Containers: ACR, ACI & Container Apps

Containers vs VMs

Simple explanation

If a VM is a house (full building with its own plumbing and electricity), a container is an apartment (shares the building’s infrastructure but has its own space).

Containers package your application with everything it needs to run — code, libraries, settings — but share the host operating system. They start in seconds (not minutes like VMs), use less resources, and are perfect for microservices.

Azure gives you three container services: Container Registry (stores your container images), Container Instances (run containers without managing servers), and Container Apps (run containers with auto-scaling and traffic management).

Azure Container Registry (ACR)

ACR is a private Docker container registry — a storage service for your container images.

SKU	Features	Storage	Use Case
Basic	Core features, limited throughput	10 GB	Dev/test, personal projects
Standard	Increased throughput and storage	100 GB	Most production workloads
Premium	Geo-replication, private link, content trust	500 GB	Enterprise, multi-region deployments

Key operations:

az acr create — create a registry
az acr build — build an image in the cloud (no local Docker needed)
docker push myregistry.azurecr.io/myapp:v1 — push an image
docker pull myregistry.azurecr.io/myapp:v1 — pull an image

Azure Container Instances (ACI)

ACI is the simplest way to run a container in Azure — no VMs, no orchestrator, no cluster management. Just run a container.

Key features:

Starts in seconds
Per-second billing
Supports both Linux and Windows containers
Container groups — multiple containers sharing resources (like a Kubernetes pod)
Can mount Azure Files for persistent storage

Limitations: No auto-scaling, no built-in load balancing, no traffic splitting. For those, use Container Apps.

Azure Container Apps (ACA)

ACA is a managed container platform built on Kubernetes (but you don’t manage Kubernetes). It adds the features ACI lacks.

Key features:

Auto-scaling — scale based on HTTP traffic, CPU, memory, or event-driven (KEDA)
Scale to zero — no instances running = no compute cost
Revision management — deploy new versions alongside old ones
Traffic splitting — route percentage of traffic to different revisions (canary deployments)
Built-in ingress — HTTPS endpoints without configuring a load balancer

Container Instances vs Container Apps
Feature	ACI (Container Instances)	ACA (Container Apps)
Best for	Simple tasks, batch jobs, testing	Web apps, APIs, microservices
Auto-scaling	No	Yes (HTTP, CPU, memory, events)
Scale to zero	No (minimum 1 instance)	Yes
Load balancing	No (manual)	Yes (built-in ingress)
Traffic splitting	No	Yes (canary deployments)
Pricing	Per-second (vCPU + memory)	Per-second + scale to zero

Exam tip: ACI vs ACA decision

The exam loves asking when to use ACI vs ACA. Simple rule: ACI for simple, short-lived tasks (batch processing, build agents, quick scripts). ACA for web apps and APIs that need scaling, load balancing, and traffic management. If the question mentions “auto-scale” or “microservices,” it’s ACA.

Question

What is Azure Container Registry (ACR) used for?

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Answer

ACR is a private Docker container registry that stores and manages container images. It integrates with ACI, ACA, AKS, and other container services. Premium SKU adds geo-replication and private link support.

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Question

What is the key advantage of Azure Container Apps over Container Instances?

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Answer

Container Apps provides auto-scaling (including scale to zero), built-in load balancing, traffic splitting between revisions, and event-driven scaling via KEDA. Container Instances lacks all these features — it's simpler but less capable for production web workloads.

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Question

What are the three ACR SKUs and when would you use Premium?

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Answer

Basic (dev/test, 10 GB), Standard (production, 100 GB), Premium (enterprise, 500 GB). Use Premium when you need geo-replication (multi-region image availability), private link (no public endpoint), or content trust (signed images). Most production workloads use Standard.

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Real-world: Meridian Financial's container architecture

Meridian Financial runs their customer-facing API on Azure Container Apps with:

ACR Premium with geo-replication (UK South + UK West) for fast pulls in both regions
3 microservices as separate Container Apps, each auto-scaling independently
Scale to zero enabled on the reporting service (only used during business hours)
Traffic splitting — 90% to current revision, 10% to the new version for canary testing
Revision mode: multiple — both old and new versions run simultaneously during deployments

Total monthly cost: ~60% less than their previous VM-based setup, with better scaling.

Knowledge check

Knowledge Check

CloudFirst Labs needs to deploy a web API that auto-scales based on HTTP traffic and can scale to zero when idle to save costs. Which service should they use?

Knowledge Check

TechCorp needs to run a nightly batch job that processes CSV files and exits when done. The job runs for about 10 minutes. Which container service is most appropriate?