Chapter 04 — Cloud Service Types: IaaS, PaaS, SaaS

Overview

Cloud services are delivered in three main models, each offering a different level of control, flexibility, and management responsibility. These are:

IaaS  →  Infrastructure as a Service
PaaS  →  Platform as a Service
SaaS  →  Software as a Service

Understanding these models — what they include, what you manage, and when to use each — is essential for the AZ-900 exam.

The Concept: "As a Service"

In traditional IT, you manage everything yourself — hardware, OS, runtime, data, and application. Cloud computing offloads parts of that stack to the provider.

The stack from bottom to top:

┌──────────────────┐
│   Application    │  ← Your code / software
│   Data           │  ← Your databases, files
│   Runtime        │  ← Node.js, Python, .NET
│   Middleware     │  ← Message queues, APIs
│   OS             │  ← Windows, Linux
│   Virtualization │  ← Hypervisor
│   Servers        │  ← Physical CPU, RAM
│   Storage        │  ← Disks, SSDs
│   Networking     │  ← Switches, cables
└──────────────────┘

Different service models hand over different layers to the provider:

                    You Manage   │  Provider Manages
────────────────────────────────┼──────────────────
On-Premises         Everything  │  Nothing
IaaS                App+Data+OS │  Hardware + Virtualization
PaaS                App + Data  │  Everything below App
SaaS                Nothing     │  Everything

IaaS — Infrastructure as a Service

What Is It?

IaaS gives you access to raw computing infrastructure — virtual machines, storage, and networking — over the internet. You rent the hardware and manage everything else yourself.

The cloud provider manages:

• Physical servers

• Storage hardware

• Networking hardware

• Virtualization layer

You manage:

• Operating system

• Runtime and middleware

• Application

• Data

Azure IaaS Examples

When to Use IaaS

• You need full control over the OS and environment

• You're migrating existing on-premises workloads to cloud ("lift and shift")

• You're running legacy applications that can't be modified

• You have specific compliance requirements for the OS configuration

IaaS Analogy

IaaS is like renting a plot of land with a foundation. You get the land (hardware), but you build the house (OS, runtime, app) yourself.

PaaS — Platform as a Service

What Is It?

PaaS gives you a managed platform to build, deploy, and run applications — without managing the underlying infrastructure or OS. The cloud provider handles everything except your application and data.

The cloud provider manages:

• Physical hardware

• Virtualization

• OS and patches

• Runtime and middleware

You manage:

• Application code

• Data

Azure PaaS Examples

When to Use PaaS

• You want to focus on writing code, not managing servers

• You're building web apps, APIs, or mobile backends

• You want automatic OS patching and updates

• You want built-in scaling and high availability without configuration

PaaS Analogy

PaaS is like renting a fully equipped kitchen. The stoves, ovens, and utilities are set up. You just cook (write code) and serve food (deploy your app).

SaaS — Software as a Service

What Is It?

SaaS delivers complete, ready-to-use software applications over the internet. You don't manage anything — just use the software through a browser or app.

The cloud provider manages:

• Everything — hardware, OS, runtime, and the application itself

You manage:

• Your data (what you enter/store in the app)

• User access and permissions

Azure / Microsoft SaaS Examples

When to Use SaaS

• You need software up and running immediately with no setup

• You're using common productivity, collaboration, or business tools

• You have non-technical users who just need access to an application

• You don't want to maintain software updates, patches, or servers

SaaS Analogy

SaaS is like ordering food delivery. You don't buy ingredients, cook, or clean up — you just eat. Someone else handles everything.

Side-by-Side Comparison

Visual Model

┌───────────────────────────────────────────────────────────┐
│                        SaaS                               │
│         Provider manages EVERYTHING                       │
│         You just USE the software                         │
├───────────────────────────────────────────────────────────┤
│                        PaaS                               │
│         Provider manages infra + OS + runtime             │
│         You manage your APPLICATION + DATA                │
├───────────────────────────────────────────────────────────┤
│                        IaaS                               │
│         Provider manages physical hardware                │
│         You manage OS + middleware + app + data           │
└───────────────────────────────────────────────────────────┘

Shared Responsibility in Each Model

The more control you have, the more responsibility you carry:

On-Premises  →  IaaS  →  PaaS  →  SaaS
More control                    Less control
More work                       Less work
More flexibility                Less flexibility

Real-World Scenario Examples

Scenario 1 — A startup building a web app

Best choice: PaaS (Azure App Service)

• Developers focus on code, not servers

• Auto-scaling and patching handled by Azure

• Faster time to market

Scenario 2 — A company migrating old Windows Server apps

Best choice: IaaS (Azure Virtual Machines)

• Full control over OS (Windows Server)

• Minimal changes to existing application

• "Lift and shift" migration

Scenario 3 — A team using email and documents

Best choice: SaaS (Microsoft 365)

• No setup required

• Access from any device via browser

• Microsoft manages everything

Quick Recap

IaaS → You manage the OS and above. Great for migration.
PaaS → You manage only your app and data. Great for development.
SaaS → You manage nothing. Great for ready-made software.

More control = More responsibility
Less control = Less work, less flexibility

Official References

• Cloud service types — Microsoft Learn

• What is IaaS? — Microsoft Azure

• What is PaaS? — Microsoft Azure

• What is SaaS? — Microsoft Azure

Next Chapter → Chapter 05: Cloud Deployment Models — Public, Private, Hybrid