When to use services vs topics in ROS - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding communication patterns in ROS

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

A When you want to continuously stream sensor data to multiple nodes.

B When you want to broadcast messages without expecting a reply.

C When you need a one-time request and response interaction between nodes.

D When you want to log data asynchronously without blocking the sender.

💡 Hint

Think about whether you need a reply or just send data.

Services in ROS are used for synchronous communication where a node sends a request and waits for a response. Topics are used for asynchronous, continuous data streaming.

❓ component_behavior

intermediate

Behavior of topics vs services in ROS

What happens if a ROS node calls a service but the service server is not available?

A The client node retries the request silently without notifying the user.

B The client node waits indefinitely until the service becomes available.

C The client node publishes the request on a topic instead.

D The client node immediately receives an error indicating the service is unavailable.

💡 Hint

Consider how synchronous calls behave when the server is missing.

Service clients in ROS receive an error if the service server is not available when they call it. They do not wait indefinitely or switch to topics.

❓ state_output

advanced

Output behavior of topic subscribers

If a ROS topic publisher sends messages at 10 Hz but a subscriber processes messages at 5 Hz, what happens to the messages?

A The subscriber receives and processes every message without loss.

B The subscriber processes every other message, dropping some messages in between.

C The subscriber crashes due to message overflow.

D The subscriber queues all messages and processes them later, causing delay.

💡 Hint

Think about how message queues and processing rates affect message delivery.

If the subscriber cannot keep up, it will drop messages depending on queue size and settings. Usually, it processes the latest messages, skipping some older ones.

📝 Syntax

advanced

Correct ROS service definition syntax

Which of the following is the correct way to define a ROS service file with a request containing an int32 and a response containing a string?

A

int32 request_data
---
string response_message

B

int32 request_data
string response_message

C

string request_data
---
string response_message

D

int32 request_data
---
int32 response_message

💡 Hint

Remember the separator between request and response in service files.

ROS service files separate request and response parts with '---'. The types must match the intended data.

🔧 Debug

expert

Debugging a ROS node using topics and services

A ROS node subscribes to a topic and calls a service based on the received message. The service call never completes, and the node hangs. What is the most likely cause?

A The service server is not running or crashed, so the client waits indefinitely.

B The topic publisher is sending messages too fast, causing buffer overflow.

C The subscriber callback is not properly registered, so messages are never received.

D The service client is calling the service asynchronously without waiting for a response.

💡 Hint

Consider what happens when a service client calls a missing server.

If the service server is not available, the client waits for a response and can hang. Other issues would cause different symptoms.

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