Availability Sets are a fundamental capability in Azure that ensures high availability for your virtual machine (VM) deployments. They are a logical grouping of VMs within a datacenter, designed to protect your applications from planned maintenance events and unplanned outages. Azure strategically …Availability Sets are a fundamental capability in Azure that ensures high availability for your virtual machine (VM) deployments. They are a logical grouping of VMs within a datacenter, designed to protect your applications from planned maintenance events and unplanned outages. Azure strategically distributes these VMs across multiple fault domains and update domains within a datacenter.
Fault domains represent a unit of failure within the datacenter, like a rack of servers sharing a common power source and network switch. By spreading VMs across multiple fault domains, Availability Sets ensure that if one fault domain fails, the other VMs in the set remain operational. Update domains represent units of the infrastructure that can be rebooted at the same time during Azure maintenance. VMs within an Availability Set are distributed across update domains, meaning that if Azure needs to perform maintenance that requires a reboot, only one update domain is affected at a time. This prevents all your VMs in the set from being rebooted simultaneously.
Using Availability Sets means that your application remains accessible even during maintenance or hardware failures. Though they improve resilience, they do **not** protect against datacenter-wide outages. For that, you would use Availability Zones.
Availability Sets: Ensuring High Availability in Azure
Importance: Availability Sets are a crucial component of Azure's infrastructure for ensuring high availability and resilience of your virtual machines (VMs). They protect your application from planned maintenance events and unplanned hardware failures within the Azure data center. Without Availability Sets, your VMs could be subject to downtime during these events, impacting your application's reliability. br br What it is: An Availability Set is a logical grouping of VMs within an Azure datacenter that ensures that a specified number of VMs are available during planned or unplanned maintenance events. Azure distributes these VMs across multiple *Fault Domains* and *Update Domains*. br br *Fault Domain:* A fault domain represents a physical unit of failure, typically a rack of servers within the data center. VMs in different fault domains are less likely to fail simultaneously due to a hardware or power outage affecting a single rack. br br *Update Domain:* An update domain represents a group of VMs that can be rebooted simultaneously during planned maintenance. Azure sequentially applies updates to each update domain, ensuring that at least one instance of your application remains running during the update process. br br How it Works: When you create an Availability Set, you specify the desired number of fault domains and update domains (up to 3 fault domains in a region and up to 20 update domains). Azure then automatically distributes your VMs across these domains, ensuring that no two VMs are placed in the same fault domain or update domain. This distribution helps to minimize the impact of planned maintenance or hardware failures on your application. br br When Azure performs planned maintenance, it updates the update domains sequentially. VMs in one update domain are rebooted while VMs in other update domains remain online. In the event of a hardware failure, only the VMs in the affected fault domain are impacted. VMs in other fault domains continue to run, ensuring that your application remains available. br br Exam Tips: Answering Questions on Availability Sets: br Consider these points when answering questions about Availability Sets: br br *Focus on High Availability:* Availability Sets are primarily about achieving high availability. They are not for disaster recovery or backups. br *Understand Fault and Update Domains:* Be familiar with the difference between fault domains (physical isolation) and update domains (logical isolation for updates). br *Scalability vs. Availability:* Availability Sets are NOT about scaling your application. They are about ensuring that your application remains available even if parts of the underlying infrastructure fail. Use Virtual Machine Scale Sets for scalability. br *SLA:* Availability Sets provide a higher Service Level Agreement (SLA) compared to single VMs. Recognize this in the exam questions. br *Placement:* Azure automatically places VMs in different Fault and Update domains. Your role is to define the Availability Set and add virtual machines to it. br *Identify the Scenario:* Understand what the question is asking. Is it about preventing downtime during maintenance? Is it about protection from hardware failures? The correct answer will depend on the specific scenario mentioned in the question.