Choosing IaaS, PaaS or SaaS
😟 THE PROBLEM
One company needs to move an old app, build a new portal and give staff email.
😊 THE SOLUTION
Pick per job: IaaS for the old app, PaaS for the portal, SaaS for email.
- Legacy app needs a specific OS✓ IaaS virtual machine
- New portal, no time for servers✓ PaaS: App Service and Azure SQL
- Staff need email and chat✓ SaaS: Microsoft 365
Quick answer
Choose IaaS when you need full control of the operating system or must migrate existing servers unchanged. Choose PaaS when you are building or running your own application and want the provider to manage the platform. Choose SaaS when a ready-made application already does the job. Moving from IaaS to SaaS, control decreases and so does your management effort.
Exam objective: 1.3.4 Identify appropriate use cases for each cloud service type (IaaS, PaaS, and SaaS). (Skills measured as of July 20, 2026)
Start with a question
You are hungry. Do you cook from scratch, use a meal kit, or order delivery?
All three get you fed. They differ in how much you control (exact ingredients, seasoning) and how much work you do (shopping, cooking, washing up).
Cloud service types are the same choice for software. There is no “best” option, only the right one for each situation.
Explain it like I’m 10
The technical explanation
The three service types sit on a scale of control versus convenience:
| Responsibility | On-premises | IaaS | PaaS | SaaS |
|---|---|---|---|---|
| Information and data | You | You | You | You |
| Devices (mobile and PCs) | You | You | You | You |
| Accounts and identities | You | You | You | You |
| Identity and directory infrastructure | You | You | Shared | Shared |
| Applications | You | You | Shared | Microsoft |
| Network controls | You | You | Shared | Microsoft |
| Operating system | You | You | Microsoft | Microsoft |
| Physical hosts | You | Microsoft | Microsoft | Microsoft |
| Physical network | You | Microsoft | Microsoft | Microsoft |
| Physical datacenter | You | Microsoft | Microsoft | Microsoft |
| IaaS | PaaS | SaaS | |
|---|---|---|---|
| You get | VMs, storage, networks | A managed platform for your code and data | A finished application |
| You manage | OS, runtime, apps, data, access | Apps, data, access | Data, users, devices, settings |
| Control | Highest | Medium | Lowest |
| Management effort | Highest | Medium | Lowest |
| Azure / Microsoft examples | Virtual Machines, VM Scale Sets | App Service, Azure SQL Database, Functions | Microsoft 365, Dynamics 365 |
| Typical users | IT administrators, infrastructure teams | Developers | End users and business teams |
A decision guide
-
1 Does a SaaS product do this?
Yes → SaaS. Configure it; do not build it.
-
2 Is it your own app on a standard runtime?
Yes → PaaS or serverless.
-
3 Need containers or portability?
Yes → Container Apps or AKS.
-
4 Need OS control or no changes?
Yes → IaaS virtual machines.
Use cases at a glance
IaaS
- Lift-and-shift migration
- Test and development environments
- Custom or legacy software needing OS access
- High-performance computing
- Storage, backup and recovery
PaaS
- Building web apps and APIs
- Managed databases
- Development frameworks and DevOps
- Analytics and business intelligence
- Event-driven apps (Functions)
SaaS
- Email and messaging
- Office and collaboration tools
- CRM and ERP
- Business productivity apps
- Anything you would rather not build
Real-world business scenario
A food delivery company
- The problem
- It needs a customer ordering app, a route-optimisation engine that uses a vendor's Linux-only binary, and email and chat for 400 staff.
- What they used
- Ordering API on Azure App Service with Azure SQL Database (PaaS); route engine on Azure VMs (IaaS); Microsoft 365 for staff (SaaS).
- Why that choice
- Each component got the most managed option that still met its constraints: PaaS for custom code, IaaS where the vendor binary needs OS control, SaaS for commodity productivity.
- The result
- The small platform team manages only a handful of VMs; everything else is patched and scaled by Microsoft.
When to use each, and when not to
✓ Use it when…
- **IaaS** when you need OS control, specific software, or a fast lift-and-shift.
- **PaaS** for new custom applications, APIs and managed databases.
- **SaaS** for standard business capabilities you do not need to build.
✕ Think twice when…
- **IaaS** for simple web apps. You inherit patching and scaling you do not need.
- **PaaS** for software that needs OS-level installs or unsupported runtimes.
- **SaaS** when the process is your unique competitive advantage and needs deep customisation.
Interview preparation
Say your answer out loud first, then open the model answer. Interviewers care more about your reasoning than the exact words.
Beginner Explain IaaS, PaaS and SaaS in one sentence each.
Model answer
IaaS means renting virtual servers that you manage yourself. PaaS means renting a ready platform where you just deploy your code. SaaS means using finished software over the internet that the provider runs completely.
Interviewer follows up
Can you give me an Azure or Microsoft example for each?
You
IaaS is Azure Virtual Machines. PaaS is Azure App Service or Azure SQL Database. SaaS is Microsoft 365.
Intermediate Can one solution use all three service types?
Model answer
Yes, and most do. A company might use Microsoft 365 for email (SaaS), host its customer API on App Service with Azure SQL Database (PaaS), and run a legacy reporting server on a VM (IaaS). You choose per component, not per company.
Experienced developer How do you decide the default compute choice for a new service in an enterprise?
Model answer
I use a "most managed first" rule. Can a SaaS product do it? If not, can it run as serverless or PaaS (Functions, App Service, Container Apps)? If it needs more control or portability, containers on AKS. Only fall back to VMs when there is a concrete reason, such as OS dependencies, licensing or specialised hardware. Each step down gives more control but adds operational cost, so the burden of proof is on going lower.
Scenario A company has three needs. Migrate a legacy app quickly, build a new customer portal, and give staff email. Which service type for each?
Model answer
Legacy app, IaaS (lift and shift to VMs). Customer portal, PaaS (App Service plus a managed database). Staff email, SaaS (Microsoft 365).
Test yourself with exercises
4 original practice questions in the AZ-900 style. Pick your answers, then check them. You will see why each option is right or wrong. Score 75% or more and this topic is marked complete.
Quick revision
Key points
- Control and responsibility are highest in IaaS, then PaaS, then SaaS (lowest).
- IaaS suits OS control, lift-and-shift, custom software, test/dev and HPC.
- PaaS suits building apps and APIs, managed databases and development speed.
- SaaS suits standard business needs such as email, CRM and collaboration.
- Real solutions mix all three.
Important terminology
- Control vs convenience
- The core trade-off. IaaS gives the most control, SaaS the most convenience.
- Rehost / refactor / replace
- Migration strategies that roughly map to IaaS (rehost), PaaS (refactor) and SaaS (replace).
Common mistakes
- Picking one service type for a whole company. Decide per workload.
- Defaulting to VMs out of habit when PaaS would remove most operational work.
Exam tips
- Look for the keyword in the scenario. "Control of OS" means IaaS, "focus on code" means PaaS, "just use the app" means SaaS.
- Azure Functions is PaaS and serverless; Azure VMs are IaaS; Microsoft 365 is SaaS.
Five quick revision questions
-
1.Which service type gives the most control?Show
IaaS.
-
2.Which service type suits building a custom web app without managing servers?Show
PaaS.
-
3.Which service type is Microsoft 365?Show
SaaS.
-
4.Which service type fits a lift-and-shift migration?Show
IaaS.
-
5.Can one organisation use IaaS, PaaS and SaaS at the same time?Show
Yes. Most organisations do.
Official references
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