Ah! We haven’t tried RethinkDB, but if it supports IPv6 it should work fine.
I think you can skip the load balancer, and possibly use our built in DNS service discovery. The proxy could either be its own app, or you could roll it into the RethinkDB VMs.
If you can get a DB going with Volumes, and call it something like my-app-db, you can connect to it from either the rethink proxy or your fastify app using my-app-db.internal.
Where are you hosting RethinkDB now? You could also try putting your app + RethinkDB proxies on Fly and connect them to your existing cluster.