AZ-900 cloud concepts explained
Describe cloud concepts is the AZ-900 domain worth 25–30% of the exam. It is the only part that is not specific to Azure: it tests whether you understand what cloud computing is, who is responsible for what, which cloud model and service type fit a situation, and why organisations move to the cloud at all. Most of it is vocabulary, but the questions test whether you can apply that vocabulary to a short scenario.
The domain has three parts in the skills outline: cloud computing, the benefits of cloud services, and cloud service types.
Cloud computing
What it is
Cloud computing is renting computing resources — servers, storage, databases, networking, software — over the internet, instead of buying and running them yourself. You pay for what you use, and you can get more or less of it quickly.
The shared responsibility model
The most tested concept in the domain. Responsibility for security and operations is split between you and the cloud provider, and the split moves with the service type.
| Responsibility | On-premises | IaaS | PaaS | SaaS |
|---|---|---|---|---|
| Information and data | You | You | You | You |
| Devices, accounts and identities | You | You | You | You |
| Applications | You | You | Shared | Microsoft |
| Operating system | You | You | Microsoft | Microsoft |
| Physical hosts, network, datacenter | You | Microsoft | Microsoft | Microsoft |
Two rules answer most questions. The physical datacenter is always Microsoft’s once you are in the cloud. Your data, your devices and your accounts are always yours, whatever the service type. Everything in between shifts towards Microsoft as you move from IaaS to SaaS.
Cloud models
| Model | What it is | Typical use |
|---|---|---|
| Public cloud | Services run by a provider and shared by many customers over the internet | Fast start, no hardware purchase, variable demand |
| Private cloud | Cloud infrastructure used by one organisation, in its own datacenter or hosted | Strict control or regulation, existing investment |
| Hybrid cloud | Public and private connected, with workloads able to move between them | Keep sensitive data on-premises, burst to public cloud for peaks |
Watch for the word “both” in a scenario. If some data must stay in the company’s own datacenter while other workloads use Azure, the answer is hybrid.
The consumption-based model and pricing
Buying servers is capital expenditure (CapEx): a large up-front cost for an asset that depreciates. Cloud computing is operational expenditure (OpEx): you pay as you use it, and stop paying when you stop.
The outline asks you to compare pricing models. The main ones on Azure are:
- Pay-as-you-go: billed for actual usage, no commitment. Suits unpredictable or short-lived workloads.
- Reserved capacity: commit to one or three years of a resource for a lower price. Suits steady, predictable workloads.
- Spot pricing: use spare capacity at a discount, accepting that it can be taken back. Suits interruptible batch work.
Serverless
Serverless means you do not manage servers at all. You deploy code or configuration, the platform runs it when needed, and you pay per execution rather than for idle capacity. Azure Functions is the example the exam uses most. Servers still exist; you just never see them.
Benefits of the cloud
The outline lists eight benefits in four pairs. The exam often describes one and asks you to name it.
| Benefit | Means |
|---|---|
| High availability | The service stays up despite component failures |
| Scalability | Capacity can be added to meet demand: vertically (a bigger machine) or horizontally (more machines) |
| Reliability | The system recovers from failure and keeps working |
| Predictability | Performance and cost can be forecast |
| Security | Tools and controls to protect data, plus physical security run by the provider |
| Governance | Policies and templates keep deployments compliant with standards |
| Manageability | Management of the cloud (auto-scaling, monitoring) and in the cloud (portal, CLI, APIs) |
Elasticity is not a separate objective but appears in explanations: scaling automatically up and down with demand. Scaling up means a bigger instance; scaling out means more instances.
Cloud service types
| Type | You manage | Microsoft manages | Example |
|---|---|---|---|
| IaaS | OS, runtime, apps, data | Hardware, network, datacenter | Azure Virtual Machines |
| PaaS | Apps and data | Everything below, including OS and runtime | Azure App Service, Azure SQL Database |
| SaaS | Data and users | The whole application | Microsoft 365 |
Use cases the exam likes:
- IaaS: lift-and-shift migration, test environments, software that needs a specific OS configuration.
- PaaS: developers who want to write code without managing servers.
- SaaS: email, collaboration, CRM — software used, not built.
A good tie-breaker: the question that mentions installing or patching an operating system is IaaS. The one that mentions only writing code is PaaS. The one where nobody writes any code is SaaS.
Three practice questions
Question 1. A company runs a short marketing campaign. Traffic to its website rises tenfold for two weeks, then drops back. It wants to pay only for the extra capacity while the campaign runs. Which cloud characteristic does this describe?
- A. Capital expenditure
- B. Elasticity with a consumption-based model
- C. A private cloud
- D. High availability
Show answer
Answer: B
Paying for resources only while you use them is the consumption-based model, and adding and removing capacity as demand changes is elasticity. Capital expenditure is the up-front purchase the company is avoiding. Private cloud implies dedicated infrastructure that it would pay for all year. High availability is about staying up during failures, not matching cost to demand.
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Question 2. A development team wants to deploy a web application without managing virtual machines, operating system updates or the web server software. Which cloud service type should they use?
- A. Infrastructure as a service
- B. Software as a service
- C. Platform as a service
- D. On-premises hosting
Show answer
Answer: C
Platform as a service provides a managed environment where the team deploys code while the provider manages the operating system and runtime. IaaS would leave OS patching with the team. SaaS provides finished software rather than a place to run your own. On-premises hosting leaves everything with the team.
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Question 3. A hospital must keep patient records in its own datacenter for regulatory reasons, but wants to run its public appointment website on Azure and connect the two. Which cloud model is this?
- A. Hybrid cloud
- B. Public cloud
- C. Private cloud
- D. Multi-region deployment
Show answer
Answer: A
A hybrid cloud combines on-premises or private infrastructure with the public cloud and connects them. Public cloud alone would not keep the records on-premises. A private cloud alone would not use Azure for the website. Multi-region deployment is about Azure regions, not the mix of environments.
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How to study this domain
This domain rewards precise definitions more than any other. Write the shared responsibility table and the service type table from memory until you get them right twice. Then practise turning scenarios into terms: “pay only while it runs”, “some data must stay on-site”, “no code at all”. The study plan puts this domain in week 1, and the practice test has six questions on it.