Software-Engineering

API Fundamentals – Complete & Simple Guide

What is an API?

An API (Application Programming Interface) is a communication interface that allows two software systems to interact with each other by exposing specific functions and data, without sharing memory or internal implementation.

APIs act as a bridge between systems so that they can work together in a controlled and secure way.

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Why Do We Need APIs?

Modern software systems are built as independent processes or services.
These processes:

• Run in separate memory spaces
• Cannot directly access each other’s memory
• May be written in different languages
• May run on different machines

APIs solve this problem by providing a well-defined communication contract.

Key reasons APIs are needed:

• Process isolation and security
• Controlled data access
• Scalability (microservices)
• Platform and language independence
• Reusability of functionality

Without APIs, modern web, mobile, and cloud applications would not be possible.

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Real-World Example: Camera Access

A website wants to access the user’s camera.

Direct access is not allowed for security reasons.

Actual flow:

Website (JavaScript) ↓ Browser API (getUserMedia) ↓ (User Permission) Operating System Camera API ↓ Browser ↓ Website

Here:

• The website talks to the browser API
• The browser talks to the system API
• Permissions ensure security

This is a real-world example of APIs enabling safe and controlled access.

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How Does an API Work? (High Level)

1. A client sends a request to an API
2. The API validates the request
3. The server processes the request
4. A response is sent back to the client

Example: Client → API → Server Logic → API → Client

The client never knows:

• How data is stored
• How logic is implemented

This is called abstraction.

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Types of APIs (Based on Access)

1. Public APIs

• Open to everyone
• Example: Google Maps API

2. Private APIs

• Used internally within an organization
• Not exposed publicly

3. Internal APIs

• Used between internal services (microservices)
• Improves modular design

4. Partner APIs

• Shared with specific business partners
• Access controlled via authentication

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Types of APIs (Based on Communication Style)

Only a high-level overview is given here.
Each type will be covered in detail separately.

REST (Representational State Transfer)

• Most commonly used
• Uses HTTP protocol
• Stateless (each request is independent)
• Follows client–server architecture
• Mainly used for CRUD operations
• One-directional request–response

WebSockets

• Bi-directional communication
• Persistent connection
• Used for real-time updates (chat, live data)

SOAP

• XML-based
• Strict standards
• Used in legacy enterprise systems

gRPC

• High-performance
• Uses Protocol Buffers
• Common in microservices

GraphQL

• Client controls data shape
• Reduces over-fetching

Webhooks

• Event-driven APIs
• Server sends data automatically on events

WebRTC

• Real-time peer-to-peer communication
• Used for audio/video calls

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Core Fundamentals of API Security (Overview)

Detailed security topics will be covered in a separate document.

Basic API security practices include:

• Using HTTPS for encrypted communication
• Authentication & Authorization (OAuth 2.0, tokens)
• Rate limiting to prevent abuse
• API versioning for backward compatibility
• API Gateway for centralized control
• Proper error handling
• Input validation to prevent attacks

Security ensures APIs are:

• Safe
• Reliable
• Scalable

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Why APIs Are Important

APIs are the backbone of modern software systems.

They enable:

• Web and mobile applications
• Microservices architecture
• Cloud computing
• Third-party integrations
• Scalable and maintainable systems