What is a REST API?

🔗 REST API Lesson 1 Beginner

A REST API (Representational State Transfer Application Programming Interface) is a set of rules that allows software programs to communicate over the web using standard HTTP. It is the most widely-adopted API design architecture in modern software development.

1 The 6 REST Constraints

An API is RESTful when it follows these architectural constraints defined by Roy Fielding in 2000:

Client-Server: The UI and data storage concerns are separated. The client and server evolve independently.
Stateless: Every request from client to server must contain all information needed to understand it. The server stores no session state between requests.
Cacheable: Responses must define themselves as cacheable or non-cacheable to improve efficiency.
Uniform Interface: A consistent, standardized way to interact with the server (resources, HTTP verbs, self-describing messages).
Layered System: Clients cannot tell whether they are connected directly to the server or a middleware layer (load balancer, cache, gateway).
Code on Demand (optional): Servers can send executable code to clients (e.g. JavaScript snippets).

2 How REST Works — The Request/Response Cycle

HTTP — A REST Request Example

--- REQUEST ---
GET /api/v1/users/42 HTTP/1.1
Host: api.example.com
Authorization: Bearer eyJhbGci...
Accept: application/json

--- RESPONSE ---
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: max-age=300

{
  "id": 42,
  "name": "Balaji Nayak",
  "email": "balaji@example.com",
  "role": "admin",
  "createdAt": "2026-01-15T10:30:00Z"
}

3 REST vs Other API Styles

Feature	REST	GraphQL	SOAP	gRPC
Protocol	HTTP	HTTP	HTTP/SMTP	HTTP/2
Data Format	JSON/XML	JSON	XML	Protobuf
Flexibility	High	Very High	Low	Medium
Learning Curve	Low	Medium	High	Medium
Best For	Public APIs	Complex UIs	Enterprise	Microservices

4 Code Challenge

Challenge: Use the browser's DevTools Network tab (or a tool like Postman) to inspect a REST API call to https://jsonplaceholder.typicode.com/users/1. Identify the request method, status code, response headers, and the JSON body fields returned.