500+ REST API Interview Questions with Answers 2026

Detailed Exam Domain Coverage

This comprehensive practice test suite is engineered to thoroughly assess and reinforce your knowledge across the core domains evaluated during technical screening rounds for engineering roles.

• HTTP Protocols (20%): Deconstruction of core HTTP methods (GET, POST, PUT, DELETE, PATCH), safe vs. unsafe verbs, header manipulation, and protocol specifics.

• REST Architecture (18%): Core architectural constraints including statelessness, client-server separation, resource identification through structured URIs, and accurate mapping of HTTP status codes.

• API Design (15%): Designing predictable API endpoints, body payload formats (JSON, XML), robust error handling payload structures, versioning strategies (URI, Header, Query string), and pagination mechanics.

• Security and Authentication (12%): Implementation of OAuth workflows, signing and verifying JSON Web Tokens (JWT), SSL/TLS encryption mechanisms, input validation, and defensive API authentication strategies.

• Advanced REST Concepts (10%): Hypermedia (HATEOAS), idempotency guarantees, rate limiting algorithms, multi-level caching strategies, and content negotiation.

• Comparison with Other APIs (8%): Structural architectural evaluations comparing SOAP vs REST, GraphQL vs REST, and the distinct design principles driving RESTful web services.

• Troubleshooting and Best Practices (7%): Practical debugging techniques, standard error handling strategies, performance optimization, structural API documentation, and testing methodologies.

• Tools and Technologies (10%): Leveraging Swagger and OpenAPI specifications, working with JAX-RS ecosystems, configuring API gateways, and managing enterprise API management platforms.

About the Course

Navigating modern system design and backend engineering interviews requires a deep, production-level understanding of API lifecycle operations. Landing a role as an API Developer, Full Stack Engineer, or Software Architect depends on your ability to handle real-world edge cases—like choosing between PUT and PATCH, preventing token manipulation, handling race conditions via HTTP headers, or designing optimal rate-limiting strategies.

I designed this extensive repository of 550 highly specific, original practice questions to help you master the exact scenarios technical panels evaluate. Instead of basic definitions, this test bank dives straight into architectural tradeoffs, security implications, and error states. Every question is backed by a thorough, step-by-step breakdown explaining why the correct choice stands up to architectural standards and why the alternative selections cause bugs, performance drops, or security flaws in a live environment. Utilizing these realistic scenario-based evaluations ensures you enter your interview with the absolute confidence required to pass your technical screening rounds on your very first attempt.

Sample Practice Questions Preview

Review these three production-grade sample questions to preview the depth, structure, and quality of the explanations provided throughout this entire test preparation bank.

Question 1: Idempotency Realities in HTTP Method Execution

An engineer needs to design a set of endpoints for an e-commerce platform. While evaluating network retry behaviors, they must guarantee that duplicate incoming client requests do not cause unexpected side effects in the system state. Which of the following accurately describes the idempotency guarantees of standard HTTP methods?

• A) POST and PUT are inherently idempotent, while PATCH and DELETE are strictly non-idempotent.

• B) GET, PUT, and DELETE are idempotent methods, whereas POST is inherently non-idempotent.

• C) All standard HTTP methods are completely idempotent as long as the backend database handles transactions correctly.

• D) Only GET and HEAD requests are idempotent because they do not modify the server state at all.

• E) PUT is completely non-idempotent because it creates a new entity representation every single time it runs.

• F) DELETE is non-idempotent because running it a second time against a missing resource returns a 404 instead of a 200.

Correct Answer & Explanation:

• Correct Answer: B

• Why it is correct: By HTTP specification standards, an idempotent operation can be executed multiple times without changing the final state of the server beyond the initial request. GET is safe and idempotent as it only retrieves data. PUT replaces the target resource entirely, meaning multiple identical PUT requests result in the exact same resource state. DELETE is idempotent because once a resource is removed, subsequent delete operations leave the system in that same "removed" state, despite variations in the response code returned. POST is explicitly non-idempotent because multiple execution instances create duplicate resources.

• Why alternative options are incorrect:

  • Option A is incorrect: POST is not idempotent, and DELETE is idempotent.

  • Option C is incorrect: The HTTP specification dictates method properties, not the database layer logic.

  • Option D is incorrect: GET and HEAD are safe methods, but PUT and DELETE are also strictly idempotent even though they alter state.

