Yeah, not trolling. I haven't tried to design for exactly that, but my first shot at it would be independent pairs of app servers and redis instances, with red

Yeah, not trolling. I haven't tried to design for exactly that, but my first shot at it would be independent pairs of app servers and redis instances, with redis used as an expiring cache, fronted by a load balancer with affinity, backed by centralized regular SQL. Something like MySQL has no trouble hanging onto a scadload of records it mostly doesn't touch, and you can scale the expiry time on redis down as it gets more heavily loaded, so only the hottest tip of the hot set is kept in RAM.