49+ Most Common REST API Interview Questions

Fresher Level REST API Interview Questions

1. What is REST?

Representational State Transfer (REST) is an architectural style for designing networked applications that treat every piece of data as a “resource” addressable via URLs and manipulated through standard HTTP methods. RESTful services are stateless (no client context stored on server), use a uniform interface (URI naming, standard verbs, self-descriptive messages), and can optionally employ hypermedia links (HATEOAS) to guide clients. This simplicity and alignment with the Web’s native protocols make REST immensely popular for web and mobile APIs.

2. What is an API?

An Application Programming Interface (API) is a contract exposing an application’s functionality to other software. For web APIs, this means a set of endpoints (URIs) you call with HTTP methods, passing request bodies (JSON/XML), headers (authentication, content type), and receiving responses (status codes, payload). APIs abstract internal implementation, enabling modular development, third-party integrations, and language-agnostic access.

3. What is the difference between REST and SOAP?

REST is lighter and web-native; SOAP provides enterprise features like built-in transactions and formal contracts.

4. What are the HTTP methods used in RESTful services?

• GET: Retrieve data (no side effects).
• POST: Create new resource (non-idempotent).
• PUT: Replace or create a resource at a known URI (idempotent).
• PATCH: Apply partial updates (idempotent when properly implemented).
• DELETE: Remove a resource (idempotent).
• HEAD, OPTIONS: Retrieve metadata or supported methods.

5. Explain the difference between GET, POST, PUT, and DELETE.

6. What is a RESTful web service?

An API adhering to REST constraints (stateless, uniform interface, resource-oriented, cacheable). Endpoints represent resources via URIs and support standard verbs. For example:

• GET /articles → list
• POST /articles → create
• GET /articles/{id} → retrieve
• PUT /articles/{id} → update/replace
• DELETE /articles/{id} → delete

7. What is the HTTP status code for a successful GET request?

200 OK (with response body). If there’s nothing to return but it succeeded, 204 No Content.

8. What is the purpose of the 404 status code?

Not Found indicates the requested URI/resource doesn’t exist on the server (or is hidden due to permissions).

9. What is a URI in the context of RESTful services?

A Uniform Resource Identifier uniquely names a resource. Example:

– scheme (https), host (api.example.com), path (/users/42/friends), optional query and fragment.

10. What is the difference between PUT and PATCH?

• PUT: Replaces the entire resource. Fields omitted in the request are removed/overwritten.
• PATCH: Applies partial changes; only the supplied fields are updated.

11. What are the components of a URL in a RESTful web service?

• Scheme: http:// or https://
• Authority: Domain (and optional port), e.g., api.example.com:8080
• Path: Resource location, e.g. /orders/123
• Query string: Optional filters, e.g.,?status=shipped&page=2
• Fragment: Rarely used in APIs, e.g. #section

12. What does JSON stand for, and why is it commonly used in REST APIs?

JavaScript Object Notation – a lightweight, human-readable format. It’s language-agnostic, easy to parse/generate, and more compact than XML, making it ideal for web/mobile clients.

13. Explain the concept of statelessness in REST.

Each request contains all the information for the server to process it (authentication, parameters). Servers store no client session, simplifying scaling (any server can handle any request) and failure recovery.

14. How do you handle errors in REST APIs?

Use appropriate HTTP status codes (4XX for client errors, 5XX for server errors) and include a structured error payload, e.g.:

{
  "error": {
   "code": 400,
  "message": "Invalid user ID",
 "details": ["User ID must be numeric."]
  }
}

15. What are the different types of HTTP requests used in REST APIs?

GET, POST, PUT, PATCH, DELETE, HEAD, OPTIONS, each serving distinct CRUD or metadata retrieval functions.

16. What is the significance of HTTP headers in RESTful APIs?

Headers carry metadata and control behavior:

• Content negotiation: Content-Type, Accept
• Authentication: Authorization: Bearer <token>
• Caching: Cache-Control, ETag
• Custom: X-Request-ID, feature flags, etc.

