API development is essential for building software that can communicate and interact with other applications or services. Developers working with APIs need to understand concepts like REST, authentication, data formats, and error handling. Below are 25 commonly asked API Development interview questions with answers to help you prepare effectively.

Q1. What is an API?

An API (Application Programming Interface) is a set of rules and protocols that allows different software applications to communicate with each other.

Q2. What is REST?

REST (Representational State Transfer) is an architectural style for designing networked applications, using standard HTTP methods like GET, POST, PUT, and DELETE.

Q3. What are the main HTTP methods used in RESTful APIs?

GET (retrieve data), POST (create data), PUT (update data), DELETE (remove data), PATCH (partial update).

Q4. What is the difference between REST and SOAP?

REST is an architectural style using simple HTTP protocols and multiple data formats like JSON or XML. SOAP is a protocol with strict standards using only XML.

Q5. What is JSON and why is it commonly used in APIs?

JSON (JavaScript Object Notation) is a lightweight data interchange format that is easy to read and write, widely used for API communication.

Q6. What are API endpoints?

API endpoints are specific URLs where APIs can be accessed by clients to perform operations like data retrieval or submission.

Q7. What is authentication in APIs?

Authentication verifies the identity of a client accessing the API, often using methods like API keys, OAuth tokens, or JWT.

Q8. What is authorization in APIs?

Authorization determines what resources or actions a verified client is allowed to access or perform.

Q9. What are status codes in HTTP?

HTTP status codes indicate the result of an HTTP request, such as 200 (OK), 404 (Not Found), 401 (Unauthorized), 500 (Server Error).

Q10. What is idempotency in APIs?

An idempotent API call means that making the same request multiple times has the same effect as making it once (e.g., PUT and DELETE requests).

Q11. What is throttling in APIs?

Throttling limits the number of API requests a client can make within a certain timeframe to prevent abuse or overload.

Q12. What is CORS?

CORS (Cross-Origin Resource Sharing) is a security feature that controls how resources on a web server can be requested from another domain.

Q13. What is an API gateway?

An API gateway is a server that acts as an entry point, managing API requests, routing, authentication, and rate limiting.

Q14. What is versioning in APIs and why is it important?

Versioning manages changes in API endpoints or data formats to ensure backward compatibility for existing clients.

Q15. What are query parameters in an API request?

Query parameters are key-value pairs appended to the URL to filter, sort, or paginate data (e.g., ?page=2&limit=10).

Q16. What is the difference between PUT and PATCH?

PUT updates the entire resource, while PATCH applies partial modifications.

Q17. How do you handle errors in API development?

By returning appropriate HTTP status codes and error messages in the response body.

Q18. What is a webhook?

A webhook is a way for an API to send real-time notifications to other systems by making HTTP callbacks when certain events occur.

Q19. What tools do you use to test APIs?

Common tools include Postman, Insomnia, curl, and automated testing frameworks like REST Assured.

Q20. What is rate limiting?

Rate limiting restricts how many API requests a user can make in a specified period to protect the server.

Q21. Explain statelessness in REST APIs.

Each API request contains all necessary information, and the server does not store any client context between requests.

Q22. What is OAuth?

OAuth is an open standard for token-based authentication and authorization used to grant access without sharing credentials.

Q23. How do you secure an API?

By using HTTPS, authentication tokens, input validation, rate limiting, and proper error handling.

Q24. What is the difference between synchronous and asynchronous API calls?

Synchronous calls block until a response is received; asynchronous calls allow other processing while waiting for the response.

Q25. What are RESTful API best practices?

Use proper HTTP methods, consistent resource naming, statelessness, versioning, meaningful status codes, and thorough documentation.