Quick Note — Recommend a High Availability Solution for Compute
AZ-305 › Unit 3: Design business continuity solutions › Design for high availability › Recommend a high availability solution for compute
Quick Note — Recommend a High Availability Solution for Compute
A retail bank pushes a routine OS patch to its payment-processing fleet on a Wednesday afternoon. Twenty minutes later the platform team gets paged: half the gateway VMs are offline, transactions are queueing, and the SRE on duty cannot find the postmortem template fast enough. The cause is not the patch — the cause is that the fleet was deployed into a single fault domain on a single host. A platform that the architect had described as "highly available" turned out to be just "redundantly deployed". This lesson is about closing that gap, deliberately, before an incident exposes it.
Retrieval target
- Recommend a High Availability Solution for Compute
- Closed-book recall
- 5 minutes
- Open the full lesson after a miss
Decision anchors
| Prompt | Compact answer |
|---|---|
Availability Zone (AZ) | A physically separate datacenter inside an Azure region with independent power, cooling, and networking. Each region with AZ support has AZs. Spreading replicas across AZs gives 99.99% SLA for zone-redundant deployments. Not every region supports AZs — check the region list when designing. |
Availability Set (fault + update domains) | A logical grouping of VMs within one datacenter that spreads them across distinct fault domains (up to 3 — separate racks, power, network) and update domains (up to 20 — separate maintenance windows). Lifts the VM SLA from 99.9% → 99.95%. Doesn't protect against datacenter outage; use Availability Zones for that. |
Virtual Machine Scale Set (VMSS) | A managed group of identical, load-balanced VMs that you can scale by count (manual / scheduled / metric-based autoscale). VMSS instances can be deployed across multiple AZs for zone redundancy, into a single AZ + Availability Set for cheaper HA, or 'regional' (Microsoft picks zones). The foundation for AKS node pools, App Service hosts, and many PaaS services internally. |
| Single VM SLA ladder: $99.%$ | 99.9% — single VM with premium SSD disks for OS + data (no redundancy at the VM level). 99.95% — 2+ VMs in an Availability Set (fault/update-domain separation in one DC). 99.99% — 2+ VMs spread across Availability Zones (independent DCs in the region). Move up the ladder by changing deployment topology, not VM SKU. |
Read the answers once, then cover the right-hand column and reconstruct each one from the prompt. A useful answer names the requirement, the recommended control or service boundary, and the nearest alternative it rejects. If you can only recognize the answer after seeing it, retrieval is not yet secure.
Ninety-second explanation
Without notes, explain:
- What requirement signals this learning objective rather than a neighbouring one?
- Which two solution families are most likely to be compared?
- Which hard constraint eliminates the strongest distractor?
- What identity, network, data, or failure boundary must appear in the design?
- Which operational test would prove the recommendation works?
Then compare your explanation with the full lesson. Record the missing decision rule—not merely the missed product name—in your error log.
When to open the full lesson
Open the curriculum-linked lesson when you cannot explain a comparison, when a scenario depends on a numeric limit or SKU feature, or when the service is on a retirement path. Use current Microsoft Learn documentation for availability, limits, pricing, naming, and migration milestones; the quick note is intentionally compact.
Source and freshness
Derived from the linked AZ-305 lesson and retrieval deck, grounded in both attached course sources. Reviewed 2026-08-02. Current Microsoft documentation controls changing product contracts.