Why services handle request-response interactions in ROS

Introduction

Services let different parts of a robot talk by asking questions and getting answers. This helps robots do tasks step-by-step.

When a robot needs to ask for specific information and wait for the answer before moving on.

When you want to control a robot's action that requires confirmation, like turning on a light.

When a robot part needs to send a command and get a result immediately.

When you want to keep communication simple and clear between robot components.

When you want to avoid continuous data flow and just need one-time responses.

Syntax

ROS

ros::ServiceServer service = nh.advertiseService("service_name", serviceCallback);

bool serviceCallback(RequestType &req, RequestType::Response &res) {
    // process request
    // set response
    return true;
}

The service has a name and a callback function that handles requests and sends responses.

The callback returns true if the service worked correctly.

Examples

This example adds two numbers sent in the request and returns the sum.

ROS

bool addTwoIntsCallback(AddTwoInts::Request &req, AddTwoInts::Response &res) {
    res.sum = req.a + req.b;
    return true;
}

This line creates a service named "add_two_ints" that uses the callback above.

ROS

ros::ServiceServer service = nh.advertiseService("add_two_ints", addTwoIntsCallback);

Sample Program

This program creates a ROS service called "add_two_ints". When a request with two numbers comes in, it adds them and sends back the sum. The server waits and handles requests one by one.

ROS

#include "ros/ros.h"
#include "beginner_tutorials/AddTwoInts.h"

bool add(beginner_tutorials::AddTwoInts::Request &req,
         beginner_tutorials::AddTwoInts::Response &res) {
    res.sum = req.a + req.b;
    ROS_INFO("Request: a=%ld, b=%ld", (long int)req.a, (long int)req.b);
    ROS_INFO("Sending back response: [%ld]", (long int)res.sum);
    return true;
}

int main(int argc, char **argv) {
    ros::init(argc, argv, "add_two_ints_server");
    ros::NodeHandle nh;

    ros::ServiceServer service = nh.advertiseService("add_two_ints", add);
    ROS_INFO("Ready to add two ints.");
    ros::spin();
    return 0;
}

Important Notes

Services are synchronous: the client waits for the server to reply before continuing.

Use services for tasks that need a clear question and answer, not continuous data streams.

Make sure the service callback returns true to confirm successful handling.

Summary

Services let robot parts ask questions and get answers immediately.

They are good for commands or requests that need a quick response.

Services keep communication simple and organized in ROS.

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