?? Testing APIs: A Step-by-Step Guide for Manual Testers

API testing is a critical skill for manual testers. It ensures that applications function correctly, communicate seamlessly, and handle data securely. While automation plays a key role in API testing, manual testers are often the first line of defense in identifying defects before automated scripts take over.

This guide will walk you through how to manually test APIs, covering tools, techniques, and best practices to improve your efficiency and accuracy.

1?? Understanding API Testing

What is an API?

An Application Programming Interface (API) allows different software systems to communicate with each other. APIs define the rules for data exchange between applications.

Why is API Testing Important?

? Ensures functional correctness of API responses

? Detects integration issues before they impact users

? Improves security by identifying vulnerabilities

? Reduces reliance on UI tests, making the testing process faster

2?? Getting Started with API Testing

Before diving into testing, gather key information:

??API Documentation – Read API specs (Swagger, Postman collections, or API docs).

??Authentication Methods – Understand how to authenticate (API keys, OAuth, JWT).

??Endpoints & Methods – Identify API endpoints and their respective HTTP methods:

GET – Retrieve data

POST – Send new data

PUT/PATCH – Update existing data

DELETE – Remove data

??Request & Response Formats – Learn the data formats (JSON, XML) and status codes (200, 400, 500).

3?? Step-by-Step API Testing Process

Step 1. Choose a Testing Tool

?? Postman – User-friendly, allows sending API requests & validating responses

?? cURL – Command-line tool for API requests

?? Swagger UI – Interactive API documentation for testing

Step 2. Verify API Requests & Responses

Start testing by sending a simple request and checking the response.

Send a Request

- Choose an HTTP method (GET, POST, etc.).

- Provide necessary headers (e.g., Content-Type: application/json).

- Add query parameters or request body if needed.

Check the Response

- Ensure expected responses (e.g., 200 OK, 201 Created).

- Validate the correctness of data returned.

- Verify security headers like CORS, Cache-Control, and Content-Type.

??? Example GET Request to Retrieve User Data

GET https://api.example.com/users/123

Headers: Authorization: Bearer <token>

Expected Response (200 OK):

{

"id": 123,

"name": "John Doe",

"email": "[email protected]"

}

? If the API returns a 500 error, report it as a potential backend issue.

Step 3. Validate Edge Cases & Error Handling

Test negative scenarios to ensure API handles errors gracefully.

? Invalid Data Inputs:

- Send invalid/missing required fields in POST requests.

- Use incorrect data formats (e.g., sending a string instead of an integer).

??? Example of Testing Error Handling (Invalid ID)

GET https://api.example.com/users/xyz

Expected Response (400 Bad Request):

{

"error": "Invalid User ID"

}

Step 4. Test Authentication & Authorization

APIs often require authentication (tokens, API keys, OAuth).

? Valid Authentication:

- Use correct access tokens to verify successful requests.

? Invalid Authentication:

- Send a request without authentication – API should return 401 Unauthorized.

- Try accessing restricted resources with incorrect permissions – API should return 403 Forbidden.

Step 5. Performance & Rate Limit Testing

Check how the API handles high loads and multiple requests.

- Send rapid multiple requests (e.g., 100 requests in a minute).

- API should return 429 Too Many Requests if rate-limited.

??? Example: Testing Rate Limiting

GET https://api.example.com/users

Headers: Authorization: Bearer <token>

Expected Response (429 Too Many Requests):

{

"error": "Too many requests. Please try again later."

}

Step 6. Security Testing

Manually verify API vulnerabilities like:

SQL Injection – Try sending ' OR 1=1 in query parameters.

Cross-Site Scripting (XSS) – Input <script>alert(1)</script> and check responses.

CORS Configuration – Test if API allows unauthorized cross-origin requests.

4?? Best Practices for Manual API Testing

• Always validate responses against API documentation.
• Use different test data (valid, invalid, and edge cases).
• Check request time – Slow API responses may indicate performance issues.
• Document test results – Report any failures with details for developers.
• Test APIs before UI – This helps catch issues early before they affect the front end.

5?? Conclusion

Manual API testing is essential for identifying functional, security, and performance issues before they impact users. By following this structured step-by-step process, testers can ensure API reliability and prevent defects from reaching production.

?? Want to make API testing more efficient? TestCaseLab can help you manage and organize your test cases seamlessly! Try it free for 30 days. https://bit.ly/3O8Exmn

?? Have you encountered API testing challenges? Share your experiences in the comments!

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