The Three Service Models — Overview
Cloud computing is delivered in three fundamental service models. The key difference between them is how much of the stack you manage vs how much the cloud provider manages.
| Model | You Manage | Provider Manages | Best For |
|---|---|---|---|
| IaaS | OS, middleware, apps, data | Servers, storage, networking | Max control |
| PaaS | Apps and data only | Everything else | Developers |
| SaaS | Just your data and who uses it | Everything | End users |
The Pizza Analogy
This is the most famous analogy in cloud computing — and it makes the three models instantly clear.
Imagine you want pizza. You have four options:
| Option | What You Do | Cloud Equivalent |
|---|---|---|
| 🏠 Make at Home | Buy ingredients, make dough, bake, serve | On-Premises — you manage everything |
| 🛒 Buy & Bake | Buy a ready-made pizza, put it in your oven | IaaS — provider gives you the oven (infrastructure); you do the rest |
| 🍕 Dine In | Sit at a restaurant, order, eat | PaaS — provider manages the kitchen; you just bring your recipe (code) |
| 🛵 Home Delivery | Open the door, eat | SaaS — everything is done for you; just consume |
Infrastructure as a Service (IaaS)
IaaS gives you the raw building blocks of cloud computing. The provider gives you virtual machines, storage, and networking — and you manage everything on top of that.
Think of it as renting land and an empty building. You're responsible for fitting it out, decorating it, and maintaining the interior. The landlord (cloud provider) maintains the building structure and grounds.
What the Provider Manages (Microsoft)
- Physical servers and hardware
- Physical networking and switches
- Physical storage
- The data centre (power, cooling, security)
- Virtualisation layer (Hypervisor)
What You Manage
- Operating system (installation, patching, updates)
- Middleware and runtime environments
- Applications
- Data
- Security configuration
When to Use IaaS
- Migrating existing on-prem workloads to cloud (lift and shift)
- You need full control over the OS and environment
- Running legacy applications that need a specific OS version
- Custom networking configurations that PaaS won't support
Platform as a Service (PaaS)
PaaS removes the burden of managing infrastructure and operating systems. The cloud provider manages everything up to and including the runtime environment. You just bring your code and data.
Think of it as renting a fully equipped kitchen in a restaurant. The restaurant provides the stoves, ovens, utensils, and staff to clean up. You just bring your recipes and cook.
What the Provider Manages (Microsoft)
- Everything in IaaS (hardware, networking, storage)
- Operating system (installation and patching)
- Middleware and runtime
- Scaling infrastructure
What You Manage
- Your application code
- Your data
- Application configuration
When to Use PaaS
- You want to focus on writing code, not managing infrastructure
- Building web applications, APIs, or mobile backends
- Your team is small and can't afford a dedicated infrastructure team
- You want automatic scaling and patching without manual effort
Software as a Service (SaaS)
SaaS is complete software delivered over the internet. You don't manage any infrastructure, no OS, no platform, no application code. You just use the software through a browser or app.
Think of it as ordering food delivery. You don't cook, don't clean, don't even leave your house. Someone else handles absolutely everything — you just eat.
What the Provider Manages (Microsoft)
- Everything — hardware, OS, platform, application, updates, security, scaling
What You Manage
- Your data inside the application
- Who has access (user management)
- Application configuration settings
Common SaaS Examples
- Microsoft 365 — Word, Excel, Teams, Outlook
- Gmail — Email service
- Salesforce — CRM platform
- Zoom — Video conferencing
- Dropbox — File storage and sharing
Side-by-Side Comparison
| Component | On-Premises | IaaS | PaaS | SaaS |
|---|---|---|---|---|
| Physical hardware | You | Microsoft | Microsoft | Microsoft |
| Networking | You | Microsoft | Microsoft | Microsoft |
| Virtualisation | You | Microsoft | Microsoft | Microsoft |
| Operating system | You | You | Microsoft | Microsoft |
| Middleware / Runtime | You | You | Microsoft | Microsoft |
| Application | You | You | You | Microsoft |
| Data | You | You | You | You |
When to Use Which?
| Scenario | Best Model | Why |
|---|---|---|
| Migrate existing app with no code changes | IaaS | Lift-and-shift — you need full OS control |
| Deploy a new web app quickly | PaaS | Focus on code, not infrastructure |
| Give employees email and Office tools | SaaS | No management needed — just use it |
| Run a custom database with specific config | IaaS | Need full control over DB engine and OS |
| Build and deploy a REST API | PaaS | Azure App Service handles everything except your code |
| Video meetings for your team | SaaS | Just use Teams or Zoom — no setup needed |
Azure Examples of Each Model
Azure IaaS Services
- Azure Virtual Machines — Windows or Linux VMs, full OS control
- Azure Virtual Network — Custom network configuration
- Azure Managed Disks — Block storage for VMs
- Azure Blob Storage — Raw object storage
Azure PaaS Services
- Azure App Service — Host web apps and APIs without managing servers
- Azure SQL Database — Fully managed relational database
- Azure Functions — Serverless compute — just write functions
- Azure Kubernetes Service (AKS) — Managed Kubernetes cluster
- Azure Cosmos DB — Fully managed NoSQL database
Azure SaaS Services
- Microsoft 365 — Word, Excel, Teams, Outlook, SharePoint
- Microsoft Dynamics 365 — CRM and ERP platform
- Microsoft Intune — Device management
- Microsoft Defender — Security as a service