Request-response communication model in ROS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a ROS service client sends a request?

In ROS, when a service client sends a request to a service server, what is the expected behavior?

A The server sends a response before receiving the request from the client.

B The client sends the request and immediately continues without waiting for any response.

C The client waits for the server to process the request and send back a response before continuing.

D The client and server communicate only through topics, not services.

💡 Hint

Think about how request-response communication works in real life, like ordering food and waiting for it.

In ROS, a service client sends a request and waits for the server to process it and send back a response. This synchronous behavior ensures the client knows the result before moving on.

📝 Syntax

intermediate

Identify the correct ROS service client call syntax in Python

Which of the following Python code snippets correctly calls a ROS service named add_two_ints with two integers 5 and 3?

A response = add_two_ints_client.call(5, 3)

B response = add_two_ints_client.call(AddTwoIntsRequest(a=5, b=3))

C response = add_two_ints_client(5, 3)

D response = add_two_ints_client.request(5, 3)

💡 Hint

Remember that ROS service calls usually require a request object with named fields.

The correct way to call a ROS service client in Python is to create a request object with the required fields and pass it to the client's call() method.

🔧 Debug

advanced

Why does this ROS service client code raise an error?

Consider this Python ROS service client code snippet:

import rospy
from beginner_tutorials.srv import AddTwoInts

rospy.wait_for_service('add_two_ints')
add_two_ints = rospy.ServiceProxy('add_two_ints', AddTwoInts)
response = add_two_ints(5, '3')
print(response.sum)

Why does this code raise an error?

A Because the second argument '3' is a string instead of an integer.

B Because the service name 'add_two_ints' is incorrect.

C Because rospy.wait_for_service was not called before creating the proxy.

D Because the response object does not have a 'sum' attribute.

💡 Hint

Check the types of arguments passed to the service call.

The service expects two integers, but the second argument is a string. This type mismatch causes a runtime error.

❓ state_output

advanced

What is the output of this ROS service server callback?

Given this ROS service server callback in Python:

def handle_add_two_ints(req):
    sum_result = req.a + req.b
    return AddTwoIntsResponse(sum_result * 2)

# Assume AddTwoIntsResponse is imported correctly

If the client sends a request with a=4 and b=6, what will the client receive as the response?

A 10

B None

C 40

D 20

💡 Hint

Calculate the sum and then apply the multiplication as in the callback.

The sum of 4 and 6 is 10. The callback returns double that, so the response is 20.

🧠 Conceptual

expert

Which statement best describes the request-response communication model in ROS services?

Choose the statement that best explains how the request-response communication model works in ROS services.

A Services provide synchronous communication where clients send a request and block until the server sends back a response, ensuring immediate feedback.

B Services use topics internally to broadcast requests and responses to multiple clients simultaneously.

C Services allow asynchronous message passing where clients publish requests and servers subscribe to them without waiting for responses.

D Services are used only for streaming large amounts of data continuously between nodes.

💡 Hint

Think about how a phone call works compared to a radio broadcast.

ROS services implement synchronous request-response communication. The client waits for the server's reply before continuing, unlike asynchronous topic messaging.

Practice

1. In ROS, what does the request-response communication model primarily allow?

easy

A. One node sends a request and waits for a reply from another node

B. Nodes continuously publish data without expecting a reply

C. Nodes broadcast messages to all other nodes simultaneously

D. Nodes store data locally without communication

Solution

1. Step 1: Understand the request-response model in ROS

   This model involves one node sending a request message and waiting for a response from another node.

2. Step 2: Compare options with this model

   Only One node sends a request and waits for a reply from another node describes this direct ask-and-reply behavior, while others describe different communication types.

3. Final Answer:

   One node sends a request and waits for a reply from another node -> Option A

4. Quick Check:

   Request-response = direct ask and reply [OK]

Hint: Request-response means ask first, then wait for answer [OK]

Common Mistakes:

• Confusing request-response with continuous publishing
• Thinking request-response is broadcasting
• Assuming no reply is needed

2. Which of the following is the correct way to define a ROS service server in Python using the request-response model?

easy

A. service = rospy.Service('service_name', ServiceType, handler_function)

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

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

D. service = rospy.Client('service_name', ServiceType, handler_function)

Solution

1. Step 1: Recall ROS Python service server syntax

   In ROS, a service server is created using rospy.Service with the service name, type, and handler function.

2. Step 2: Identify correct function

   Only rospy.Service matches this pattern; Publisher and Subscriber are for topics, Client is not a rospy function.

