OpenAPI Specification (Swagger) in Rest API

Introduction

OpenAPI Specification helps you describe your API clearly so others can understand and use it easily.

You want to share your API details with other developers.

You need to create automatic API documentation.

You want to test your API endpoints quickly.

You want to generate code or client libraries from your API.

You want to keep your API design consistent and clear.

Syntax

Rest API

openapi: 3.0.0
info:
  title: Sample API
  version: 1.0.0
paths:
  /items:
    get:
      summary: Get list of items
      responses:
        '200':
          description: Successful response
          content:
            application/json:
              schema:
                type: array
                items:
                  type: string

The specification is usually written in YAML or JSON format.

It starts with the OpenAPI version and includes info, paths, and components.

Examples

This example shows a simple GET endpoint that returns a greeting as plain text.

Rest API

openapi: 3.0.0
info:
  title: Simple API
  version: 1.0.0
paths:
  /hello:
    get:
      summary: Returns a greeting
      responses:
        '200':
          description: OK
          content:
            text/plain:
              schema:
                type: string

This is the same API written in JSON format, showing a POST endpoint to create a user.

Rest API

{
  "openapi": "3.0.0",
  "info": {
    "title": "JSON API",
    "version": "1.0.0"
  },
  "paths": {
    "/users": {
      "post": {
        "summary": "Create a user",
        "responses": {
          "201": {
            "description": "User created"
          }
        }
      }
    }
  }
}

Sample Program

This OpenAPI spec describes a GET endpoint at /books that returns a list of books with id, title, and author.

Rest API

openapi: 3.0.0
info:
  title: Book Store API
  version: 1.0.0
paths:
  /books:
    get:
      summary: List all books
      responses:
        '200':
          description: A list of books
          content:
            application/json:
              schema:
                type: array
                items:
                  type: object
                  properties:
                    id:
                      type: integer
                    title:
                      type: string
                    author:
                      type: string

Important Notes

You can use tools like Swagger UI to turn this spec into interactive documentation.

Keep your API spec updated as your API changes to avoid confusion.

Summary

OpenAPI Specification describes APIs in a clear, standard way.

It helps with documentation, testing, and code generation.

You write it in YAML or JSON format with sections like info and paths.

Practice

1. What is the main purpose of the OpenAPI Specification in REST APIs?

easy

A. To store user data securely

B. To execute API requests automatically

C. To replace the need for API servers

D. To describe the API endpoints, methods, and data formats in a standard way

Solution

1. Step 1: Understand OpenAPI's role

   OpenAPI Specification is designed to describe APIs clearly and consistently.

2. Step 2: Identify the correct purpose

   It documents endpoints, HTTP methods, parameters, and data formats for APIs.

3. Final Answer:

   To describe the API endpoints, methods, and data formats in a standard way -> Option D

4. Quick Check:

   OpenAPI = API description standard [OK]

Hint: OpenAPI is for describing APIs, not running or storing data [OK]

Common Mistakes:

• Confusing OpenAPI with API execution tools
• Thinking OpenAPI stores user data
• Believing OpenAPI replaces servers

2. Which of the following is the correct way to define an HTTP GET method for the path /users in OpenAPI YAML?

easy

A. paths:
get:
/users:
summary: Get all users

B. paths:
/users:
get:
summary: Get all users

C. get:
paths:
/users:
summary: Get all users

D. paths:
/users:
post:
summary: Get all users

Solution

1. Step 1: Check OpenAPI path structure

   Paths are keys under paths, then HTTP methods like get are nested under each path.

2. Step 2: Validate correct nesting and method

   paths: /users: get: summary: Get all users correctly places /users under paths and get under /users.

3. Final Answer:

   paths: /users: get: summary: Get all users -> Option B

4. Quick Check:

   Paths > /users > get [OK]

Hint: Remember: paths > endpoint > method > details [OK]

Common Mistakes:

• Swapping method and path levels
• Using wrong HTTP method for GET
• Incorrect YAML indentation

3. Given this OpenAPI snippet, what is the response code defined for a successful GET request to /items?

paths:
  /items:
    get:
      responses:
        '200':
          description: Successful response
        '404':
          description: Not found

medium

A. 200

B. 201

C. 500

D. 404

Solution

1. Step 1: Locate response codes under GET method

   The snippet shows responses under get for /items.

2. Step 2: Identify success response code

   Code 200 is labeled as "Successful response", indicating success.

3. Final Answer:

   200 -> Option A

4. Quick Check:

   Success = 200 HTTP code [OK]

Hint: Success responses usually use 200 code [OK]

Common Mistakes:

• Choosing 404 as success code
• Confusing 201 (created) with 200
• Ignoring response codes listed

4. Identify the error in this OpenAPI YAML snippet:

paths:
  /login:
    post:
      summary: User login
      responses:
        200:
          description: Login successful

medium

A. Summary field is not allowed under POST

B. Missing requestBody for POST method

C. Response code 200 should be a string '200', not a number

D. Path should start with a capital letter

Solution

1. Step 1: Check response code format

   OpenAPI requires response codes as strings, e.g., '200', not numbers.

2. Step 2: Verify other fields

   Summary is allowed; requestBody is optional; path case is flexible.

3. Final Answer:

   Response code 200 should be a string '200', not a number -> Option C

4. Quick Check:

   Response codes must be strings [OK]

Hint: Always quote response codes as strings in YAML [OK]

Common Mistakes:

• Using numeric response codes
• Assuming requestBody is mandatory
• Thinking summary is invalid

5. You want to document an API that returns a list of products with optional filtering by category. Which OpenAPI feature helps you define the optional category filter parameter for the GET /products endpoint?

hard

A. Use a query parameter with required: false under the GET method

B. Add a path parameter named category in the URL

C. Include category in the requestBody schema

D. Define category as a header parameter

Solution

1. Step 1: Understand parameter types

   Optional filters are usually query parameters, not path or header parameters.

2. Step 2: Define optional query parameter

   Set required: false for the category query parameter under GET.

3. Final Answer:

   Use a query parameter with required: false under the GET method -> Option A

4. Quick Check:

   Optional filters = query params with required false [OK]

Hint: Filters usually go as optional query parameters [OK]

Common Mistakes:

• Using path parameters for optional filters
• Putting filters in requestBody for GET
• Defining filters as headers