Service definition (.srv files) in ROS - Mini Project: Build & Apply

Create a ROS Service Definition (.srv file)

📖 Scenario: You are building a simple ROS (Robot Operating System) service that adds two integers. This service will receive two numbers and return their sum.

🎯 Goal: Create a ROS service definition file named AddTwoInts.srv that defines the request and response structure for adding two integers.

📋 What You'll Learn

Create a service definition file named AddTwoInts.srv

Define two integers named a and b in the request part

Define one integer named sum in the response part

Separate request and response parts with --- line

💡 Why This Matters

🌍 Real World

ROS services allow robots to perform tasks on request, like adding numbers or controlling hardware.

💼 Career

Understanding .srv files is essential for robotics developers working with ROS to define communication between nodes.

1

Create the request part of the service

In the AddTwoInts.srv file, write two lines to define the request part with two integers named a and b.

ROS

# Define two integers a and b for the request
# Your code here

Hint

The request part lists the inputs. Use int64 for integers.

2

Add the separator between request and response

Add a line with exactly --- below the request part to separate request and response in AddTwoInts.srv.

ROS

int64 a
int64 b
# Add separator line '---' below
# Your code here

Hint

The separator --- divides request and response parts.

3

Define the response part of the service

Below the separator line ---, define one integer named sum as the response in AddTwoInts.srv.

ROS

int64 a
int64 b
---
# Define one integer sum for the response
# Your code here

Hint

The response part lists the output. Use int64 sum for the result.

4

Complete the AddTwoInts.srv file

Ensure the AddTwoInts.srv file has the request part with int64 a and int64 b, the separator ---, and the response part with int64 sum.

ROS

int64 a
int64 b
---
int64 sum
# Final check: file complete
# Your code here

Hint

Make sure all parts are present exactly as specified.

Practice

1. What is the main purpose of a .srv file in ROS?

easy

A. To define the structure of a service with request and response parts

B. To list all the topics a node subscribes to

C. To specify the launch configuration for nodes

D. To store parameters for the robot

Solution

1. Step 1: Understand what a .srv file represents

   A .srv file defines a service interface in ROS, which includes a request and a response part.

2. Step 2: Match the purpose to the options

   Only To define the structure of a service with request and response parts correctly states that .srv files define the structure of a service with request and response parts.

3. Final Answer:

   To define the structure of a service with request and response parts -> Option A

4. Quick Check:

   .srv files define service structure = C [OK]

Hint: Remember .srv files split request and response with --- [OK]

Common Mistakes:

• Confusing .srv files with launch files
• Thinking .srv files list topics
• Assuming .srv files store parameters

2. Which of the following is the correct syntax to separate request and response fields in a .srv file?

easy

A. Using a colon ':' between request and response

B. Using a comma ',' between request and response

C. Using a semicolon ';' between request and response

D. Using three dashes '---' between request and response

Solution

1. Step 1: Recall the .srv file format

   The .srv file separates request and response fields with three dashes '---'.

2. Step 2: Identify the correct separator

   Only Using three dashes '---' between request and response correctly uses '---' to separate the two parts.

3. Final Answer:

   Using three dashes '---' between request and response -> Option D

4. Quick Check:

   Request and response separated by --- = A [OK]

Hint: Look for '---' as the separator in .srv files [OK]

Common Mistakes:

• Using commas or semicolons instead of ---
• Confusing with YAML or JSON syntax
• Missing the separator line entirely

3. Given this .srv file content:

int32 a
int32 b
---
int32 sum

What will the response contain when the service is called with request values a=3 and b=4 if the service adds them?

medium

A. sum = 12

B. sum = 34

C. sum = 7

D. sum = 0

Solution

1. Step 1: Understand the request and response fields

   The request has two integers a and b; the response has one integer sum.

2. Step 2: Apply the service logic

   The service adds a and b, so sum = 3 + 4 = 7.

3. Final Answer:

   sum = 7 -> Option C

4. Quick Check:

   3 + 4 = 7 [OK]

Hint: Add request fields to get response sum [OK]

Common Mistakes:

• Concatenating numbers as strings
• Multiplying instead of adding
• Returning zero by mistake

4. Identify the error in this .srv file:

float64 x
float64 y
float64 z
---
float64 result

medium

A. No error, this is a valid .srv file

B. Response field should be on the same line as request fields

C. Missing separator '---' between request and response

D. Request fields must be integers, not float64

Solution

1. Step 1: Check the separator

   The separator '---' is present between request and response fields.

2. Step 2: Validate field types and format

   Using float64 for request and response fields is valid in .srv files.

3. Final Answer:

   No error, this is a valid .srv file -> Option A

4. Quick Check:

   Correct separator and valid types = A [OK]

Hint: Look for '---' and valid types to confirm syntax [OK]

Common Mistakes:

• Thinking separator is missing when it is present
• Believing response must be on same line
• Assuming only integers allowed

5. You want to define a service that takes a string command and an integer timeout as request, and returns a boolean success and a string message as response. Which of these .srv files correctly defines this?

hard

A. string command
int32 timeout
---
bool success, string message

B. string command
int32 timeout
---
bool success
string message

C. string command
int32 timeout
bool success
string message

D. string command
int32 timeout
---
string message
bool success

Solution

1. Step 1: Check for correct separator and field order

   The request fields are before '---' and response fields after. string command int32 timeout --- bool success string message correctly uses '---' and lists request then response fields.

2. Step 2: Validate field types and syntax

   string command int32 timeout --- bool success string message lists request fields (string, int32) and response fields (bool, string) correctly without syntax errors.

3. Final Answer:

   string command int32 timeout --- bool success string message -> Option B

4. Quick Check:

   Request before --- and response after = D [OK]

Hint: Request fields above ---; response fields below --- [OK]

Common Mistakes:

• Placing response fields before separator
• Missing separator line
• Using commas inside .srv files