AZ-104 Part 3 — Compute

The spectrum from IaaS to PaaS

More control   Virtual Machines   →   VM Scale Sets   →   ACI  →  AKS  →  App Service   Less control
Less managed                                                                             More managed

Virtual Machines (IaaS)

az vm create -g myrg -n webvm \
  --image Ubuntu2204 --admin-username azureuser --generate-ssh-keys

• OS disk — required (28 GB min).
• Data disks — attach separately, live longer than the VM.
• Temp disk — fast, but wiped on stop.

Availability — the SLA cheat sheet

Setup	Uptime SLA
Single VM with Premium SSD	99.9%
Availability Set (spread across racks)	99.95%
Availability Zone (spread across zones)	99.99%

Availability Sets protect from rack failure; Availability Zones protect from datacenter failure. Zones win when the exam mentions "datacenter outage".

VM Scale Sets (VMSS)

A group of identical VMs behind a Load Balancer that grows/shrinks based on load. Perfect for stateless web tiers.

az vmss create -g myrg -n webscale --image UbuntuLTS --instance-count 3

App Service (PaaS web hosting)

• App Service Plan = the machine you're renting (Free, B1, S1, P1v3…).
• Web App = the code that runs on it.
• Deployment Slots = staging → swap to production with zero downtime.
• Scale Up = bigger machine. Scale Out = more machines.

Containers on Azure

Service	Use it when
ACI (Container Instances)	One-shot job / small container, no orchestration
AKS (Kubernetes)	Real production, many services, autoscale, DevOps
ACR (Container Registry)	Private Docker Hub for your images

Real-world example

Your college project runs on a B1 App Service (₹1000/mo). Traffic spikes on results day → you scale up to P1v3 for 24 h then back to B1. No servers touched, no downtime, invoice barely moves.

Exam gotchas

• Availability Set = same region, different racks (99.95%).
• Availability Zones = different datacentres in the region (99.99%).
• Deployment slots exist on Standard, Premium, Isolated App Service plans — not on Free/Basic.
• ACI billed per second; AKS billed per node VM.