Availability set

As explained in Microsoft Azure in Action

An Azure availability set is a configuration that distributes virtual machines across update and fault domains to improve availability during maintenance and hardware failures.

An Azure availability set spreads virtual machines across separate fault and update domains, reducing the chance that maintenance or a hardware problem takes them all offline. section 3.2.1 section 3.5.2

The two domains handle different kinds of disruption:

• Fault domains separate VMs across hardware groups that do not share power, cooling, or network switches. section 3.2.1
• Update domains group VMs for planned maintenance; Azure reboots only one update domain at a time. section 3.5.2
• Create the availability set before creating its VMs, then accept the cost of running multiple instances for improved resilience. section 3.2.1 section 3.5.2

A small resilient VM service

A set containing nine VMs can use three fault domains and six update domains. During a platform update, only one update domain is rebooted, while the remaining VMs continue serving users. More fault domains also reduce the impact of a rack-level power, cooling, or networking failure. section 3.2.1 section 3.5.2

Figure 3.14 An availability set with nine VMs, six update domains, and three fault domains

Use the diagram to compare VM placement across update and fault domains. section 3.2.1

How Microsoft Azure in Action covers it

How deep it goes

Defines fault and update domains, explains their behavior, and stops short of detailed deployment commands.

Examples

Uses diagrams showing nine VMs across three fault and six update domains, plus three fault and five update domains.

What it assumes

Assumes familiarity with virtual machines and Azure regions while defining fault and update domains directly.

What you can do after

Operate/configure it by creating an availability set before its VMs and choosing redundancy appropriate to the workload’s criticality.

How it connects

Connects virtual machines with Azure regions, availability zones, fault domains, update domains, maintenance, and infrastructure redundancy.

liveBookmanning.com

Read Microsoft Azure in Action on liveBook Get the book at manning.com

Related concepts

• virtual machinehas part · A virtual machine is a software-based environment that emulates or implements an abstract computer system, providing an interface on which an operating system or programs can execute.
• Azure Diskconfigures · Azure Disk is a Microsoft Azure block-storage service providing persistent disks for virtual machines and applications while abstracting underlying storage infrastructure.
• update domainhas part · An update domain is a logical group of virtual machines that can be updated or rebooted together during platform maintenance.
• failure domainhas part · A failure domain is a group of infrastructure components that share a potential failure boundary and can fail together.

This explainer was generated by Manning's knowledge graph from the text of Microsoft Azure in Action. The section links point to the parts of the book it draws on.