AZ-900 Describe Azure architecture and services Practice Question
A company is building a mobile app backend that handles HTTP requests from thousands of users. The traffic pattern is highly unpredictable: there can be sudden spikes to hundreds of requests per second followed by long periods of no activity. The development team wants a solution that automatically scales from zero to hundreds of instances based on demand, and they want to pay only for the compute time consumed when the code is actually executing. They do not want to manage any underlying infrastructure, such as virtual machines or containers. Which Azure compute service should they choose?
⚠ Common exam trap
It's easy for candidates to confuse Azure Functions with Azure App Service, assuming both can scale to zero, but App Service requires always-on resources and cannot scale down to zero instances, whereas Functions on a consumption plan can.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Azure Functions
Azure Functions is the correct choice because it is a serverless compute service that automatically scales from zero to hundreds of instances based on incoming HTTP requests, and you pay only for the compute time consumed when your code executes. It handles unpredictable traffic patterns with sudden spikes and idle periods without requiring any infrastructure management, as it runs on a consumption plan that provides dynamic scaling.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Azure Functions
Why this is correct
Azure Functions is a serverless compute service that runs code on-demand without requiring you to provision or manage infrastructure. It automatically scales out to handle incoming HTTP requests and scales down to zero when idle, so you are billed only for the actual execution time and resources consumed during each invocation. This makes it the ideal choice for a mobile app backend that needs to handle sporadic or variable HTTP traffic while minimizing cost.
- ✗
Azure App Service
Why it's wrong here
Incorrect. Azure App Service can autoscale, but in most plans it requires at least one instance running continuously (always-on). It does not scale to zero and you incur charges even when no requests are processed.
When this WOULD be correct
A company needs to deploy a web application with a consistent traffic pattern, requires built-in auto-scaling based on metrics like CPU or memory, and wants to manage the app code without worrying about the underlying OS or infrastructure, but does not need the extreme scale-to-zero capability.
- ✗
Azure Kubernetes Service (AKS)
Why it's wrong here
Incorrect. AKS is a container orchestration service that runs on virtual machines. You are responsible for managing the node pool (VMs) and paying for the underlying compute resources even when no containers are running. It does not scale to zero and is not a serverless solution in its standard form.
When this WOULD be correct
A company needs to run containerized microservices with custom scaling rules, requires portability across clouds, and has a team to manage the Kubernetes cluster. The question would specify that the team can handle container orchestration and needs consistent scaling policies.
- ✗
Azure Virtual Machines
Why it's wrong here
Incorrect. Azure Virtual Machines require you to manage the operating system and scaling configuration. Even if you use VM scale sets with autoscale, the VMs remain allocated and incur costs even when idle. Scaling to zero is not possible without manually deallocating all VMs.
When this WOULD be correct
A company needs to run a legacy application that requires full control over the operating system, custom software installations, or specific VM configurations. They are willing to manage scaling and pay for provisioned VMs regardless of usage.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The AZ-900 exam frequently reuses these exact scenarios with slightly different constraints.
✓Azure FunctionsCorrect answer▾
Why this is correct
Azure Functions is a serverless compute service that runs code on-demand without requiring you to provision or manage infrastructure. It automatically scales out to handle incoming HTTP requests and scales down to zero when idle, so you are billed only for the actual execution time and resources consumed during each invocation. This makes it the ideal choice for a mobile app backend that needs to handle sporadic or variable HTTP traffic while minimizing cost.
✗Azure App ServiceWrong answer — click to see why▾
Why this is wrong here
Azure App Service does not support scaling from zero instances to handle unpredictable traffic; it requires a minimum number of always-on instances and does not offer true consumption-based billing where you pay only when code executes.
★ When this WOULD be the correct answer
A company needs to deploy a web application with a consistent traffic pattern, requires built-in auto-scaling based on metrics like CPU or memory, and wants to manage the app code without worrying about the underlying OS or infrastructure, but does not need the extreme scale-to-zero capability.
Why candidates choose this
Candidates may confuse App Service's auto-scaling and serverless-like management with the true serverless, event-driven, and consumption-based model of Azure Functions, overlooking the requirement for scaling from zero and paying only per execution.
✗Azure Kubernetes Service (AKS)Wrong answer — click to see why▾
Why this is wrong here
Azure Kubernetes Service (AKS) requires managing container orchestration and does not scale to zero instances; it also incurs costs for the underlying node VMs even when idle, contradicting the pay-per-execution requirement.
★ When this WOULD be the correct answer
A company needs to run containerized microservices with custom scaling rules, requires portability across clouds, and has a team to manage the Kubernetes cluster. The question would specify that the team can handle container orchestration and needs consistent scaling policies.
Why candidates choose this
Candidates may think AKS is serverless because it abstracts VMs, but it still requires node management and does not offer true pay-per-execution or scale-to-zero like Azure Functions.
✗Azure Virtual MachinesWrong answer — click to see why▾
Why this is wrong here
Azure Virtual Machines require manual scaling and management of VMs, and you pay for allocated compute resources even when idle, not just for code execution time. They do not automatically scale from zero or provide a serverless, pay-per-execution model.
★ When this WOULD be the correct answer
A company needs to run a legacy application that requires full control over the operating system, custom software installations, or specific VM configurations. They are willing to manage scaling and pay for provisioned VMs regardless of usage.
Why candidates choose this
Candidates may think VMs are the most flexible option for handling unpredictable traffic, not realizing that serverless services like Azure Functions are designed specifically for such patterns with automatic scaling and consumption-based pricing.
Analysis generated from the official AZ-900blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Quick reference
Cloud Service Model Comparison
| Model | You Manage | Provider Manages | Examples |
|---|---|---|---|
| IaaS | OS, runtime, apps, data | Hardware, hypervisor, networking | EC2, Azure VMs, GCP Compute Engine |
| PaaS | Apps and data | OS, runtime, middleware, hardware | Elastic Beanstalk, Azure App Service |
| SaaS | Data and settings only | Everything else | Microsoft 365, Salesforce, Workday |
| FaaS / Serverless | Function code only | Infra, scaling, runtime | Lambda, Azure Functions, Cloud Run |
| CaaS | Containers and apps | Kubernetes, OS, hardware | EKS, AKS, GKE |
Go deeper
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Azure Regions and Geographies
Key term
Azure Functions
Azure Functions is a serverless compute service that lets you run event-driven code without managing infrastructure, automatically scaling to meet demand.
Key term
Serverless
Serverless is a cloud computing model where the cloud provider manages the servers, and you only pay for the actual compute time your code uses, without having to worry about provisioning or maintaining infrastructure.
About these practice questions
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This AZ-900 practice question is part of Courseiva's free Microsoft certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the AZ-900 exam.