platformUpdateDomainCount Value in Availability Set ARM Template

AZ-104 By islever Editorial Last updated

Create and configure virtual machines Automate deployment of resources by using Azure Resource Manager (ARM) templates or Bicep files

Short answer

B Set platformUpdateDomainCount to 20, the maximum update domains an Azure availability set supports for the best maintenance resilience.

Question

Your company has an Azure subscription.
You need to deploy a number of Azure virtual machines (VMs) using Azure Resource Manager (ARM) templates. You have been informed that the VMs will be
included in a single availability set.
You are required to make sure that the ARM template you configure allows for as many VMs as possible to remain accessible in the event of fabric failure or
maintenance.
Which of the following is the value that you should configure for the platformUpdateDomainCount property?
  1. 10
  2. 20 Correct Answer
  3. 30
  4. 40

Key Takeaways

The question tests the maximum allowed update domain count in an availability set, and the trap is choosing 30 or 40 because Azure limits platformUpdateDomainCount to 20 (while fault domains max out at 3).

When deploying Azure VMs into an availability set with an ARM template, the platformUpdateDomainCount property sets how many update domains the availability set uses. Azure caps this value at 20, so configuring 20 maximizes how many VMs stay online during planned maintenance and platform updates.

Common mistake: Test-takers pick D (40) by assuming a higher number always improves resilience, or C (30) by confusing update domains with other limits; Azure rejects any value above 20 update domains per availability set, so those deployments fail.

Expert Analysis

Why the Answer Is Correct

Azure documents that each availability set can be configured with up to 3 fault domains and 20 update domains, so 20 is the highest legal value for platformUpdateDomainCount. Update domains are rebooted in rolling batches during planned maintenance, so more update domains means fewer VMs are taken down simultaneously and more remain accessible. Setting the property to 20 therefore gives the maximum possible resilience against fabric maintenance, exactly what the question asks for. Any value above 20 is out of range for an availability set and would be rejected by the ARM template deployment.

Why the Other Options Are Wrong

Option A (10) is a valid but suboptimal value — it works, yet halves the update-domain granularity and lets more VMs reboot together than necessary. Options C (30) and D (40) exceed the documented maximum of 20 update domains per availability set, so ARM validation fails and no VMs deploy at all. Note that 3 is the fault-domain limit, not an update-domain limit, so it cannot justify the higher numbers either. Since the requirement is "as many VMs as possible remain accessible," only the maximum valid value satisfies it.

Community Comment Notes

Comments consistently converge on B, with tubby04 quoting the official wording, "Each availability set can be configured with up to three fault domains and twenty update domains." Mehedi007 links the same Learn documentation line, and elishlomo explains the mechanism: with fewer update domains, batches of VMs go down together during maintenance. Push_Harder adds that "the best value you could configure against the update domain will be its maximum of 20," while MSExpert and Pradh both ask the site to correct a displayed answer of 40.

Official Reference

https://learn.microsoft.com/en-us/azure/virtual-machines/availability-set-overview#how-do-availability-sets-work

This link opens the official vendor documentation in a new tab.

Exam Strategy

Memorize the availability set pair of limits: 3 fault domains and 20 update domains. If an ARM template question asks for maximum resilience, set platformFaultDomainCount to 3 and platformUpdateDomainCount to 20.

Frequently Asked Questions

Why can't platformUpdateDomainCount be set to 30 or 40?

Azure limits an availability set to a maximum of 20 update domains (and 3 fault domains), so values above 20 fail ARM template validation and the deployment is rejected.

How do update domains keep VMs accessible during maintenance?

Update domains are logical batches rebooted one at a time during planned maintenance, so with 20 domains only a fifth of the VMs are affected at any moment and the rest stay online.

