High availability is crucial for any production environment either it is in on-premises datacenter or in cloud. If you will go in detail of high availability, you will observer that HA can be achieved in the following levels from compute perspective:

Hardware level

Hypervisor/VM level

Operating System level

In this blogpost, I’ll cover high availability options available in MS Azure. First, make it clear that OS level HA has no difference between on-premises or in cloud. Now, let me provide the overview of H/W, and Hypervisor/VM level HA in on-premises datacenter or private cloud.

Hardware level HA: Dual or Quad processor, dual power supply, multi memory channel, multiple network slots, and multiple PCI card slots etc.

Hypervisor/VM level: All type 1 hypervisors provide high-availability configuration options like operating systems. Once, you configure HA for the hypervisor the VM can be created on top of that and hypervisor HA configuration maintains the availability for guest VMs if any host goes down.

Apart from the H/W and hypervisor level, all the datacenter components can be configured in high available mode such as multiple racks and power supply units etc. but when it comes to public cloud you can’t define the above configurations by your-self. However, cloud service provider does all these configurations for you in advance and to simply the things for you it just provides an availability feature.

Microsoft Azure provides “Availability Set” to provide high-availability at the VM level. This availability set feature takes care of both planned and unplanned failures. To define these planned and unplanned failures, availability set allows you to configure update domain and fault domains.

In simple words, availability set is a logical grouping of two or more virtual machines. When you setup the availability set keep the following principles in your mind:

Setup an availability set for one type of VMs. For example, in 3 tier application architecture create different availability sets for each tier.

For high availability of VMS, create multiple VMs in an availability set.

Attach load balancer with availability sets. It helps you to divide the load among the VMs.

Now, let me explain you update domain and fault domain.

Update domain: An update domain allows VMs to maintain availability during planned maintenance. Each update domain contains set of virtual machines and associated physical hardware that can be updated and rebooted at the same time. It allows Azure to perform incremental or rolling upgrades across a deployment. Once, you create an availability set then you can observe that there are five update domains by default set but you can configure up to twenty update domains.

Fault domain: A fault domain allows VMs to maintain availability during unplanned hardware failures, network outages, power failures and software updates. Fault domain describes the datacenter level components such as network switches and power supply serving a single rack can become a single point of failure for one rack or multiple racks. To avoid these circumstance, VMs in an availability set can have at least two fault domains. Many Azure region only supports two fault domains while other Azure regions can have maximum three fault domains.

If you would like to setup an availability set, follow the step by step availability sets blogpost.