During the recent outage of Azure, a company I am aware of lost it's federated services farm as the underlying VMs lost access to their storage. I've been asked to suggest ways of adding High Availability to their current, textbook implementation (really, textbook https://docs.microsoft.com/en-us/windows-server/identity/ad-fs/deployment/how-to-connect-fed-azure-adfs ).
About the only way I can think of that would work would be a redundant Federated Services farm, in a different Azure region - but I can't find any MS documentation that implies that this is even possible. You certainly don't appear to be able to have multiple URIs for federating a domain via multiple federation server farms.
Any ideas people?