AZ-900 Describe Azure architecture and services Practice Question
A company deploys two Azure virtual machines in an availability set. The application requires that at least one VM remains running during Azure platform-initiated maintenance, such as operating system updates to the underlying host. Which component of the availability set directly ensures that the VMs are not updated at the same time?
⚠ Common exam trap
Candidates often confuse fault domains (hardware failure isolation) with update domains (maintenance sequencing), leading them to incorrectly select fault domains when the question specifically asks about platform-initiated maintenance updates.
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
✓
Update domains
Update domains (B) are the correct component because they logically group VMs that are updated together during Azure platform-initiated maintenance. By placing VMs in different update domains, Azure ensures that only one update domain is taken offline at a time, guaranteeing that at least one VM remains running during host OS updates.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Fault domains
Why it's wrong here
Fault domains represent physically separate server racks within an Azure datacenter, providing independent power, cooling, and network switches to protect against hardware failures. They isolate VMs from single points of physical infrastructure failure, but they do not influence the order of OS or guest updates during planned maintenance. Update sequencing is managed exclusively by update domains, not fault domains.
When this WOULD be correct
A question asking: 'Which component ensures VMs are placed on different physical racks to survive a rack-level failure?' would make fault domains the correct answer, as they isolate VMs across distinct hardware to mitigate single points of failure.
- ✓
Update domains
Why this is correct
Update domains logically segment VMs within an availability set so that during Azure platform-planned maintenance, only one update domain's VMs are rebooted and updated at a time. This sequencing ensures at least one replica of your workload remains running throughout the maintenance window, preserving the availability SLA. Because Azure may reboot VMs during a planned update, spreading VMs across multiple update domains is essential for high availability.
- ✗
Proximity placement groups
Why it's wrong here
Proximity placement groups are a logical grouping that places VMs in the same Azure region and availability zone to achieve the lowest possible network latency between them. They are used for performance-sensitive, tightly coupled workloads like HPC or low-latency applications, not for managing platform maintenance or improving availability during updates. These groups do not define any maintenance behavior and, in fact, may reduce fault tolerance if not carefully designed.
When this WOULD be correct
A question that asks: 'Which Azure feature should you use to minimize network latency between two VMs running a latency-sensitive application?' would have proximity placement groups as the correct answer.
- ✗
Availability zones
Why it's wrong here
Availability zones are physically separate datacenters within a region and are not a component of an availability set. They provide high availability across datacenters, but the question specifically asks about the component within an availability set that handles maintenance updates.
When this WOULD be correct
A question that asks: 'Which Azure feature provides resilience by placing VMs in physically separate locations within a region to protect against datacenter failures?' would have availability zones as the correct answer.
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.
✓Update domainsCorrect answer▾
Why this is correct
Update domains logically segment VMs within an availability set so that during Azure platform-planned maintenance, only one update domain's VMs are rebooted and updated at a time. This sequencing ensures at least one replica of your workload remains running throughout the maintenance window, preserving the availability SLA. Because Azure may reboot VMs during a planned update, spreading VMs across multiple update domains is essential for high availability.
✗Fault domainsWrong answer — click to see why▾
Why this is wrong here
Fault domains protect against physical hardware failures (e.g., rack or power supply issues) by distributing VMs across separate racks, but they do not control the sequencing of platform-initiated maintenance like OS updates; update domains handle that.
★ When this WOULD be the correct answer
A question asking: 'Which component ensures VMs are placed on different physical racks to survive a rack-level failure?' would make fault domains the correct answer, as they isolate VMs across distinct hardware to mitigate single points of failure.
Why candidates choose this
Candidates confuse fault domains with update domains because both are availability set concepts, and they may think 'fault' covers all types of failures including maintenance, not realizing maintenance is specifically managed by update domains.
✗Proximity placement groupsWrong answer — click to see why▾
Why this is wrong here
Proximity placement groups are used to ensure VMs are physically close to reduce network latency, not to control maintenance sequencing. They do not affect the order of platform-initiated updates.
★ When this WOULD be the correct answer
A question that asks: 'Which Azure feature should you use to minimize network latency between two VMs running a latency-sensitive application?' would have proximity placement groups as the correct answer.
Why candidates choose this
Candidates may confuse the concept of grouping VMs for maintenance (update domains) with grouping for physical proximity, as both involve organizing VMs within an availability set or placement group.
✗Availability zonesWrong answer — click to see why▾
Why this is wrong here
Availability zones are physically separate datacenters within a region, designed to protect against datacenter-level failures, not against platform-initiated maintenance that updates underlying hosts within the same datacenter.
★ When this WOULD be the correct answer
A question that asks: 'Which Azure feature provides resilience by placing VMs in physically separate locations within a region to protect against datacenter failures?' would have availability zones as the correct answer.
Why candidates choose this
Candidates may confuse availability zones with update domains because both involve distributing VMs for high availability, but zones address broader infrastructure failures while update domains handle planned maintenance.
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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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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