REST API Interview Questions and Answers (2026)
If you are preparing for a backend or full-stack developer interview, REST API questions are almost guaranteed to come up. Most modern applications — whether they are web apps, mobile apps, or microservices — communicate using REST APIs.
In many interviews, the interviewer doesn’t just check if you know definitions. They want to see if you understand how APIs actually work in real applications.
In this guide, I’ve compiled 40 REST API interview questions that developers commonly face, along with simple explanations.
Interview Question And Answers For Fresher To Experience :
1. What is a REST API?
Answer : REST API is a way for applications to communicate with each other using HTTP.
For example, when a mobile app shows a list of users, it doesn’t directly access the database. Instead, it sends a request to a server through an API.
The server processes the request and returns data, usually in JSON format.
Example : GET /api/users
2. What does REST stand for?
Answer : REST stands for Representational State Transfer.
It is not a technology or library. Instead, it is an architectural style used to design APIs that are simple, scalable, and easy to maintain.
3. What is a resource in REST?
Answer : In REST, everything is treated as a resource.
Examples of resources:
users
products
orders
comments
4. What is an endpoint?
Answer : An endpoint is the specific URL where an API can be accessed.
Example: https://example.com/api/users
Here /api/users is the endpoint used to fetch user data.
5. What are HTTP methods in REST APIs?
Answer : REST APIs mainly use these HTTP methods:
GET – retrieve data
POST – create new data
PUT – update existing data
PATCH – update partial data
DELETE – remove data
6. What is statelessness in REST?
Answer : REST APIs are stateless, which means the server does not store client session information.
Every request must contain all the information required to process it.
7. What format is commonly used in REST APIs?
Answer : Most REST APIs use JSON (JavaScript Object Notation) because it is lightweight and easy to read.
Example :
“id”: 1,
“name”: “Rahul”,
“email”: “rahul@email.com”
}
8. What are HTTP status codes?
Answer : Status codes indicate whether a request was successful or not.
Some commonly used :
200 – Request successful
201 – Resource created
400 – Bad request
401 – Unauthorized
404 – Resource not found
500 – Server error
9. What is the difference between PUT and PATCH?
Answer : Both are used to update data, but they behave differently.
PUT : Updates the entire resource.
Example : PUT /api/users/5
PATCH : Updates only specific fields.
PATCH is usually preferred when you only need to modify a few fields.
10. What is API versioning?
Answer : Over time, APIs change. But older applications might still rely on older versions.
To avoid breaking existing clients, APIs use versioning.
11. What is idempotency?
Answer : An operation is called idempotent if repeating it multiple times produces the same result.
12. What is caching in REST APIs?
Answer : Caching stores API responses temporarily so that the server does not need to process the same request repeatedly.
Benefits of caching:
faster response time
reduced server load
improved performance
13. What is authentication in APIs?
Answer : Authentication verifies who the user is.
Common methods :
API keys
JWT tokens
OAuth
14. What is authorization?
Answer : Authorization determines what a user is allowed to do after authentication.
Example:
Admin can delete users
Normal users cannot
15. What are query parameters?
Answer : Query parameters help filter data returned by an API.
Example : /api/products?category=mobile
16. What is pagination?
Answer :When an API returns a large dataset, pagination is used to divide it into smaller pages.
17. What is CORS?
Answer : CORS stands for Cross-Origin Resource Sharing.
It allows APIs to be accessed from a different domain.
For example:
A frontend running on: localhost:3000
can access an API hosted on: api.example.com
18. What is an API gateway?
Answer : An API gateway acts as a single entry point for multiple APIs.
19. What is rate limiting?
Answer : Rate limiting restricts how many API requests a client can send within a certain time.
20. What is the difference between REST and SOAP?
Answer :REST is lightweight and usually uses JSON.
SOAP is a protocol that uses XML and is more complex.
Because REST is simpler and faster, it is widely used in modern web development.
21. What is a URI?
Answer : URI stands for Uniform Resource Identifier. It is a string used to identify a resource on the internet.
A URI can represent anything such as a web page, an API endpoint, or a file.
22. What is a request body?
Answer : A request body is the data sent from the client to the server when making an API request.
It is usually used with HTTP methods like POST, PUT, or PATCH.
For example, when creating a new user:
23. What is an API client?
Answer : An API client is a tool or application that sends requests to an API and receives responses.
Examples of API clients include:
Web browsers
Mobile applications
Postman
Frontend applications
24. What tools are used for API testing?
Answer : API testing tools help developers send requests and inspect responses.
Some popular tools include:
Postman – most widely used API testing tool
Swagger UI – test APIs directly from documentation
Insomnia – lightweight API client
REST Assured – used for automated API testing
25. What is Swagger?
Answer : Swagger is a tool used to document and test APIs.
It automatically generates a web interface where developers can:
View API endpoints
See request and response formats
26. What is API documentation?
Answer : API documentation explains how developers can use an API.
Good API documentation usually includes:
API endpoints
request parameters
request body format
response examples
authentication methods
Tools like Swagger and Redoc are commonly used to generate API documentation.
27. What is content negotiation?
Answer : Content negotiation allows the client to specify which format it wants the response in.
For example, a client might request JSON or XML.
This is usually done using the Accept header.
28. What is throttling?
Answer : Throttling is a mechanism used to control the number of requests a client can make to an API.
It helps prevent server overload.
29. What is API security?
Answer : API security refers to protecting APIs from unauthorized access and attacks.
Common API security practices include:
authentication using tokens
authorization checks
input validation
rate limiting
HTTPS encryption
Without proper security, APIs can expose sensitive data.
30. Why should APIs use HTTPS?
Answer : HTTPS encrypts data transmitted between the client and the server.
This prevents attackers from intercepting sensitive information such as:
login credentials
tokens
personal data
For this reason, most modern APIs only allow HTTPS requests.