1.4 The Consumption-Based Model

Quick Answer: The consumption-based (pay-as-you-go) model charges only for what you actually use — metered by the second, by the GB, or per transaction. No upfront cost, no wasted idle hardware; stop a resource and the meter stops. You can optionally commit to 1- or 3-year terms for large discounts.

Why It Matters

In traditional IT you buy hardware up front against a guess about future demand. Overestimate and you waste money on idle servers; underestimate and you cannot serve users. The consumption model removes the guess: capacity follows demand, and billing follows capacity. AZ-900 will not ask you to calculate a bill — it asks which pricing option fits a scenario and which billing rules apply.

Core principles

1. No upfront cost — no hardware purchase or mandatory contract (unless you choose a discount commitment).
2. No wasted resources — scale down or deallocate when demand falls.
3. Pay for what you use — billing tracks real consumption.
4. Stop paying when you stop using — deallocate compute and the compute meter halts.

Azure Pricing Options

Pay-As-You-Go (PAYG)

The default: pay per second/hour/transaction with no commitment.

• Best for: unpredictable or short-term workloads, dev/test, proofs of concept.
• Flexibility: maximum — change or stop anytime.
• Discount: 0% (full list price).

Reserved Instances (RIs)

Commit to a specific resource (e.g., a VM size in a region) for 1 or 3 years.

• Best for: steady-state, 24/7, predictable workloads.
• Flexibility: low — tied to the reserved capacity (exchange/refund rules apply).
• Discount: up to ~72% vs PAYG.

Azure Savings Plans (for compute)

Commit to a fixed hourly dollar amount of compute spend for 1 or 3 years; the discount flexes automatically across VM families and regions.

• Best for: consistent overall compute spend but changing VM mix.
• Flexibility: medium — locked to a spend rate, not a specific SKU.
• Discount: up to ~65% vs PAYG.

Spot VMs

Buy unused Azure capacity at deep discounts; Azure can reclaim it whenever capacity is needed.

• Best for: interruptible, fault-tolerant work — batch jobs, rendering, CI/CD, dev/test.
• Flexibility: none guaranteed — Azure evicts with a minimum 30 seconds' notice.
• Discount: up to ~90% vs PAYG.

Pricing Option Comparison

How Azure Meters Usage

Billing rules you must memorize

• Inbound data (into Azure): generally free.
• Outbound data (leaving Azure): metered and billed after a free monthly allowance.
• Deallocated (stopped) VM: compute charges stop, but disk storage charges continue.
• Deleted resources: all charges stop. To zero out a VM's cost, delete the VM and its disks.

On the Exam: The classic trap question is "a stopped VM stops all charges." False — deallocation halts compute only; the OS/data disks still cost money until deleted.

Factors That Affect Cost

Cost-management tooling (also tested): use the Azure Pricing Calculator to estimate before deploying, the Total Cost of Ownership (TCO) Calculator to compare on-prem vs Azure, Azure Cost Management + Billing to track and analyze actual spend, and budgets/alerts plus resource tags to govern and attribute costs.

On the Exam: You do not compute prices. You match a workload to the right option (steady 24/7 ⇒ Reserved Instance; interruptible batch ⇒ Spot; unpredictable/short ⇒ PAYG) and recall the billing rules above.

Worked Scenario: Picking the Right Commitment

An analytics company runs three distinct workloads. Its production database servers run 24 hours a day, every day, with steady load — these are textbook Reserved Instance candidates, locking in up to ~72% savings for a 1- or 3-year term because the demand is fully predictable. Its data-science team runs large, restartable model-training jobs overnight that tolerate interruption — perfect for Spot VMs at up to ~90% off, since an eviction merely reschedules the job.

Its developers spin up short-lived test environments a few hours at a time on an unpredictable schedule — best left on Pay-As-You-Go, because committing would waste the discount on capacity they do not consistently use. If the company has consistent total compute spend but keeps changing VM families, a Savings Plan captures up to ~65% without locking a specific SKU.

Azure Hybrid Benefit and Other Savings Levers

Beyond the four pricing options, AZ-900 expects you to recognize Azure Hybrid Benefit: organizations that already own Windows Server or SQL Server licenses with Software Assurance can apply them to Azure VMs and pay only the base (Linux) compute rate, avoiding a second license charge. Combined with reservations, Hybrid Benefit can stack for substantial savings. Other levers that affect the bill include choosing a cheaper region, right-sizing the VM SKU, and reducing egress by keeping traffic within a region.

Cost-Management Tooling Summary

Common Trap: The Pricing Calculator estimates a proposed deployment, while Cost Management analyzes existing spend, and the TCO Calculator compares on-prem to Azure. Match the tool to whether the scenario is planning, comparing, or monitoring — exam stems hinge on that distinction.