What Is a RESTful API and How Does It Work?

Concept

A RESTful API (Representational State Transfer) is a widely adopted architectural style for building stateless, client-server communication systems over the HTTP protocol.
It focuses on resources — such as users, orders, or products — each represented by a unique URL and manipulated using standard HTTP verbs.

REST was introduced by Roy Fielding in 2000 as part of his doctoral dissertation, emphasizing simplicity, scalability, and decoupling between clients and servers.
Today, REST powers most of the web’s APIs — from payment platforms like Stripe to social networks like Twitter.

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1. Core Principles of REST

1.1 Client–Server Architecture

• The client handles user interactions and UI rendering.
• The server provides resources and handles business logic.
  This separation allows independent evolution and scaling of both sides.

1.2 Statelessness

• Each request from the client must contain all information required for the server to process it.
• The server does not retain session data between requests.
• This improves scalability since servers can handle requests independently.

Example:
Each request includes authentication tokens (like Authorization: Bearer <token>) rather than relying on stored sessions.

1.3 Uniform Interface

REST relies on a consistent, predictable structure for communication:

HTTP Method	Operation	Description
GET	Read	Retrieve a resource
POST	Create	Add a new resource
PUT	Update	Replace an existing resource
PATCH	Partial Update	Modify part of a resource
DELETE	Delete	Remove a resource

This predictability allows developers to work with REST APIs intuitively.

1.4 Resource Identification

• Every resource is identified via a URI (Uniform Resource Identifier).
• Resources are represented in formats such as JSON, XML, or YAML.

Example (safe for MDX):

GET /api/users/101

This retrieves the user with ID 101 in a standardized structure like:

{
  "id": 101,
  "name": "Alice Johnson",
  "email": "alice@example.com"
}

1.5 Cacheability

• Responses must define whether data is cacheable using headers (Cache-Control, ETag).
• Proper caching reduces redundant server calls and improves performance.

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2. Example: RESTful API Workflow

1. The client (e.g., web or mobile app) sends a request:

   POST /api/orders
   Content-Type: application/json
   

2. The server validates input, processes business logic, and interacts with the database.

3. The server responds with a standardized payload and status code:

   { "id": 5021, "status": "created", "amount": 199.99 }
   

4. Clients use status codes to interpret results:

   • 200 OK → Success
   • 201 Created → Resource added
   • 400 Bad Request → Invalid input
   • 404 Not Found → Resource unavailable
   • 500 Internal Server Error → Server issue

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3. REST in Practice

Example Endpoints

GET /users           → Retrieve all users
POST /users          → Create a new user
GET /users/{id}      → Get specific user
PUT /users/{id}      → Replace user data
DELETE /users/{id}   → Remove user

Real-World Implementations

• Stripe API: Offers REST endpoints for payment processing.
• GitHub API: Provides access to repositories, commits, and issues.
• Twitter API: Uses REST principles for tweet and user data retrieval.

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4. Advantages of REST

• Scalable: Stateless design supports horizontal scaling.
• Interoperable: Language-agnostic communication via HTTP and JSON.
• Simple: Easy for clients to integrate and debug using tools like Postman or curl.
• Cacheable: HTTP caching enhances performance.
• Separation of Concerns: UI and server logic evolve independently.

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5. Limitations and Alternatives

• Over-fetching/Under-fetching: Clients may receive too much or too little data.

• Tight coupling to endpoints: Complex APIs can become difficult to maintain.

• Alternatives:

  • GraphQL: Provides flexible queries for exact data retrieval.
  • gRPC: Uses binary data and protocol buffers for low-latency communication.
  • WebSockets: Enables persistent, real-time bidirectional connections.

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6. REST API Design Best Practices

• Use nouns, not verbs, for endpoints (/orders, not /createOrder).
• Return standard HTTP status codes.
• Include versioning (e.g., /v1/orders).
• Use pagination for large datasets.
• Secure APIs using HTTPS, OAuth2, and rate limiting.

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7. Common Interview Follow-Ups

• Difference between REST and SOAP (REST is simpler, lighter, and stateless).
• How idempotent methods (PUT, DELETE) differ from non-idempotent ones (POST).
• How REST handles authentication and authorization (tokens, JWT, OAuth2).
• Discuss trade-offs vs GraphQL or RPC-style APIs.

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Summary Insight

RESTful APIs power modern web ecosystems by enforcing simplicity, statelessness, and resource-based structure. They are the backbone of scalable, distributed systems — predictable for developers, reliable for clients, and efficient for machines.