carbonxx · Anjor99 · Feb 14, 2026 · Feb 14, 2026 · Feb 14, 2026
diff --git a/API/api_testing.md b/API/api_testing.md
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+# API (Application Programming Interface)
+
+## Table of Contents
+- [What is an API?](#what-is-an-api)
+- [How APIs Work](#how-apis-work)
+- [Client-Server Architecture](#client-server-architecture)
+- [HTTP Methods](#http-methods)
+- [Request and Response](#request-and-response)
+- [Status Codes](#status-codes)
+- [REST API](#rest-api)
+- [Authentication](#authentication)
+- [API Testing](#api-testing)
+
+---
+
+## What is an API?
+
+An **API (Application Programming Interface)** is a set of rules and protocols that allows different software applications to communicate with each other. It acts as an intermediary that enables data exchange between systems.
+
+### Real-World Analogy
+
+Think of an API like a **waiter in a restaurant**:
+- **You (Client)**: You look at the menu and decide what you want
+- **Waiter (API)**: Takes your order to the kitchen
+- **Kitchen (Server)**: Prepares your food
+- **Waiter (API)**: Brings the food back to you
+
+The waiter (API) is the messenger between you and the kitchen, just like an API is the messenger between your application and a server.
+
+### Why APIs Matter
+
+- **Integration**: Connect different services (payment gateways, weather data, social media)
+- **Efficiency**: Reuse existing functionality instead of building from scratch
+- **Scalability**: Separate frontend and backend concerns
+- **Security**: Control access to data and functionality
+
+---
+
+## How APIs Work
+
+APIs enable communication through **requests** and **responses**:
+
+1. **Client** sends a request to the API
+2. **API** processes the request and communicates with the server
+3. **Server** performs the operation and sends data back
+4. **API** returns the response to the client
+
+```
+[Client Application] 
+       ↓ (Request)
+    [API Layer]
+       ↓
+   [Server/Database]
+       ↓
+    [API Layer]
+       ↓ (Response)
+[Client Application]
+```
+
+---
+
+## Client-Server Architecture
+
+### Client
+The **client** is the application or device that requests data:
+- Web browsers
+- Mobile apps
+- Desktop applications
+- Other servers
+
+### Server
+The **server** is the system that provides data or services:
+- Stores data in databases
+- Processes business logic
+- Manages authentication
+- Returns responses to clients
+
+### Communication Flow
+```
+Client → Request → API → Server → Database
+                             ↓
+Client ← Response ← API ← Server
+```
+
+---
+
+## HTTP Methods
+
+APIs use HTTP methods to perform different operations. See [http.md](http.md) for detailed information.
+
+### Quick Reference
+
+| Method | Purpose | Example |
+|--------|---------|---------|
+| **GET** | Retrieve data | Get user profile |
+| **POST** | Create new data | Create new user |
+| **PUT** | Update entire resource | Replace user data |
+| **PATCH** | Partial update | Update user email only |
+| **DELETE** | Remove data | Delete user account |
+
+---
+
+## Request and Response
+
+Every API interaction consists of a **request** from the client and a **response** from the server. See [req_&_resp.md](req_&_resp.md) for complete details.
+
+### Request Components
+- **URL/Endpoint**: Where to send the request
+- **Method**: What operation to perform
+- **Headers**: Metadata about the request
+- **Body**: Data being sent (for POST/PUT/PATCH)
+
+### Response Components
+- **Status Code**: Success or error indicator
+- **Headers**: Metadata about the response
+- **Body**: Data returned from the server
+
+---
+
+## Status Codes
+
+HTTP status codes indicate the result of an API request. See [status_codes.md](status_codes.md) for the complete list.
+
+### Categories
+
+- **2xx Success**: Request succeeded
+- **3xx Redirection**: Further action needed
+- **4xx Client Error**: Problem with the request
+- **5xx Server Error**: Server failed to process request
+
+---
+
+## REST API
+
+**REST (Representational State Transfer)** is an architectural style for designing APIs. See [rest_api.md](rest_api.md) for comprehensive coverage.
+
+### Key Principles
+- **Stateless**: Each request contains all necessary information
+- **Resource-based**: Everything is a resource with a unique URL
+- **Standard methods**: Use HTTP methods appropriately
+- **JSON format**: Typically use JSON for data exchange
+
+---
+
+## Authentication
+
+Authentication ensures only authorized users can access API resources. See [auth.md](auth.md) for security details.
+
+### Common Methods
+- **API Keys**: Simple token passed in headers
+- **OAuth**: Industry-standard authorization framework
+- **JWT (JSON Web Tokens)**: Encoded tokens with user data
+- **Basic Auth**: Username and password encoded in headers
+
+---
+
+## API Testing
+
+Testing ensures your API works correctly and handles errors properly. See [api_testing.md](api_testing.md) for testing strategies.
+
+### Popular Tools
+- **Postman**: GUI-based API testing
+- **cURL**: Command-line tool
+- **Thunder Client**: VS Code extension
+- **Insomnia**: API design and testing platform
+
+---
+
+## Quick Start Example
+
+### Python Example
+```python
+import requests
+
+# GET request
+response = requests.get("https://api.example.com/users")
+print(response.json())
+
+# POST request
+new_user = {"name": "John Doe", "email": "john@example.com"}
+response = requests.post("https://api.example.com/users", json=new_user)
+print(response.status_code)
+```
+
+### JavaScript Example
+```javascript
+// GET request
+fetch("https://api.example.com/users")
+  .then(res => res.json())
+  .then(data => console.log(data));
+
+// POST request
+fetch("https://api.example.com/users", {
+  method: "POST",
+  headers: {
+    "Content-Type": "application/json"
+  },
+  body: JSON.stringify({
+    name: "John Doe",
+    email: "john@example.com"
+  })
+})
+  .then(res => res.json())
+  .then(data => console.log(data));
+```
+
+---
+
+## Further Reading
+
+- [HTTP Methods](http.md) - Detailed guide to GET, POST, PUT, DELETE, PATCH
+- [Request & Response](req_&_resp.md) - Structure and examples
+- [Status Codes](status_codes.md) - Complete reference guide
+- [REST API](rest_api.md) - Design principles and best practices
+- [Authentication](auth.md) - Security methods and implementation
+- [API Testing](api_testing.md) - Tools and techniques