  • Option E is incorrect: PUT updates or replaces the resource at a specific URI, making its final state predictable across retries.

  • Option F is incorrect: The returned status code variation (200 OK vs 404 Not Found) does not break idempotency; what matters is that the backend server state remains identical after the first deletion.

Question 2: Designing Graceful Versioning Strategies for Enterprise Clients

A team needs to transition an active production API from version 1 to version 2 due to breaking changes in the response payload structure. The architect wants to implement a versioning approach that does not break existing client integrations, supports browser caching mechanisms out of the box, and keeps the resource URIs clean. Which strategy fits these requirements best?

• A) Use URI path versioning exclusively by prefixing all endpoints with /v1/ and /v2/.

• B) Use custom header versioning by requiring clients to send an X-API-Version key in their requests.

• C) Use content negotiation via the standard Accept header to define payload schemas.

• D) Duplicate the entire server infrastructure to run the new API version on a separate subdomain.

• E) Avoid explicit versioning by writing complex conditional statements inside a single unified endpoint controller code block.

• F) Implement query string versioning by appending a version parameter onto the end of every URL string.

Correct Answer & Explanation:

• Correct Answer: C

• Why it is correct: Content negotiation versioning leverages the standard Accept header (e.g., Accept: application/vnd. company.v2+json). This keeps the resource URI completely clean and consistent, which aligns with true REST principles. It also supports standard HTTP caching engines by utilizing the Vary: Accept response header to distinguish cached schemas cleanly.

• Why alternative options are incorrect:

  • Option A is incorrect: URI path versioning alters the URL, meaning the exact same conceptual resource has two different identifiers, which violates REST principles and complicates long-term caching strategies.

  • Option B is incorrect: Custom headers hide version details from the URL, but they break standard proxy caching because most downstream internet proxies do not cache separate variants based on custom headers.

  • Option D is incorrect: Splitting entire infrastructure environments creates massive operational overhead and does not resolve individual application-layer caching rules.

  • Option E is incorrect: Omitting versioning rules and relying on internal conditional blocks degrades code maintainability and causes severe technical debt.

  • Option F is incorrect: Query string versioning compromises clean URI patterns and frequently conflicts with custom query filtering or sorting arrays.

Question 3: Handling Token Validation Faults inside an API Gateway Layer

An API Gateway intercepts an incoming request containing an expired JSON Web Token (JWT) in the Authorization header. The gateway layer must halt processing immediately to prevent the request from hitting downstream microservices. According to REST architectural design principles, what is the most appropriate HTTP response payload response?

• A) Return a status code of 400 Bad Request along with a plain text string describing the token expiration.

• B) Return a status code of 401 Unauthorized accompanied by a WWW-Authenticate challenge header detailing the error.

• C) Return a status code of 403 Forbidden because the user identity inside the expired token payload is recognized.

• D) Return a status code of 500 Internal Server Error because validation failed inside the core gateway system logic.

• E) Return a status code of 404 Not Found to obscure the security architecture layout from potential malicious actors.

• F) Return a status code of 200 OK with a custom internal error code parameter embedded inside the JSON body.

Correct Answer & Explanation:

• Correct Answer: B

• Why it is correct: A 401 Unauthorized response is specifically intended for scenarios where authentication is missing, invalid, or has expired. The HTTP specification also states that a 401 response should include a WWW-Authenticate header to instruct the client on how to properly authenticate or refresh their tokens.

• Why alternative options are incorrect:

  • Option A is incorrect: 400 Bad Request indicates malformed request syntax, not a failure of valid credentials.

  • Option C is incorrect: 403 Forbidden means the server understands who the user is, but the authenticated user explicitly lacks authorization rights for that specific resource.

  • Option D is incorrect: 500 Internal Server Error implies a server-side bug or system crash, whereas token expiration is an expected validation outcome.

  • Option E is incorrect: Using 404 to hide endpoints complicates client debugging and degrades the predictability of the API interface.

  • Option F is incorrect: Returning a 200 OK for an authentication failure violates the core mapping rules of HTTP status codes and breaks client-side interceptor logic.

What to Expect

• Welcome to the Interview Questions Tests to help you prepare for your REST API Interview Questions Practice Test.

• You can retake the exams as many times as you want

• This is a huge original question bank

• You get support from instructors if you have questions

• Each question has a detailed explanation

• Mobile-compatible with the Udemy app

I hope that by now you're convinced! And there are a lot more questions inside the course.