We do not need to worry about the high availability of DC, because AD's multi master replication has taken into account high availability.
Refer to the following connection:
- Features of the Replication Model for Active Directory Domain Services
https://learn.microsoft.com/en-us/windows/win32/ad/features-of-the-replication-model-for-active-directory-domain-services - Why Active Directory Domain Services Uses This Replication Model
https://learn.microsoft.com/en-us/windows/win32/ad/why-active-directory-domain-services-uses-this-replication-model
“Active Directory Domain Services are a free-form system for the following reasons:
Customers require a highly distributed solution in which parts of the directory can be spread across their networks and administered locally.
Large customers often grow to millions of objects, hundreds or thousands of replicas, or both.
Many customer networks provide only intermittent connectivity to some locations; for example, remote oil drilling platforms and ships at sea, so the system must be tolerant of partly connected or disconnected operations.”
https://learn.microsoft.com/en-us/windows/win32/ad/features-of-the-replication-model-for-active-directory-domain-services