I'm having trouble understanding the purpose of these edge services. Granted, I've only handled small - medium sized infrastructures, and never really experienced the issues at scale, but if I were to get to that point, wouldn't it be easier to just hire a Infrastructure Engineer who could deal with the replication / sharding, not to mention having that engineer also deal with site reliability and dev ops? If a user…
You can replicate across datacenters on Fly.io for free. No infra engineer required. And very little code change required: https://fly.io/blog/run-ordinary-rails-apps-globally/