17. What are the common HTTP methods used in RESTful web services?

GET, POST, PUT, PATCH, DELETE, plus HEAD and OPTIONS for metadata.

18. What is an API endpoint?

A specific URL (URI) exposing a resource’s functionality (e.g., GET /products, POST /products).

19.How would you test a simple REST API?

• Manual: LambdaTest KaneAI, Insomnia
• CLI: curl or HTTPie
• Automated: Unit/integration tests with Jest, pytest, JUnit
• Mock: Simulate upstream/downstream systems

20. What is the importance of using HTTPS instead of HTTP in RESTful APIs?

HTTPS encrypts traffic via TLS, preventing eavesdropping and tampering, and ensuring data integrity and confidentiality.

Intermediate Level Rest API Interview Questions

21. Explain the difference between SOAP and RESTful Web Services.

SOAP is a heavyweight protocol with strict XML envelopes, WSDL contracts, and built-in WS-Security/transactions. REST is lightweight, uses native HTTP verbs, flexible formats (JSON/XML), and stateless interactions.

22.What is API authentication, and why is it important?

Verifies caller identity to prevent unauthorized access. Common schemes: API keys, HTTP Basic Auth, OAuth 2.0, JWT.

23. What is OAuth 2.0, and how does it work in REST APIs?

An authorization framework where clients obtain access tokens via flows (authorization code, client credentials). Tokens (Bearer) are presented in Authorization headers; servers validate scopes, expiration, and signatures.

24. Explain the use of the Authorization header in HTTP requests.

Carries credentials:

Authorization: Bearer <token>


Authorization: Basic <base64(user:pass)>

Custom schemes like ApiKey <key>

25. What is a JSON Web Token (JWT), and how is it used in REST APIs?

A compact token carrying claims (user ID, roles, expiration). It has three parts (header, payload, signature). Clients store JWTs and send them with requests; servers verify the signature and claims statelessly.

26. What are the different HTTP status codes used in RESTful services?

Common codes:

• 2XX: 200 OK, 201 Created, 204 No Content
• 4XX: 400 Bad Request, 401 Unauthorized, 403 Forbidden, 404 Not Found, 409 Conflict, 422 Unprocessable Entity, 429 Too Many Requests
• 5XX: 500 Internal Server Error, 503 Service Unavailable

27. How do you implement pagination in REST APIs

• Offset-based: ?page=3&limit=20 with metadata (total, pages)
• Cursor-based: ?cursor=eyJpZCI6MTAwfQ==&limit=20 returning nextCursor

28. What is CORS (Cross-Origin Resource Sharing), and how does it affect REST APIs?

Browsers block cross-origin AJAX unless servers include headers like:

Access-Control-Allow-Origin: https://app.example.com

• Access-Control-Allow-Methods: GET, POST
• Access-Control-Allow-Headers: Authorization, Content-Type

29. What are the advantages of using RESTful APIs over other types of APIs?

• Simplicity (HTTP/URLs)
• Lightweight (JSON)
• Statelessness (easy scaling)
• Language-agnostic
• Built-in caching
• Rich tooling (Swagger, KaneAI)

30. Explain how to version a REST API.

• URI versioning: /v1/users vs /v2/users
• Header versioning: Accept: application/vnd.example.v2+json
• Query param: ?version=2

31. What is HATEOAS (Hypermedia As The Engine Of Application State)?

Servers include links in responses:

{
  "id": 5,
  "name": "Item",
  "links": [
    { "rel":"self", "href":"/items/5" },
    { "rel":"orders","href":"/items/5/orders" }
  ]
}

Clients navigate via these links instead of hard-coding URIs.

32. What are the main design constraints of REST?

• Client–Server separation
• Statelessness
• Cacheability
• Uniform interface
• Layered system
• (Optional) Code on Demand

33. What is the purpose of the Accept and Content-Type headers in a REST API request?

• Content-Type: media type of the request body (e.g., application/json).
• Accept: desired response format (e.g., application/json, application/xml).

