A virtual machine scale set is a group of identical virtual machines that are automatically distributed across fault domains and update domains in one or more placement groups1. A fault domain is a logical group of underlying hardware that share a common power source and network switch, and a failure in one fault domain will not affect virtual machines in other fault domains2. An update domain is a logical group of underlying hardware that can undergo maintenance or be rebooted at the same time3.
By creating a virtual machine scale set with 12 instances, you can ensure that App1 has high availability and scalability. You can configure the scale set to have a minimum number of instances that must always be running, and a maximum number of instances that can be scaled up or down based on demand or a schedule. You can also configure the scale set to use automatic OS image upgrades, which will apply updates to the virtual machines in batches, ensuring that at least one instance is always running during the upgrade process.