Describe cloud concepts Study Guide — AZ-900

Describe Cloud Concepts – AZ-900 Study Guide

This domain accounts for 25–30% of the exam and tests whether you understand what cloud computing is, how responsibility is split between you and Microsoft, and when each cloud model and service type is the right choice. AZ-900 questions in this domain are almost never about memorizing product names — they describe a scenario and ask which concept, model, or benefit applies. If you can distinguish similar-sounding terms (elasticity vs. scalability, reliability vs. availability, hybrid vs. private), you will bank easy marks here.

What Cloud Computing Is

Cloud computing is the delivery of computing services — compute, storage, networking, databases, and more — over the internet, with pay-as-you-go pricing. Two ideas anchor everything else in this domain:

• You rent, you don't own. The provider owns and maintains the physical infrastructure; you consume services from it.
• The consumption-based model. You pay only for what you use, when you use it. No upfront hardware purchase, no paying for idle capacity, stop paying when you stop using it.

CapEx vs. OpEx — On-premises IT is capital expenditure: big upfront purchases of depreciating assets, plus guessing future capacity. Cloud is operational expenditure: an ongoing usage-based cost. When the exam asks which model "avoids large upfront investment" or "converts fixed costs to variable costs," the answer is the cloud consumption model / OpEx.

Cloud pricing model comparison — know these at a concept level:

Serverless — the extreme end of consumption-based: the platform allocates and scales compute automatically, you deploy only code, and billing follows executions/consumption. There are no servers for you to manage (servers still exist — that's a classic distractor). Azure Functions is the flagship example.

The Shared Responsibility Model

The single most-tested concept in this domain. Responsibility divides between Microsoft and the customer depending on service type:

Three things to burn in:

1. Data, identities, and devices are ALWAYS yours — in every model, no exceptions.
2. Physical infrastructure is ALWAYS Microsoft's — you never patch a hypervisor or replace a disk in an Azure datacenter.
3. The OS is the pivot point: IaaS → you patch the guest OS; PaaS/SaaS → Microsoft does. "Which responsibility shifts when moving from IaaS to PaaS?" → OS/runtime maintenance.