Why services handle request-response interactions in ROS - See It in Action

Understanding ROS Services for Request-Response Interactions

📖 Scenario: You are building a simple robot control system using ROS (Robot Operating System). You want the robot to perform specific actions only when requested, and get a clear response back confirming the action or providing data.

🎯 Goal: Build a ROS service that handles a request from a client and sends back a response, demonstrating how services manage request-response interactions in ROS.

📋 What You'll Learn

Create a ROS service server node with a specific service name

Define a service type with request and response fields

Implement the service callback function to process requests and send responses

Create a ROS service client node that sends a request and waits for the response

💡 Why This Matters

🌍 Real World

Robots often need to perform specific actions only when asked, such as moving to a position or reading a sensor value. ROS services let one part of the robot ask another part to do something and wait for the answer.

💼 Career

Understanding ROS services is essential for robotics developers to build modular and interactive robot software that communicates reliably.

1

Create the ROS service definition file

Create a ROS service definition file named AddTwoInts.srv with two integers a and b as request fields, and one integer sum as the response field.

ROS

# Your code here

Hint

This file defines the request and response parts separated by ---.

2

Create the ROS service server node

Write a Python ROS node named add_two_ints_server.py that imports rospy and the AddTwoInts service. Initialize the node with rospy.init_node('add_two_ints_server') and create a service named 'add_two_ints' using rospy.Service with the service type AddTwoInts and a callback function handle_add_two_ints.

ROS

import rospy
from your_package.srv import AddTwoInts, AddTwoIntsResponse

# Your code here

Hint

The callback function takes the request and returns the response.

3

Create the ROS service client node

Write a Python ROS node named add_two_ints_client.py that imports rospy and the AddTwoInts service. Initialize the node with rospy.init_node('add_two_ints_client'). Wait for the service 'add_two_ints' to be available using rospy.wait_for_service('add_two_ints'). Create a service proxy with rospy.ServiceProxy('add_two_ints', AddTwoInts) and call it with a=5 and b=3 to get the response.

ROS

import rospy
from your_package.srv import AddTwoInts

# Your code here

Hint

The client sends a request and waits for the response from the server.

4

Print the response from the service

Add a line to print the sum field from the response object returned by the service call in add_two_ints_client.py.

ROS

import rospy
from your_package.srv import AddTwoInts

rospy.init_node('add_two_ints_client')
rospy.wait_for_service('add_two_ints')
add_two_ints = rospy.ServiceProxy('add_two_ints', AddTwoInts)
response = add_two_ints(a=5, b=3)
# Your code here

Hint

Access the sum attribute from the response object to display the result.

Practice

1. Why do ROS services handle request-response interactions instead of continuous data streams?

easy

A. Because services continuously send data without waiting

B. Because services are designed for immediate answers to specific requests

C. Because services only work with sensor data streams

D. Because services store data for later use

Solution

1. Step 1: Understand the purpose of ROS services

   ROS services are meant to handle specific requests and provide immediate responses, unlike topics which handle continuous data streams.

2. Step 2: Compare services with other communication methods

   Services wait for a request, process it, and send back a response, making them ideal for commands or queries needing quick answers.

3. Final Answer:

   Because services are designed for immediate answers to specific requests -> Option B

4. Quick Check:

   Request-response = immediate answers [OK]

Hint: Services = ask once, get answer once [OK]

Common Mistakes:

• Confusing services with continuous data streams
• Thinking services store data permanently
• Assuming services only handle sensor data

2. Which of the following is the correct way to define a ROS service in code?

easy

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

B. service = rospy.Publisher('add_two_ints', AddTwoInts, queue_size=10)

C. service = rospy.Subscriber('add_two_ints', AddTwoInts, callback)

D. service = rospy.ActionServer('add_two_ints', AddTwoInts, execute_cb)

Solution

1. Step 1: Identify the correct ROS service creation syntax

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

2. Step 2: Differentiate from other communication types

   Publisher and Subscriber are for topics, ActionServer is for actions, not services.

3. Final Answer:

   service = rospy.Service('add_two_ints', AddTwoInts, handle_add_two_ints) -> Option A

4. Quick Check:

   rospy.Service creates services [OK]

Hint: Services use rospy.Service, not Publisher or Subscriber [OK]

Common Mistakes:

• Using Publisher or Subscriber instead of Service
• Confusing ActionServer with Service
• Missing the handler function argument

3. Given this ROS service client code snippet, what will be the output if the service returns sum=7?

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(3, 4)
print(response.sum)

medium

A. 3

B. 4

C. 7

D. None

Solution

1. Step 1: Understand the service call and response

   The client calls the service with arguments 3 and 4, expecting the service to add them and return the sum.

2. Step 2: Check the printed output

   The response.sum contains the result from the service, which is 7, so print(response.sum) outputs 7.

3. Final Answer:

   7 -> Option C

4. Quick Check:

   3 + 4 = 7 output [OK]

Hint: Service response holds result in response.sum [OK]

Common Mistakes:

• Printing input arguments instead of response
• Assuming response is None before service call
• Confusing service call with topic publish

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

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

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

medium

A. The handler function is missing parameters

B. The service name is invalid

C. rospy.spin() should be called before service creation

D. The handler must return a response object, not just a sum

Solution

1. Step 1: Check the handler function return type

   The handler must return a response object of the service type, not just a raw sum value.

2. Step 2: Understand ROS service response structure

   Returning only req.a + req.b causes an error; the correct return is AddTwoIntsResponse(req.a + req.b).

3. Final Answer:

   The handler must return a response object, not just a sum -> Option D

4. Quick Check:

   Handler returns response object [OK]

Hint: Handler returns response object, not raw value [OK]

Common Mistakes:

• Returning raw values instead of response objects
• Calling rospy.spin() too early
• Incorrect service name format

5. You want to design a ROS system where a robot arm must receive a command and confirm completion immediately. Why is using a service better than a topic for this interaction?

hard

A. Because services provide a direct request-response, ensuring the command was received and processed

B. Because topics automatically guarantee message delivery and response

C. Because topics are faster for one-time commands

D. Because services can send continuous sensor data streams

Solution

1. Step 1: Analyze the communication needs for command and confirmation

   The robot arm needs to know immediately if the command was received and completed, requiring a synchronous interaction.

2. Step 2: Compare services and topics for this use case

   Services provide a request-response pattern, confirming completion, while topics are asynchronous and do not guarantee immediate feedback.

3. Final Answer:

   Because services provide a direct request-response, ensuring the command was received and processed -> Option A

4. Quick Check:

   Request-response ensures confirmation [OK]

Hint: Use services for commands needing immediate confirmation [OK]

Common Mistakes:

• Thinking topics guarantee immediate response
• Using services for continuous data streaming
• Confusing speed with communication pattern