Q. Can I cluster Azure AD Sync?
A. Azure AD Sync (AAD Sync) enables the replication of identities between on-premises AD and Azure AD. This replaces the old DirSync technology, which is not supported to cluster AAD Sync.
If the AAD Sync instance is unavailable, you can either stand up a new AAD Sync instance (which is very fast to do), or have another instance in a staging mode ready to take over if required.