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Ranit Saha
Ranit Saha

Posted on • Originally published at coderooz.in

How APIs Work: A Beginner-Friendly Guide to REST, GraphQL, Authentication & More

#api #webdev #backend #beginners

Introduction

Every modern application relies on APIs.

From weather apps and payment systems to AI-powered tools like ChatGPT — everything communicates through APIs behind the scenes.

Yet many developers use APIs daily without fully understanding how they actually work.

In this guide, we’ll break down APIs from first principles — including:

  • The request–response cycle
  • Types of APIs (REST, GraphQL, SOAP)
  • Authentication
  • Rate limiting
  • Error handling

Let’s simplify it.

What Is an API?

API stands for Application Programming Interface.

At its core, an API is a contract between two systems.

It defines:

  • How a client asks for something
  • How a server responds
  • What format the data must follow

APIs are not magic. They are structured communication rules.

The Request–Response Cycle (The Core Concept)

Every API interaction follows the same pattern:

  1. The client sends a request
  2. The server processes it
  3. The server sends a response
  4. The client uses the response

This happens every time you:

  • Log in to an app
  • Refresh a page
  • Submit a form
  • Check the weather

The entire modern web runs on this cycle.

Types of APIs You’ll Encounter

Not all APIs are designed the same way.

REST APIs (Representational State Transfer)

REST is the most widely used API style on the web.

It relies on standard HTTP methods:

  • GET
  • POST
  • PUT
  • DELETE

Data is typically exchanged in JSON format, making REST lightweight and easy to work with.

Most web and mobile applications rely on REST APIs.

GraphQL APIs

GraphQL allows the client to request exactly the data it needs.

Unlike REST — where the server defines the structure — GraphQL gives more control to the client.

This helps reduce over-fetching and under-fetching of data.

SOAP APIs

SOAP (Simple Object Access Protocol) is older and more rigid.

  • Uses XML
  • Strict standards
  • Strong built-in security
  • Common in enterprise systems

More formal. More structured. More verbose.

Public APIs vs Internal APIs

Public APIs

Public APIs are exposed to external developers.

Examples include:

  • Payment APIs
  • Weather APIs
  • Cloud storage APIs

They allow third-party applications to integrate services.

Internal APIs

Internal APIs are used within an organization.

They are not exposed publicly.

They help teams:

  • Maintain scalability
  • Improve communication between systems
  • Structure internal services

How APIs Work in Real Applications

At a basic level:

  • The client sends a request
  • The server processes it
  • The server returns data (or an error)
  • The client updates the UI or performs an action

The language you use (JavaScript, Python, etc.) may differ — but the underlying concept remains the same.

Authentication (Proving Who You Are)

APIs don’t allow unlimited anonymous access.

They verify identity using:

API Keys

Simple unique identifiers sent with requests.

OAuth

A secure method that allows apps to access data without exposing credentials.

If you’ve used “Sign in with Google,” you’ve used OAuth.

Authentication protects systems from misuse and tracks usage.

Rate Limiting (Preventing Abuse)

Rate limiting controls how often an API can be used.

If a client sends too many requests too quickly, the server may respond with:

429 – Too Many Requests
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This keeps systems stable and fair for all users.

Error Codes (Understanding What Happened)

APIs communicate results using status codes.

Some indicate success.
Others indicate failure.

Status codes remove guesswork and provide standardized communication.

Understanding them is essential for debugging effectively.

Why Understanding APIs Matters

APIs are foundational to backend development.

If you understand:

  • Request–response flow
  • API types
  • Authentication
  • Rate limiting
  • Error handling

You understand how modern software systems communicate.

That’s not just theory — that’s real-world architecture.

Final Thoughts

APIs can seem complex at first.

But when broken down step-by-step, they follow predictable patterns.

Mastering API fundamentals builds a strong backend foundation — and makes learning advanced topics much easier.

If you want the full expanded version with detailed explanations and examples, you can read it here:

👉 https://www.coderooz.in/content/apis-explained-for-developers

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