AZ-900 Describe Azure architecture and services Practice Question
A company deploys three Azure virtual machines (VMs) that host a critical line-of-business application. All three VMs are located in the same Azure region. The company notices that during planned maintenance events triggered by the Azure platform, such as host OS updates, all three VMs are updated simultaneously, causing the application to become unavailable. The company requires that during such maintenance, at least two VMs remain running to preserve application uptime. Which Azure feature should the company implement to logically group the VMs and ensure they are updated in separate batches?
⚠ Common exam trap
Watch out — candidates often confuse Availability Zones with Availability Sets, thinking that zones provide the same update batching behavior, but zones only guarantee physical separation across data centers, not the sequential update domain logic that ensures VMs are updated in separate batches during planned maintenance.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Azure Availability Set
An Azure Availability Set logically groups VMs to protect against platform updates and faults. By placing VMs into an availability set, Azure assigns them to different update domains (default 5) and fault domains (default 2). During planned maintenance, only one update domain is rebooted at a time, ensuring that at most one of the three VMs is updated simultaneously, thus keeping at least two VMs running.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Azure Availability Zone
Why it's wrong here
Availability Zones protect against an entire datacenter outage within a region by placing VMs in physically separate zones, but they do not control the sequencing of updates during planned maintenance. Zonal VMs are still subject to individual zone updates. Therefore, this does not guarantee that VMs are updated in separate batches.
When this WOULD be correct
A company deploys three VMs in the same region and requires high availability against a complete datacenter failure. Which feature should they use to place VMs in physically separate locations within the region?
- ✓
Azure Availability Set
Why this is correct
Availability Sets logically group VMs into update domains and fault domains. Update domains ensure that only a subset of VMs is taken offline during planned Azure platform maintenance, preventing simultaneous downtime. Fault domains distribute VMs across separate hardware racks to guard against local hardware failures. This meets the requirement of keeping at least two VMs running during updates.
- ✗
Azure Virtual Machine Scale Set
Why it's wrong here
Azure Virtual Machine Scale Sets (VMSS) are incorrect because their primary purpose is to deploy and manage a group of identical, load-balanced VMs for scaling applications, not to logically group existing, potentially distinct, critical VMs to ensure separate update batches. While VMSS instances are distributed across update domains, the scenario describes existing VMs needing protection from simultaneous updates. This option is tempting as VMSS does distribute instances across update domains, which helps with availability during maintenance. It would be the correct choice for deploying a new, scalable application where all instances are identical and managed as a single unit.
When this WOULD be correct
A company needs to automatically scale the number of VMs based on demand (e.g., CPU usage) and distribute traffic across them. The correct answer would be Virtual Machine Scale Set when the question focuses on scaling and load balancing, not on maintenance isolation.
- ✗
Azure Site Recovery
Why it's wrong here
Azure Site Recovery is a disaster recovery service that replicates workloads to a secondary region to protect against region-wide outages. It does not address planned maintenance events within the primary region and cannot ensure that VMs are updated in separate batches.
When this WOULD be correct
A company requires business continuity and disaster recovery (BCDR) for its Azure VMs, needing to replicate workloads to a different Azure region to ensure failover during a regional outage or disaster.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The AZ-900 exam frequently reuses these exact scenarios with slightly different constraints.
✓Azure Availability SetCorrect answer▾
Why this is correct
Availability Sets logically group VMs into update domains and fault domains. Update domains ensure that only a subset of VMs is taken offline during planned Azure platform maintenance, preventing simultaneous downtime. Fault domains distribute VMs across separate hardware racks to guard against local hardware failures. This meets the requirement of keeping at least two VMs running during updates.
✗Azure Availability ZoneWrong answer — click to see why▾
Why this is wrong here
Azure Availability Zones protect against datacenter-level failures by distributing VMs across physically separate zones within a region, but they do not control the order of platform maintenance updates across VMs within the same zone. The question requires grouping VMs to ensure they are updated in separate batches during planned maintenance, which is a feature of Availability Sets, not Availability Zones.
★ When this WOULD be the correct answer
A company deploys three VMs in the same region and requires high availability against a complete datacenter failure. Which feature should they use to place VMs in physically separate locations within the region?
Why candidates choose this
Candidates may confuse 'high availability' scenarios, assuming that distributing VMs across zones also controls maintenance update sequencing, or they may not fully understand that Availability Zones address physical isolation, not update domain grouping.
✗Azure Virtual Machine Scale SetWrong answer — click to see why▾
Why this is wrong here
Azure Virtual Machine Scale Set provides auto-scaling and load balancing across multiple VMs but does not guarantee that VMs are updated in separate batches during planned maintenance; it does not offer update domains like an Availability Set.
★ When this WOULD be the correct answer
A company needs to automatically scale the number of VMs based on demand (e.g., CPU usage) and distribute traffic across them. The correct answer would be Virtual Machine Scale Set when the question focuses on scaling and load balancing, not on maintenance isolation.
Why candidates choose this
Candidates may confuse the scaling and distribution capabilities of a scale set with the update domain isolation provided by an Availability Set, assuming that spreading VMs across a scale set inherently separates updates.
✗Azure Site RecoveryWrong answer — click to see why▾
Why this is wrong here
Azure Site Recovery is a disaster recovery solution that replicates VMs to a secondary region for failover during outages, not a feature to control platform maintenance updates within a single region.
★ When this WOULD be the correct answer
A company requires business continuity and disaster recovery (BCDR) for its Azure VMs, needing to replicate workloads to a different Azure region to ensure failover during a regional outage or disaster.
Why candidates choose this
Candidates may confuse Site Recovery as a way to maintain availability during maintenance events, not realizing it is designed for cross-region disaster recovery rather than intra-region update domain management.
Analysis generated from the official AZ-900blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Go deeper
Related to this question
Learn chapter
Azure Regions and Geographies
Key term
Availability set
An Availability set is a cloud computing feature that groups virtual machines across multiple fault domains and update domains to ensure high availability during hardware failures or planned maintenance.
Key term
Update domain
An update domain is a logical grouping of resources in a cloud or datacenter environment that can be patched or updated together without causing downtime to the entire application.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This AZ-900 practice question is part of Courseiva's free Microsoft certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the AZ-900 exam.