What is Core Compute — Virtual Machines & Scale Sets in Microsoft Azure?

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Microsoft Azure

Core Compute — Virtual Machines & Scale Sets

Run Windows/Linux VMs and scale them elastically with VM Scale Sets behind a load balancer.

Azure VMs are the rented servers. The interview is about the fleet around one, not the one.

Leasing identical machines from a depot that automatically adds or removes machines as the workload changes and instantly swaps any that fail — and gives a loyalty discount (hybrid benefit) if you already own the software licences.

Key Concepts

The series are named for what they are tuned for.

    B   burstable        cheap, accrues credits while idle. Dev,
                         low-traffic sites. Runs out under load.
    D   general purpose  web and application servers
    E   memory optimised caches, in-memory databases
    F   compute optimised CPU-bound work
    N   GPU              inference, training, rendering

B-series is the one that bites. It is cheap because it banks credits when idle and spends them when busy. A sustained load exhausts them and the VM is throttled to its baseline — which can be a fraction of a core.

Scale Sets manage the fleet. A VMSS keeps an identical group at a target size, spread across zones, replacing anything that fails its health probe:

    autoscale   CPU > 70% for 5 min -> add 2, max 20
                CPU < 30% for 10 min -> remove 1, min 3
    schedule    scale up before a known peak

Zones versus sets is the Azure-specific availability question:

    Availability Zone  separate DATACENTRES in the region
    Availability Set   fault and update domains inside ONE datacentre

Zones are the stronger guarantee and the modern default.

Flexible orchestration is now the default VMSS mode — it can mix sizes and handle VMs more like individual machines, where Uniform mode required them identical.

Pricing is a lever, not a footnote.

    pay-as-you-go     no commitment
    Reserved / Savings Plan   1 or 3 years, up to ~72%
    Spot              deep discount, evicted with 30 seconds' notice
    Hybrid Benefit    reuse owned Windows and SQL licences --
                      often the largest single saving on Azure

What stays yours. The operating system, patching and the image pipeline. Choosing VMs over App Service, Container Apps or Functions should be a decision about needing that control, not a default.

What the interviewer is probing.1. "Your B-series VM is fast in testing and slow in production. Why?" Probing: burst credits, which is an Azure-specific trap. Stalls: "It needs a bigger size." Moves up: B-series banks credits while idle and spends them under load; sustained traffic exhausts them and throttles the VM to a baseline that can be a fraction of a core.

2. "Availability Set or Availability Zone?" Probing: the difference in what they protect against. Stalls: "They are the same thing." Moves up: a set spreads across fault and update domains inside one datacentre; zones are separate datacentres, so only zones survive a datacentre failure.

3. "What is Azure Hybrid Benefit?" Probing: the biggest Azure-specific saving. Stalls: "A discount." Moves up: applying Windows Server and SQL licences you already own to cloud compute, often halving the cost — and most migrating organisations already hold them.

4. "How does a scale set replace a failed instance?" Probing: auto-healing. Stalls: "Someone restarts it." Moves up: the health probe marks it unhealthy and the scale set terminates and relaunches from the template, keeping the target count.