34. How do you handle rate limiting in REST APIs?

• Use token/leaky-bucket algorithms.
• Return headers:

X-RateLimit-Limit: 1000
X-RateLimit-Remaining: 250
X-RateLimit-Reset: 1616161616

• On exceed: 429 Too Many Requests with Retry-After.

• Use plural nouns for resources (/users).
• Follow HTTP semantics.
• Consistent naming and status codes.
• Support filtering, sorting, pagination.
• Version from day one.
• Secure with HTTPS and proper auth.
• Document via OpenAPI/Swagger.

36. Explain the concept of RESTful routing and its importance.

Predictable routes make APIs intuitive.

37. What are the potential security vulnerabilities in REST APIs, and how do you mitigate them?

• SQL Injection: Use parameterized queries.
• XSS: sanitize outputs.
• Broken Auth: strong tokens, multi-factor.
• IDOR: verify permissions per resource.
• CSRF: same-site cookies, anti-CSRF tokens.
• Mitigation: input validation, HTTPS, rate limiting, regular audits.

38. What is a REST API framework? Can you name some popular frameworks?

Libraries that scaffold routing, validation, and serialization:

• Node.js: Express, NestJS, Koa
• Python: Django REST Framework, Flask-RESTful, FastAPI
• Java: Spring Boot, JAX-RS (Jersey)
• Ruby: Rails, Grape
• PHP: Laravel, Lumen
• Go: Gin, Echo

39. Explain how caching works in REST APIs and why it’s important.

• Use standard headers:Cache-Control: public, max-age=3600
• ETag: "33a64df"
• Clients and proxies cache safe GET responses and revalidate with If-None-Match. Caching reduces latency and server load.

Advanced Level REST API Interview Questions

40. What is the difference between a stateless and a stateful API?

• Stateless: no server session, each request is independent. Pros: easy scaling; Cons: clients must send full context.
• Stateful: server retains session info. Pros: simplifies complex workflows; Cons: harder to scale, requires session management.

41. How do you handle concurrency in REST APIs?

• Optimistic locking: include a version/timestamp; reject updates on mismatch.
• Pessimistic locking: lock resource until operation completes (rare in REST).
• ETags & If-Match: use ETag headers to ensure client updates only current versions.

42. What is the role of content negotiation in RESTful APIs?

Servers inspect the client’s Accept header and choose the best representation (JSON, XML, CSV). This lets one endpoint serve multiple formats seamlessly.

43. How do you ensure the scalability of REST APIs?

• Horizontal scaling: stateless servers behind load balancers.
• Caching: CDNs, reverse proxies, client caching.
• Asynchronous processing: message queues for heavy tasks.
• Database optimizations: indexing, sharding, read replicas.
• Microservices: decouple components for independent scaling.

44.What is the role of API gateways in microservices architecture?

A single entry point that handles routing, authentication, rate limiting, caching, and request aggregation, decoupling clients from individual microservices.

45. What is the use of WebSockets in RESTful services?

For real-time, bi-directional communication (chat, live feeds) complementing REST’s request/response model. Use WebSockets for updates and REST for CRUD.

46. How can you achieve idempotency in REST APIs?

• Idempotent methods: GET, PUT, DELETE by default.
• Idempotency keys: clients supply a unique key (e.g., header Idempotency-Key); the server stores and reuses the response for duplicate requests.

47. What are the main principles of RESTful API design, and how do they impact scalability?

• Statelessness: simplifies load balancing.
• Uniform interface: enables caching, tooling.
• Cacheability: reduces redundant processing.
• Layered system: allows intermediaries (proxies, gateways) to offload tasks.

48. Explain the concept of lazy loading in REST APIs.

Delay the retrieval of non-essential or large nested data until requested. Techniques:

• Field selection: ?fields=name,email
• Expansions: ?expand=orders,profile

Saves bandwidth and improves perceived performance.

49. How do you handle transactional integrity in RESTful APIs?

Distributed or multi-step operations require patterns like:

• Two-Phase Commit: guarantees atomicity but can block resources.
• Saga pattern: chain of local transactions with compensating actions on failure.
• Eventual consistency: let services synchronize asynchronously via events, accepting temporary divergence.