Top 10 API Testing Interview QnA

API Testing: 10 Commonly Asked Interview Questions and Answers

API testing is a critical aspect of software development. Here are the 10 most commonly asked interview questions and answers to help you prepare:

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1. What is API Testing?

API Testing involves verifying application programming interfaces to ensure they meet functionality, reliability, performance, and security requirements. It focuses on sending requests to APIs and validating the responses.

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2. What is the difference between REST and SOAP APIs?

Feature	REST	SOAP
Protocol	Architectural style	Protocol-based
Data Format	JSON or XML	XML only
State	Stateless	Supports stateful operations
Transport	HTTP/HTTPS	HTTP/HTTPS, SMTP, etc.
Performance	Lightweight, faster	Heavy, slower

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3. Explain common HTTP methods used in API testing.

• GET: Retrieve data from the server.
• POST: Submit data to the server to create a resource.
• PUT: Update or replace an existing resource.
• DELETE: Remove a resource from the server.

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4. What are the standard HTTP status codes, and what do they mean?

Code	Meaning
200	OK (Success)
400	Bad Request (Client Error)
401	Unauthorized
500	Internal Server Error

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5. What is the difference between API Testing and Unit Testing?

• API Testing: Validates end-to-end business logic and data flow across systems.
• Unit Testing: Focuses on individual components of code to ensure they work as expected.

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6. Which tools are commonly used for API Testing?

• Postman: For manual testing and script automation.
• Rest Assured: A Java-based library for automated API testing.
• SoapUI: Suitable for both SOAP and REST API testing.

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7. What are the different types of API authentication?

• OAuth: Token-based secure authentication.
• Basic Auth: Encoded username and password.
• API Key: A unique identifier to access APIs.

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8. What are some best practices for writing API test cases?

• Cover all API endpoints.
• Test for valid and invalid inputs.
• Validate response status codes, headers, and payloads.
• Include edge case scenarios and performance tests.

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9. How do you handle errors in APIs?

• Validate error codes like 4xx (client errors) and 5xx (server errors).
• Ensure appropriate error messages are returned.
• Test scenarios for timeout and rate-limiting issues.

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10. What is a Mock API, and why is it used?

A Mock API simulates the behavior of a real API. It is used when the actual API is unavailable or under development, enabling developers and testers to continue their work without delays.

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Examples:

Example 1: Validating a GET request with Postman

1. Send a GET request to /users.
2. Verify the response status code is 200.
3. Check that the response contains expected fields like id, name, and email.

Example 2: Testing error handling for invalid inputs

1. Send a POST request to /login with an incorrect password.
2. Validate the response code is 401.
3. Ensure the error message is descriptive (e.g., "Invalid credentials").

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Conclusion:

Preparing for API testing interviews requires understanding the fundamentals, knowing the tools, and practicing common scenarios. These questions and answers provide a solid foundation to help you succeed.