What Is a REST API? – REST API Basics

A REST API (Representational State Transfer) is a set of rules that allows one piece of software or system to interact with another. It’s stateless and it uses standard HTTP methods. And REST API is the backbone of modern web and mobile applications.

A visual representation of REST API response body in postman tool

Understand the basics of REST API:

  • Learn about resources: Think it like nouns (Examples: "users", "products" etc.).
  • Understand HTTP methods like GET, POST, PUT, DELETE — these represent some actions on resources.
  • Remember that REST is stateless, that means each API call is independent and it doesn’t rely on past API calls.
  • Explore JSON format — JSON is the most commonly used data structure in REST APIs.

Related: What is REST?

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Core REST API Concepts for Interviews

This section is focused on key REST concepts for interviews.

Flowchart diagram showing how the REST API works – with some basic HTTP response code

Prepare by mastering these key areas:

  • Resource Naming: Use nouns in plural (e.g., /users, not /getUser)
  • HTTP Status Codes:
    • 200 OK
    • 201 Created
    • 400 Bad Request
    • 404 Not Found
    • 500 Internal Server Error
  • Idempotency: Learn how PUT and DELETE are idempotent, but POST is not idempotent.
  • Versioning APIs: Include versions in the URL (/v1/users) or headers.
  • HATEOAS (Hypermedia as the Engine of Application State): It’s an advanced topic — so mention it briefly. know details about it.

Pro Tip: Never return 200 ok when a 500 occurs in the server. Or never return 404 when it’s 401 or 403. This is a common malpractice. This can result in tech debt. Avoid this.

Related: HTTP request methods

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Top REST API Interview Questions and Answers

Whether you are a beginner or expert, you will be asked these questions in any REST API interview:

Most Common REST API Questions

Q1. What is a RESTful API and how it is different from SOAP?
Pro Tip: This shows your understanding on system architecture. You should mention statelessness, simplicity, and JSON use in REST.

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Q2. When should you use GET vs. POST methods?
Pro Tip: Interviewers want to check your knowledge of idempotency and data retrieval vs creation.

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Q3. How do you handle error responses in REST APIs?
Pro Tip: Demonstrates how well you can handle issues through proper status codes and error bodies.

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Q4. When is it appropriate to use PUT and when to use PATCH?
Pro Tip: PUT replaces the entire resource, while PATCH updates part of the resource — many developers confuse this.

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Q5. What are some common security threats in REST APIs? How to prevent that?
Pro Tip: Mention authentication, rate limiting, throttling and input validation.

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Q6. How do you document a REST API?
Pro Tip: Mention some tools like Swagger, how it matters in real-world projects.

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Q7. Can you explain the concept of versioning an API?
Pro Tip: This shows you understand long-term API stability. Mention URI versioning (/v1/resource), header versioning, and also don’t forget backward compatibility.

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Q8. How would you test a REST API?
Pro Tip: Mention tools like Postman, curl, and automated testing tools. Demonstrate edge cases as bonus points.

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A system design lead engineer is testing REST API against it’s performance and security

Q9. When do you cache API responses and how?
Pro Tip: Caching reduces load and boosts speed making your system more efficient. Talk about using Cache-Control, ETag, and reverse proxies like Varnish or CDN caching.

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Q10. What is CORS ? how do you handle CORS  in a REST API?
Pro Tip: Explain the Same-Origin Policy and Cross Origin request policy. Explain how to set correct headers (Access-Control-Allow-Origin). This shows your understanding of  cross-domain security.

 

Q11. How would you ensure your API scales with traffic?
Pro Tip: Discuss load balancing and horizontal scaling. Emphasize caching, async processing – it’s use cases etc. Talk about API monitoring tools like Prometheus or AWS CloudWatch.

 

Q12. What tools do you use to test and monitor APIs?
Pro Tip: Go beyond Postman. Include tools like New Relic, Datadog, Swagger Inspector, and even you can mention CI pipelines for test automation. Mention all the tools you know about – this at least shows your knowledge range.

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Q13. When should you return 204 - No Content?
Pro Tip: HTTP response code 204 is suitable  when request is successful but no response body is needed. Use cases- after a DELETE request or after a successful but silent PATCH operation.

 

Q14. When will you use GraphQL over REST?
Use GraphQL when the client needs flexible queries or cases you want to avoid over-fetching and under-fetching of data. GraphQL  becomes ideal when your UI is complex. So you need to fetch data in various structure and various types of data. It’s  also frequently needed for mobile apps, or situations where frontend teams need more control over the data structure.

Pro Tip: It shows you understand modern API design. Choosing GraphQL demonstrates that you're thinking in terms of client efficiency and scalable architecture.

Related: REST API Interview Questions

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REST API Security and Error Handling

You must show that you don’t just build APIs — you take the responsibility to protect them.

A messy codebase is being inspected using a magnifying glass symbolizing security pitfall inside REST API

Follow best practices:

  • Use HTTPS for all communications.
  • Implement authentication — via OAuth2, JWT tokens, or API keys.
  • Avoid exposing sensitive data like passwords, access tokens, or internal error stack traces. Disable verbose log in your framework.
  • Use proper status codes for errors (e.g., 401 Unauthorized, 403 Forbidden, 422 Unprocessable Entity).
  • Limit brute force attacks using rate limiting or throttling.

Pro Tip: Interviewers want to know you understand both prevention and graceful failure handling.

Related: OWASP API Security Project

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Performance Optimization and Best Practices

Fast and clean APIs are crucial in production.

A coder girl is sitting at a desk with two computers – optimizing REST API performance

Checklist:

  • Use pagination for large dataset (Example: /api/v2/users?page=2&size=20).
  • Compress response by  enabling Gzip.
  • Implement caching with Cache-Control and ETag headers.
  • Avoid deep nesting in responses. Make it flat as much as possible.
  • Use bulk endpoints when you are handling large updates or uploads.
  • Monitor response time.

Pro Tip: This signals your awareness of real-world performance concerns.

Related: REST API Tutorial

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REST API Interview Preparation Tips and Resources

Prepare smartly, not just hard:

  • Practice explaining REST API concepts. Remember you’ll be asked to explain, not just implement.
  • Use tools like Postman to simulate real API calls.
  • Write a dummy API using Node.js or Flask to show your understanding.
  • Write and host documentation with Swagger or Redoc.
  • Solve API challenges from platforms like LeetCode and HackerRank.
  • Review code samples that show clean REST API practices.

Related: HackerRank

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A man is interviewing a person for coding profession -  asking REST API interview questions

Final Touch

From CRUD apps to enterprise systems, REST API is essential. Both front-end and back-end engineers need it. Prepare in the right way. Focus on fundamentals, security, performance, and real-world concern — you will crack your next REST API interview.

So, here's your next move:

  • Revisit the REST API Interview Questions regularly.
  • Focus on clarity when explaining concepts.
  • Build and test your own APIs — even small ones.
  • Take every interview as a practicing session to get sharper - whether you get hired or not.

Remember: Your ability to clearly explain REST APIs reflects your understanding depth. Clarity is king in interviews. And real-world impacts matter more than just code.