3. Final Answer:

   service = rospy.Service('service_name', ServiceType, handler_function) -> Option A

4. Quick Check:

   Service server uses rospy.Service [OK]

Hint: Service server uses rospy.Service, not Publisher or Subscriber [OK]

Common Mistakes:

• Using Publisher or Subscriber instead of Service
• Confusing client and server creation
• Wrong function names like rospy.Client

3. Given this ROS Python service client code snippet, what will be printed if the service responds with response.result = 42?

import rospy
from example.srv import AddTwoInts

def call_service():
    rospy.wait_for_service('add_two_ints')
    try:
        add_two_ints = rospy.ServiceProxy('add_two_ints', AddTwoInts)
        response = add_two_ints(20, 22)
        print(response.result)
    except rospy.ServiceException as e:
        print("Service call failed", e)

call_service()

medium

A. Service call failed

B. 42

C. 20

D. None

Solution

1. Step 1: Understand the service client call

   The client waits for the service, calls it with 20 and 22, and expects a response with a 'result' attribute.

2. Step 2: Analyze the response printing

   The code prints response.result, which is given as 42 in the question.

3. Final Answer:

   42 -> Option B

4. Quick Check:

   Print response.result = 42 [OK]

Hint: Prints the response.result value returned by the service [OK]

Common Mistakes:

• Confusing request parameters with response
• Expecting print of input values instead of response
• Assuming exception without error

4. What is wrong with this ROS Python service server code snippet?

import rospy
from example.srv import Multiply

def handle_multiply(req):
    return req.a * req.b

rospy.init_node('multiply_server')
service = rospy.Service('multiply', Multiply, handle_multiply)
rospy.spin()

medium

A. The service type Multiply is not imported correctly

B. The service name cannot be 'multiply'

C. The handler must return a response object, not just a number

D. rospy.spin() is missing

Solution

1. Step 1: Check the handler function return type

   In ROS services, the handler must return a response message object, not a raw value.

2. Step 2: Identify the error in return statement

   Returning req.a * req.b (a number) is incorrect; it should return MultiplyResponse(req.a * req.b) or equivalent.

3. Final Answer:

   The handler must return a response object, not just a number -> Option C

4. Quick Check:

   Handler returns response object, not raw value [OK]

Hint: Handler must return response message object, not raw data [OK]

Common Mistakes:

• Returning raw values instead of response objects
• Forgetting to import response message class
• Missing rospy.spin() (though present here)

5. You want to implement a ROS service that returns the sum of two integers only if both are positive; otherwise, it returns zero. Which handler function correctly implements this request-response logic?

hard

A. def handle_sum(req):
return SumTwoIntsResponse(req.a + req.b)

B. def handle_sum(req):
return SumTwoIntsResponse(req.a + req.b if req.a > 0 or req.b > 0 else 0)

C. def handle_sum(req):
if req.a > 0 or req.b > 0:
return SumTwoIntsResponse(req.a + req.b)
else:
return SumTwoIntsResponse(0)

D. def handle_sum(req):
if req.a > 0 and req.b > 0:
return SumTwoIntsResponse(req.a + req.b)
else:
return SumTwoIntsResponse(0)

Solution

1. Step 1: Understand the condition for sum

   The sum should be returned only if both req.a and req.b are positive (greater than zero).

2. Step 2: Check each option's condition logic

   def handle_sum(req): if req.a > 0 and req.b > 0: return SumTwoIntsResponse(req.a + req.b) else: return SumTwoIntsResponse(0) uses 'and' to check both positive, returning sum or zero correctly. def handle_sum(req): return SumTwoIntsResponse(req.a + req.b if req.a > 0 or req.b > 0 else 0) uses 'or' incorrectly and missing parentheses in ternary. def handle_sum(req): if req.a > 0 or req.b > 0: return SumTwoIntsResponse(req.a + req.b) else: return SumTwoIntsResponse(0) uses 'or' which is wrong logic. def handle_sum(req): return SumTwoIntsResponse(req.a + req.b) always returns sum ignoring condition.

3. Final Answer:

   def handle_sum(req): if req.a > 0 and req.b > 0: return SumTwoIntsResponse(req.a + req.b) else: return SumTwoIntsResponse(0) -> Option D

4. Quick Check:

   Both positive check with 'and' = def handle_sum(req): if req.a > 0 and req.b > 0: return SumTwoIntsResponse(req.a + req.b) else: return SumTwoIntsResponse(0) [OK]

Hint: Use 'and' to check both positive before summing [OK]

Common Mistakes:

• Using 'or' instead of 'and' for both positive check
• Returning sum without condition
• Incorrect ternary syntax in return