To ensure high availability for a web app across multiple virtual machines, what should you deploy?

To ensure high availability for a web app across multiple virtual machines, deploying all virtual machines in a single Availability Set is the preferred approach. An Availability Set is a grouping of virtual machines that ensures that during planned maintenance or unplanned outages, at least one instance remains available. This is achieved by distributing the VMs across multiple physical servers, storage units, and network switches within a data center.

By placing VMs in an Availability Set, Azure guarantees that VMs are not only spread across multiple fault domains but also across update domains. A fault domain represents a physical server, while an update domain is concerned with scheduled maintenance, ensuring that not all VMs are taken down at the same time during updates. This configuration is instrumental in reducing the risk of downtime for the application.

While other options, like placing VMs in separate Availability Zones, can also provide high availability on a broader geographic level by isolating infrastructure, placing VMs in a single Availability Set within the same data center is generally more optimal for ensuring availability during localized failures. Therefore, deploying all virtual machines in a single Availability Set effectively addresses the requirements for high availability of a web app within the constraints of a single data center environment.