5.2 Database Service & Compute Tier Design
Key Takeaways
- vCore is Microsoft's recommended purchasing model for new deployments because it exposes compute/storage separately and is the only model that supports Azure Hybrid Benefit.
- General Purpose uses remote storage for balanced cost; Business Critical uses local SSD with a built-in Always On availability group for lowest latency and one free readable secondary; Hyperscale decouples storage from compute for databases up to 128 TB with near-instant backups.
- Serverless is a compute option inside General Purpose only — it autoscales, auto-pauses during inactivity, and bills per second, fitting intermittent workloads that can tolerate a brief resume delay.
- DTU bundles compute, memory, and I/O into fixed Basic/Standard/Premium tiers and cannot use Azure Hybrid Benefit or reach Business Critical/Hyperscale tiers.
- Match the tier to the defining constraint: latency and built-in HA point to Business Critical; size and fast backup/restore point to Hyperscale; cost-sensitive intermittent use points to serverless General Purpose.
Why This Topic Matters
Once you have picked a relational engine (5.1), AZ-305 tests the next design decision under the same official bullet family: "Recommend a database service tier and compute tier." This is a right-sizing and cost/performance trade-off question — Azure SQL Database and Managed Instance both offer more than one purchasing model and more than one service tier, and the exam expects you to match workload characteristics (predictability, latency sensitivity, size, budget) to the correct combination.
Purchasing Models: DTU vs. vCore
| Model | How it works | Best for |
|---|---|---|
| DTU (Database Transaction Unit) | A blended measure of CPU, memory, and I/O bundled into preset tiers (Basic, Standard, Premium) with fixed storage per tier | Simple, predictable workloads where you do not want to reason about compute and storage separately; smaller/legacy deployments already sized in DTUs |
| vCore | You independently choose the number of virtual cores, the hardware generation, and the storage amount | Workloads that need transparent, granular control over compute vs. storage cost, want Azure Hybrid Benefit (reusing existing SQL Server licenses for a discount), or need service tiers not available under DTU (Business Critical, Hyperscale) |
Key fact: vCore is Microsoft's recommended model for new deployments because it exposes hardware and licensing transparently and unlocks Azure Hybrid Benefit, which the DTU model cannot use. On the exam, a scenario that mentions "the organization has existing SQL Server licenses with Software Assurance and wants to reduce Azure costs" is a strong signal to recommend the vCore model with Azure Hybrid Benefit, not DTU.
vCore Service Tiers
| Tier | Storage architecture | Characteristics | Best for |
|---|---|---|---|
| General Purpose | Remote (network) storage, single compute node | Balanced compute/storage cost, 99.99% SLA | Most business workloads without extreme latency or built-in read-replica needs |
| Business Critical | Local SSD storage, built-in Always On availability group across replicas | Lowest latency, highest resilience, includes one free readable secondary replica for read scale-out | Mission-critical, latency-sensitive OLTP (e.g., trading, ticketing) |
| Hyperscale | Decoupled storage and compute via a scale-out page-server architecture | Storage autoscaling up to 128 TB, near-instant backups and fast restores independent of database size, up to 30 named replicas with independently sized compute | Fast-growing databases, large analytical or mixed workloads, scenarios needing rapid backup/restore regardless of size |
The Serverless Compute Tier
Serverless is a compute option available only in the vCore General Purpose tier (not a separate service tier). Its defining behaviors:
- Autoscaling: compute automatically scales between a configured minimum and maximum vCore count based on workload demand.
- Auto-pause: after a configurable period of inactivity (the auto-pause delay), the database pauses completely — only storage is billed while paused.
- Per-second billing: you pay for vCores consumed and memory, billed per second of active compute, rather than for a fixed provisioned size around the clock.
- Automatic resume: the first new connection triggers a resume, which takes several seconds to a couple of minutes depending on cache state.
Serverless is the correct recommendation for workloads with intermittent, unpredictable usage and tolerance for a brief resume delay — classic examples are dev/test databases, low-traffic line-of-business apps used only during business hours, and new applications where usage patterns are not yet known. It is the wrong choice for workloads that cannot tolerate any cold-start latency (e.g., a customer-facing checkout database that must always be warm) — for those, use provisioned compute in the appropriate tier instead.
Exam Scenarios
Scenario A: A finance team runs a batch reporting database used only 9am-5pm on weekdays; it sits idle every night and weekend. Cost efficiency matters more than instant availability. Answer: vCore General Purpose tier with serverless compute and an appropriate auto-pause delay.
Scenario B: A stock-trading platform requires sub-millisecond storage latency and automatic failover with zero data loss if the primary node fails. Answer: vCore Business Critical tier — its local SSD storage and built-in Always On availability group deliver both requirements.
Scenario C: An analytics database is growing rapidly toward multiple terabytes, and the team needs backups and restores to remain fast regardless of size, plus the ability to add dedicated read replicas for reporting without affecting the primary. Answer: Hyperscale — its decoupled storage/compute design keeps backup/restore times roughly constant as data grows, and named replicas isolate reporting load.
Scenario D: An organization already owns SQL Server Enterprise Edition licenses with active Software Assurance and wants to minimize Azure spend on a new deployment. Answer: vCore purchasing model with Azure Hybrid Benefit applied — impossible under the DTU model.
Common Traps
- Recommending DTU just because a scenario is "simple" — if Azure Hybrid Benefit, Hyperscale, or Business Critical is mentioned or implied, vCore is required.
- Confusing serverless with a separate service tier — it is a compute option within General Purpose, not its own tier, and it is not available in Business Critical or Hyperscale.
- Picking Business Critical purely for "high availability" when Hyperscale is a better fit for a workload whose defining constraint is size and backup/restore speed rather than sub-millisecond latency.
A company already owns SQL Server Enterprise Edition licenses with active Software Assurance and wants to reduce the cost of a new Azure SQL Database deployment. Which purchasing model and feature should you recommend?
A development database is used only during business hours and sits idle nights and weekends. The team wants to minimize cost and can tolerate a short resume delay after idle periods. Which configuration should you recommend?
Which Azure SQL Database service tier uses a decoupled storage-and-compute architecture that keeps backup and restore times roughly constant as a database grows to tens of terabytes?