Introduction to APIs

What is an API?
An API (Application Programming Interface) is a set of rules that allows different software systems to communicate with each other.
Think of it like a waiter in a restaurant:
   ● You (client) place an order
   ● The waiter (API) sends it to the kitchen (server)
   ● The kitchen prepares food and sends it back via the waiter
Example:
When you use a mobile app to check weather:
   ● The app sends a request to a weather API
   ● The API returns data (temperature, humidity, etc.)

Types of APIs
1. REST API (Most Common)
   ● Uses HTTP protocol
   ● Data format: JSON
   ● Lightweight and fast
   ● Stateless (no memory of previous requests)
Example:
GET /users/1

2. SOAP API
   ● Uses XML format
   ● Strict standards and security
   ● Heavier than REST
Common in banking and enterprise systems

3. GraphQL API
   ● Client decides what data it wants
   ● Avoids over-fetching or under-fetching
   ● Flexible and efficient
Example:
Instead of getting full user data, you request only:
name, email

Feature		API				Web Service
---------------------------------------------------------------------
Definition	Interface for communication	API over web (HTTP)
Protocol	Any (HTTP, TCP, etc.)		Only HTTP
Scope		Broader				Subset of API

Key Point:
All web services are APIs, but not all APIs are web services.

Feature		API Testing	UI Testing
------------------------------------
Layer		Backend		Frontend
Speed		Fast		Slow
Stability	More stable	Breaks easily
Focus		Data & logic	User interface

Example:
API testing checks if login API returns success
UI testing checks if login button works visually

Key:
   ● API = communication bridge between systems
   ● REST is most widely used
   ● API testing focuses on backend logic
   ● APIs power almost every modern application