When to use services vs topics in ROS - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a ROS topic publisher.

ROS

publisher = node.create_publisher(std_msgs.msg.String, '[1]', 10)

Drag options to blanks, or click blank then click option'

A/service_call

B/request

C/get_data

D/chatter

Attempts:

3 left

💡 Hint

Common Mistakes

Using service names for topic publishers

Confusing topic and service naming conventions

The /chatter topic is a common example name for publishing messages in ROS topics.

2fill in blank

medium

Complete the code to call a ROS service client.

ROS

client = node.create_client([1], 'add_two_ints')

Drag options to blanks, or click blank then click option'

Aexample_interfaces.srv.AddTwoInts

Bgeometry_msgs.msg.Twist

Csensor_msgs.msg.Image

Dstd_msgs.msg.String

Attempts:

3 left

💡 Hint

Common Mistakes

Using message types instead of service types

Confusing topics with services

Services require a service type like AddTwoInts from example_interfaces.srv.

3fill in blank

hard

Fix the error in the code to correctly wait for a service to be available.

ROS

if not client.[1](timeout_sec=5.0):
    node.get_logger().error('Service not available')

Drag options to blanks, or click blank then click option'

Await_for_service

Bwait_for_topic

Cwait_for_message

Dwait_for_response

Attempts:

3 left

💡 Hint

Common Mistakes

Using topic or message wait methods for services

Misspelling the method name

The method wait_for_service waits until the service is ready to be called.

4fill in blank

hard

Fill both blanks to create a topic subscriber with a callback function.

ROS

subscription = node.create_subscription([1], '[2]', callback, 10)

Drag options to blanks, or click blank then click option'

Astd_msgs.msg.String

B/service_call

C/chatter

Dexample_interfaces.srv.AddTwoInts

Attempts:

3 left

💡 Hint

Common Mistakes

Using service types for subscribers

Using service names as topic names

Subscribers use message types like std_msgs.msg.String and topic names like /chatter.

5fill in blank

hard

Fill all three blanks to define a service server with a callback.

ROS

service = node.create_service([1], '[2]', [3])

Drag options to blanks, or click blank then click option'

Aexample_interfaces.srv.AddTwoInts

B/add_two_ints

Chandle_add_two_ints

Dstd_msgs.msg.String

Attempts:

3 left

💡 Hint

Common Mistakes

Using message types instead of service types

Confusing topic names with service names

Not providing a callback function

Service servers need the service type, service name, and a callback function.

Practice

1. In ROS, when should you use a service instead of a topic?

easy

A. When you want to continuously stream sensor data.

B. When you need a single request and a single response.

C. When you want to broadcast messages to many nodes.

D. When you want to log data over time.

Solution

1. Step 1: Understand service communication

   Services in ROS are designed for one-time request and reply interactions between nodes.

2. Step 2: Compare with topic usage

   Topics are used for continuous data streaming or broadcasting messages to multiple subscribers.

3. Final Answer:

   When you need a single request and a single response. -> Option B

4. Quick Check:

   Service = single request-reply [OK]

Hint: Services = one-time ask, Topics = ongoing talk [OK]

Common Mistakes:

• Using topics for single request-response needs
• Using services for continuous data streaming
• Confusing broadcast with one-time communication

2. Which of the following is the correct syntax to define a ROS service client in Python?

easy

A. client = rospy.Subscriber('service_name', ServiceType, callback)

B. client = rospy.Service('service_name', ServiceType)

C. client = rospy.ServiceProxy('service_name', ServiceType)

D. client = rospy.Publisher('service_name', ServiceType, queue_size=10)

Solution

1. Step 1: Recall ROS Python service client syntax

   The correct way to create a service client is using rospy.ServiceProxy with the service name and type.

2. Step 2: Identify incorrect options

   rospy.Service defines a service server, rospy.Subscriber subscribes to topics, and rospy.Publisher publishes to topics.

3. Final Answer:

   client = rospy.ServiceProxy('service_name', ServiceType) -> Option C

4. Quick Check:

   Service client uses rospy.ServiceProxy [OK]

Hint: Service client uses rospy.ServiceProxy() [OK]

Common Mistakes:

• Confusing service client with service server syntax
• Using Subscriber or Publisher for services
• Missing service type argument

3. Given the following ROS Python code snippet, what will be printed?

import rospy
from std_srvs.srv import SetBool

def call_service():
    rospy.wait_for_service('toggle_led')
    try:
        toggle_led = rospy.ServiceProxy('toggle_led', SetBool)
        response = toggle_led(True)
        print(response.success)
    except rospy.ServiceException as e:
        print('Service call failed:', e)

call_service()

medium

A. True if the service call succeeds, otherwise an error message.

B. Always prints False.

C. SyntaxError due to missing import.

D. No output because the service is not called.

Solution

1. Step 1: Analyze service call behavior

   The code waits for the 'toggle_led' service, then calls it with argument True, and prints the success field from the response.

2. Step 2: Understand possible outputs

   If the service call succeeds, response.success will be True or False depending on service logic; if it fails, an exception message prints.

3. Final Answer:

   True if the service call succeeds, otherwise an error message. -> Option A

4. Quick Check:

   Service call prints success or error [OK]

Hint: ServiceProxy call prints success field if no error [OK]

Common Mistakes:

• Assuming service call always returns False
• Confusing service call with topic subscription
• Ignoring exception handling

4. You wrote a ROS node that uses a service client but it never receives a response. What is the most likely cause?

medium

A. The service server is not running or not advertised.

B. The topic name is misspelled.

C. The node is publishing to the wrong topic.

D. The callback function is missing in the subscriber.

Solution

1. Step 1: Identify service client behavior

   A service client waits for a service server to be available and respond to requests.

2. Step 2: Diagnose no response issue

   If no response arrives, the service server might not be running or the service name is incorrect, so the client cannot connect.

3. Final Answer:

   The service server is not running or not advertised. -> Option A

4. Quick Check:

   No response means server missing [OK]

Hint: No service response? Check if server is running [OK]

Common Mistakes:

• Confusing service client with topic subscriber
• Checking wrong topic instead of service
• Ignoring service server status

5. You want to control a robot arm to move to a specific position and wait for confirmation before sending the next command. Which ROS communication method should you use and why?

hard

A. Use a topic because continuous streaming of positions is required.

B. Use a service because topics do not support multiple subscribers.

C. Use a topic because services cannot send commands.

D. Use a service because you need a request and a reply for each move command.

Solution

1. Step 1: Understand the control requirement

   The robot arm needs to receive a move command and confirm completion before the next command.

2. Step 2: Match communication type to requirement

   Services provide a request-response pattern ideal for commands needing confirmation, unlike topics which are for continuous data streaming without guaranteed replies.

3. Final Answer:

   Use a service because you need a request and a reply for each move command. -> Option D

4. Quick Check:

   Command with confirmation = service [OK]

Hint: Commands needing reply? Use services [OK]

Common Mistakes:

• Choosing topics for commands needing confirmation
• Thinking services can't send commands
• Confusing subscriber limits with communication type