Completely agreee. Highly mutable => in memory ( then add a secondary stream for colder storage backups just in case).

A relational db is originaly meant for fetching data from a slow storage to a fast one, and the other way around, in a smart way. Doing it 500 times a second isn't a scenario for any db.

Build your own in memory data + process tructure, maybe using something like and agent network and using eg akka or erlang for failovers.

I'm curious as to why uber had to rely on a relational db for this case...