More AZ-104 FAQ →

Discussion 82 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 30 of 82

  1. tubby04 👍 123
    Correct answer is B. 20 'Each virtual machine in your availability set is assigned an update domain and a fault domain by the underlying Azure platform. Each availability set can be configured with up to three fault domains and twenty update domains.' https://docs.microsoft.com/en-us/azure/virtual-machines/availability-set-overview
  2. Pradh 👍 77
    Admin of this Website ... Please Update the answer to "B" . its giving negative impact on people who think of buying Contributor Access seeing such mistakes .
  3. creepted 👍 18
    this is the way the site is suppose to run because if this site give all the correct answers then MS will shut it down. that is why you need to read the discussions and analyze the answer by yourself.
  4. Mentalfloss 👍 6
    Really? Is that how sites like this exist? I had assumed just being out of the country was enough. Wutever. This is my first time back in 18 months and the new comment voting system is DA BOMB! lol
  5. MSExpert 👍 5
    Correct answer is B. 20 Admin or Moderator please change this answer to 20. Why it is still showing 40 ?
Show 25 more comments
  1. elishlomo 👍 4 Answered B
    The platformUpdateDomainCount is a property that defines how many update domains there are in the availability set. The upper limit is 20. Suppose the platformUpdateDomainCount is set to 3, and you have 15 virtual machines in the availability set. In that case, it means that 5 VMs can be updated and unavailable, but the remaining 10 VMs are always available.
  2. Mehedi007 👍 4 Answered B
    "Each availability set can be configured with up to 3 fault domains and 20 update domains." Ref: https://learn.microsoft.com/en-us/azure/virtual-machines/availability-set-overview#how-do-availability-sets-work
  3. Push_Harder 👍 4
    Answer: B Each availability set can be configured with up to 3 fault and 20 update domains. So, the best value you could configure against the update domain will be its maximum of 20.
  4. Swathi_Devi 👍 3
    Are you in the thought of learning cloud computing, especially Azure? Are you Preparing for the Az-104 exam? Glad you reading this! Practice Microsoft Azure Administrator Exam Az-104 for FREE!! WHAT IS SPECIAL? 400+ realistic Exam questions for practice with correct answers. wHAT IS NEW? The 400+ questions are grouped "MODULE-wise". You can be prepared for one module and practice the test for that one module before moving to the next module. ANYTHING MORE? Each question comes with a correct answer and also the reason why the remaining answers are not correct. ANY BONUS? Almost every question has a link to Microsoft's official pages for you to know more information. Why wait, all the above @FREE of cost, HURRY UP!! Just click the link https://www.udemy.com/course/microsoft-azure-administrator-practice-test/?couponCode=AZ104OFFER Share it with your friends, Offer ends soon!! Note: This is @FREE of cost to get your valuable feedback which is costly. Please drop in your feedback.
  5. ralphmas 👍 3
    When deploying Azure virtual machines in an availability set using ARM templates, the platformUpdateDomainCount property specifies the number of update domains to use. An update domain is a logical group of underlying physical hardware that can undergo maintenance or updates at the same time without affecting the availability of the virtual machines in the same availability set. The maximum number of update domains allowed is 20, and the default value is 5. To ensure that the ARM template allows for as many VMs as possible to remain accessible in the event of fabric failure or maintenance, it is recommended to set the platformUpdateDomainCount property to the maximum value of 20. Therefore, the answer is B: 20.
  6. ki01 👍 3
    i started thinking the same, if they had only current questions and 100% correct answers MS might sue them. and if they are out of country MS could petition backbone companies to block/remove the site. that's why you see some websites changing domains almost monthly like .to .is. net. .cc and etc. cloudflare received complains about 35k domains in 2021 and actioned a significant amount of them. so the old times of needing to nuke the website itself are gone.
  7. serenity404 👍 3
    The answer is "B. 20", as this is the maximum Update domains. https://docs.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-orchestration-modes
  8. Bapan 👍 3
    Given answer is wrong. Each availability set can be configured with up to three fault domains and twenty update domains.
