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We have the possibility that our services (for example, 2 VMs for Web App) could both go offline at the same time if we we do not use availability sets. So, availibity set is to tell Microsoft that we have the solution that is going to be highly available.
WM Host Infra : Microsoft ensures that they are not going to sit on the same infrastructure → Fault Domain = Hardware that shares common sources of failure.
VM Host Infra : Platform services updated / restarted by Microsoft → Update Domain.
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