Why High Availability?
Azure guarantees uptime through Service Level Agreements (SLAs).
To achieve these SLAs, you must design workloads with redundancy:
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Across hardware (racks).
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Across datacenters (zones).
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Across VM instances (scale sets).
Availability Sets (Within a Datacenter)
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Logical grouping of VMs in the same region/datacenter.
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Protects against:
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Fault Domain (FD): Hardware racks (power/network isolation).
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Update Domain (UD): OS/hypervisor updates.
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Guarantees at least one VM remains up during maintenance.
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SLA: 99.95% uptime with 2+ VMs in an Availability Set.
Availability Zones (Across Datacenters)
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Physically separate datacenters within the same region.
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Each zone has independent power, cooling, networking.
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Place VMs across multiple zones to survive a datacenter-level outage.
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SLA: 99.99% uptime with 2+ VMs across Zones.
VM Scale Sets (Scaling Instances)
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Group of identical, load-balanced VMs that can auto-scale.
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Supports scaling up/down based on demand (CPU, memory, metrics).
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Works with Availability Zones for both resiliency and elasticity.
Confusion Buster 🚨
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Availability Set vs Zone
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Set = resilience inside one datacenter.
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Zone = resilience across multiple datacenters.
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Zone vs Region
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Zone = datacenter.
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Region = collection of zones.
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Scale Set vs Availability Feature
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Scale Set = scaling (number of VMs).
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Set/Zone = resiliency (where VMs run).
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Exam trap: If scenario says “protect against datacenter outage” → Zone, not Set. If it says “scale web servers automatically” → VM Scale Set.
Simple Example
An e-commerce company:
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Runs web servers in a VM Scale Set across 3 Availability Zones → handles traffic spikes + survives datacenter failure.
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Runs SQL Database VMs in an Availability Set → ensures patching/updates don’t bring all nodes down at once.
Exam Tip
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“Protect against hardware failure inside a datacenter” → Availability Set.
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“Protect against datacenter outage” → Availability Zones.
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“Auto-scale web servers based on demand” → VM Scale Sets.
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“Achieve 99.99% SLA for VMs” → Deploy across Availability Zones.
What to Expect in the Exam
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Direct Q: “Which feature distributes VMs across racks in a datacenter?” → Availability Set.
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Scenario: “Company wants resiliency if a whole datacenter goes down.” → Availability Zones.
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Scenario: “Web servers must scale automatically during peak traffic.” → VM Scale Sets.
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Trick Q: “Availability Sets protect against regional outages.” (False — that’s Zones).