  9. goodboy5569 👍 3
    The explanation however does say (Resource Manager deployments can then be increased to provide up to 20 update domains)
  10. Mev4953 👍 3
    platformFaultDomainCount : max 2-3 platformUpdateDomainCount : max 20
  11. pakman 👍 3
    Incorrect. The correct answer here is B (20)
  12. Vkom 👍 3 Answered B
    20 is the max value available to choose
  13. Chi1987 👍 3
    Am I missing something? why not 20 ?
  14. ddon1999 👍 3
    40!!! omg. who is answering these??
  15. Sara_Mo 👍 3 Answered B
    The correct answer is B
  16. zaaaaaak 👍 3
    Wrong, Answer is B - 20
  17. sachin007 👍 3
    Max 20 only
  18. ruterjunior 👍 3
    Set to 20
  19. Anonymous learner 👍 2 Answered A
    Availability Set You can deploy any number of VMs, but they are distributed across a maximum of 10 update domains (meaning Azure updates VMs in batches of up to 10 at a time). If you deploy more than 10 VMs, they will be placed into the same update domains, meaning some VMs may be updated simultaneously. Azure Virtual Machine Scale Set (VMSS) You can deploy hundreds or thousands of VMs, and Azure automatically distributes them across up to 20 update domains (in some cases). This means larger deployments can have better update distribution, reducing the risk of downtime during maintenance. So, update domain count does not limit the number of VMs, but it affects how updates are applied. If your goal is high availability with manual VM management, use Availability Sets (max 10 update domains). If you need auto-scaling and better update distribution, use Scale Sets (up to 20 update domains).
  20. maqibali 👍 2 Answered B
    The platformUpdateDomainCount property specifies the number of update domains to be used by the availability set. An update domain is a group of virtual machines and underlying physical hardware that can be rebooted at the same time. By distributing virtual machines across update domains, you can ensure that no single update event can take down all of the virtual machines at once. In Azure, the maximum value for platformUpdateDomainCount is 20. This means that an availability set can have up to 20 update domains. The minimum value for platformUpdateDomainCount is 1. To make sure that the ARM template allows for as many VMs as possible to remain accessible in the event of fabric failure or maintenance, you should set the platformUpdateDomainCount property to its maximum value of 20.
  21. Madbo 👍 2
    The platformUpdateDomainCount property in an Azure Resource Manager (ARM) template specifies the number of update domains used by the virtual machines in an availability set. Update domains indicate groups of VMs that can be updated and restarted in phases to minimize downtime during maintenance. The correct value to configure for platformUpdateDomainCount depends on the number of VMs in the availability set, with a maximum of 20 update domains allowed. The recommended value is at least five times the number of VMs in the availability set, but not more than 20. Therefore, the correct answer would be either: B. 20
  22. himanshu007007 👍 2 Answered A
    In an Azure Availability Set, update domains (UDs) help ensure that virtual machines (VMs) remain accessible during planned maintenance. The platformUpdateDomainCount property defines the number of update domains within an availability set. Update domains help distribute VMs so that Azure does not restart all VMs at once during maintenance. The maximum number of update domains supported in Azure is 20, but the default is 5, and the recommended value is 10 for most cases.
  23. Jig77 👍 2
    Answer is A. To maximize VM availability during maintenance and minimize downtime in the event of fabric failure, you should configure the platformUpdateDomainCount property to 10 (Option A). This will distribute your VMs across 10 update domains, ensuring that as many VMs as possible remain unaffected during planned maintenance and providing optimal availability.
  24. thalasi 👍 2 Answered B
    Azure Availability Sets protect virtual machines from: Hardware/fabric failures Fault Domains Planned maintenance events Update Domains This question asks specifically about: platformUpdateDomainCount so that as many VMs as possible remain accessible during maintenance. Azure supports a maximum of: 20 update domains
  25. JIGT 👍 2
    20. Each availability set can be configured with up to three fault domains and twenty update domains.

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