Availability is a critical matter when it refers to systems. The ideal scenario for an administrator would be 100% availability for a virtual server/appliance.
However this is not achievable when we deal with infrastructure as we need to close the systems on purpose (patching, maintenance tasks, etc) but also disasters or other nonphysical causes create a downtime.
When we deal with Azure cloud we have the following options that we can use:
- Availability sets
- Availability zone
An Availability Zone is a high-availability offering that protects your applications and data from datacenter failures. Availability Zones are unique physical locations within an Azure region. Each zone is made up of one or more datacenters equipped with independent power, cooling, and networking. To ensure resiliency, there’s a minimum of three separate zones in all enabled regions
Availability sets are another datacenter configuration to provide VM redundancy and availability. This configuration within a datacenter ensures that during either a planned or unplanned maintenance event, at least one virtual machine is available and meets the 99.95% Azure SLA. For more information, see the SLA for Virtual Machines.
When you create a new VM resource on Azure, you should choose between availability sets or availability zone (depending on the region on which you deployed your server)
However as stated in the first article from Microsoft:
To achieve comprehensive business continuity on Azure, build your application architecture using the combination of Availability Zones with Azure region pairs. You can synchronously replicate your applications and data using Availability Zones within an Azure region for high-availability and asynchronously replicate across Azure regions for disaster recovery protection.
So the final image is the below. Based on your needs you could choose whichever option is more appropriate for you. However as recommended from Microsoft, you should use availability zones (newer feature than availability sets)
Below are some very good resources that Microsoft provides: