IaaS, PaaS, SaaS and the shared responsibility model
Almost every AZ-900 question starts from one idea: who is responsible for what. Knowing the three service models and the shared responsibility model answers a large share of the cloud concept questions.
The three service models
| Model | You manage | Microsoft manages | Azure example |
|---|---|---|---|
| IaaS | OS, runtime, apps, data | Physical hosts, storage, network | Virtual Machines |
| PaaS | Apps and data | OS, runtime, infrastructure | App Service, Azure SQL Database |
| SaaS | Your data, users, settings | Everything else | Microsoft 365, Dynamics 365 |
Moving from IaaS to PaaS to SaaS, customer responsibility goes down and convenience goes up. Control goes down too: in IaaS you choose the OS, in SaaS you only configure the product.
Serverless (Azure Functions) is a form of PaaS where you pay only when your code runs.
Shared responsibility
Microsoft is always responsible for the physical datacenter, hosts, and physical network. The customer is always responsible for information and data, devices, and accounts and identities. Everything in between (operating system, network controls, applications) shifts depending on the model:
- On-premises: you own all of it.
- IaaS: you own OS, network configuration and above.
- PaaS: you share network controls and applications.
- SaaS: you mostly own data, identities and access.
If the exam asks you to order models from most to least customer responsibility, the order is on-premises, IaaS, PaaS, SaaS.
Public, private and hybrid cloud
| Type | Meaning |
|---|---|
| Public | Resources owned by the provider (Azure), shared hardware, no capital cost |
| Private | Cloud resources used by a single organization, often in its own datacenter |
| Hybrid | Public and private combined, so workloads can move between them |
Azure Arc and Azure Stack support hybrid and multicloud setups. A multicloud environment uses more than one public cloud provider.
CapEx versus OpEx
Capital expenditure (CapEx) is a large up-front spend on assets such as servers, with value written off over years. Operational expenditure (OpEx) is spending on services as you use them, billed regularly. Cloud consumption is OpEx: no up-front hardware cost, and you stop paying when you stop using the resource. This is the consumption-based model.
Exam traps
- Azure SQL Database is PaaS, but SQL Server installed on a VM is IaaS.
- Microsoft 365 is SaaS, but the customer is still responsible for its data and user accounts.
- Hybrid means public plus private. Using two public clouds is multicloud.
- In IaaS, Microsoft maintains the hardware; the customer installs and patches the OS.
- Customer responsibility for data never goes away, even in SaaS.
Practice questions
Choose an answer, then review the explanation to check your understanding.
1. A company runs virtual machines in Azure and patches the guest operating system itself. Which service model is this?
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Answer: C.In IaaS the customer manages the operating system, patches and applications. Microsoft manages the physical hosts and datacenter.
2. A team wants to publish a web app without managing servers or operating system updates. Which model fits best?
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Answer: B.PaaS (for example Azure App Service) lets you deploy code while Microsoft manages the OS and runtime.
3. Which responsibility always stays with the customer, in every cloud service model?
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Answer: C.Information and data, devices, accounts and identities are always the customer's responsibility, whichever model you use.
4. A company stops buying servers up front and instead pays monthly for what it consumes. Which expenditure model is this?
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Answer: B.Pay-as-you-go consumption is OpEx. CapEx is a large up-front purchase of hardware.