal_shutdown() triggers a clean shutdown of the node, stopping the service server from accepting new requests.

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