IaaS, PaaS, SaaS and cloud deployment models
Almost every MS-900 scenario starts by asking you to name the cloud model. Get these definitions right and a large share of the “cloud concepts” questions become easy.
The three service models
The difference is how much you manage versus how much Microsoft manages.
| Model | You manage | Provider manages | Example |
|---|---|---|---|
| IaaS | OS, runtime, apps, data | Servers, storage, network, virtualization | Azure Virtual Machines |
| PaaS | Apps and data | Everything below, including OS and runtime | Azure App Service, Azure SQL Database |
| SaaS | Your data, users and settings | The whole application stack | Microsoft 365, Dynamics 365 |
A quick rule: if you need to patch the operating system, it is IaaS. If you only deploy code, it is PaaS. If you simply sign in and use it, it is SaaS.
Deployment models
- Public cloud: resources owned by the provider and shared across customers (Azure, Microsoft 365). No hardware to buy.
- Private cloud: resources used by one organization, hosted on-premises or by a provider. The organization keeps full control but also the cost.
- Hybrid cloud: a mix of on-premises or private resources and public cloud, connected so they work together. Typical when some apps cannot move, for legal or technical reasons.
Cloud benefits
Expect questions on the benefits of cloud computing:
- High availability and reliability: services stay up through redundancy and service level agreements (SLAs).
- Scalability and elasticity: add or remove capacity on demand.
- Agility: deploy in minutes instead of weeks.
- Disaster recovery: data and services can fail over to another region.
CapEx versus OpEx
Capital expenditure is an upfront spend on physical assets, such as buying servers. Operational expenditure is paying as you go for a service. Moving to the cloud shifts spending from CapEx to OpEx. Consumption-based pricing means you pay for what you use.
Shared responsibility
In every model the customer is always responsible for their data, accounts and devices. Responsibility for the physical datacenter always stays with the provider. The further you move from IaaS to SaaS, the more the provider takes on, for example the OS and application patching.
Exam traps
- Microsoft 365 is SaaS even though it includes some PaaS-like features such as Power Apps. Do not pick PaaS for Exchange Online.
- Hosting a website with no server management is PaaS, not IaaS.
- A private cloud is not the same as on-premises. It can be hosted by a provider.
- Hybrid means a mix of environments connected together, not simply “using two clouds”.
- Data and identities are always the customer’s responsibility, even in SaaS.
Practice questions
Choose an answer, then review the explanation to check your understanding.
1. A company wants to host a website and let the provider manage the operating system and runtime, while the company only deploys its code. Which model fits?
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Answer: B.PaaS (for example Azure App Service) manages the OS and runtime. You manage only the application and data.
2. Microsoft 365 apps such as Exchange Online and Teams are delivered as which service model?
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Answer: C.Microsoft 365 is SaaS: Microsoft runs the whole stack and you just use the app and configure it.
3. A company keeps an on-premises application server but wants to use some Microsoft 365 services with it. Which deployment model is this?
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Answer: C.Hybrid combines on-premises (or private) resources with public cloud services.
4. Which cost model describes paying only for the resources you consume?
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Answer: B.Cloud is mostly OpEx: consumption-based, no large upfront hardware purchase.