[inopinatus]

That's a damn fine question. There's probably a book in it.

Briefly, though, keep it federated; layering integrity is super important for systematic consistency and quality, so wherever possible, entities should register using the tools provided by the PaaS/IaaS layers that the SaaS is inevitably running on. This includes any home-grown PaaS/IaaS, which are pretty common in SaaS at scale. I like to delegate a DNS subdomain to each subsystem, which is then served either natively from that subsystem (e.g. because its control plane used etcd) or using script-generated zones served by nsd. The only central aspect I want is the meta-directory that contains, ideally, nothing but delegations under the infrastructure domain name.