AZ-900 Describe Azure architecture and services Practice Question
Which Azure service allows you to run code on-demand without managing servers, paying only for execution time?
⚠ Common exam trap
Candidates often confuse Azure App Service (PaaS) with serverless because it abstracts server management, but App Service still runs on a continuously billed plan, whereas Azure Functions on a consumption plan is truly serverless with pay-per-execution billing.
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 a serverless compute service that executes code in response to events (e.g., HTTP requests, timers, queue messages) without requiring you to provision or manage virtual machines or infrastructure. You are billed only for the resources consumed during code execution, measured in gigabyte-seconds, making it ideal for on-demand, event-driven workloads.
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 Virtual Machines
Why it's wrong here
Azure Virtual Machines is an infrastructure-as-a-service (IaaS) offering where you are responsible for provisioning, patching, and maintaining the guest operating system and application stack. You also handle capacity planning, availability sets, and scaling rules, which means there is no automated, event-driven execution model. Because billing is per hour of consumed compute, you pay for allocated resources even when the VM is idle, making it unsuitable for a serverless, pay-per-use scenario.
- ✗
Azure App Service
Why it's wrong here
Azure App Service is a platform-as-a-service (PaaS) for hosting web apps, REST APIs, and mobile backends, but it runs on an App Service Plan that reserves dedicated virtual machines. You select a pricing tier that dictates the number of instances, their size, and always-on behavior, so you are billed for the plan's compute capacity rather than per individual code execution. While the platform patches the OS and runtime, it lacks the event-driven, consumption-based billing characteristic of serverless computing.
- ✓
Azure Functions
Why this is correct
Azure Functions is a serverless compute service that executes code in response to triggers such as HTTP requests, timers, queue messages, or blob uploads. It runs on the Consumption Plan by default, where Azure dynamically allocates instances only when events occur and scales down to zero between executions. You are billed only for the duration and number of function executions, so no infrastructure management is needed and you never pay for idle, provisioned capacity.
- ✗
Azure Kubernetes Service
Why it's wrong here
Azure Kubernetes Service (AKS) provides managed Kubernetes orchestration, but you remain responsible for managing the worker nodes, including their virtual machine size, OS patches, node pools, and autoscaling configuration. The control plane is managed by Azure, yet the worker nodes are still provisioned and continuously billed even if no containers are actively running. AKS is not event-driven; it is a container orchestration platform that requires ongoing cluster and node management, unlike a true serverless model.
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
Related to this question
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Azure Regions and Geographies
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.
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.
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