For those interested in a keep-everything-hot approach like this, it's worth checking out the 25-year-old library Prevayler. Full ACID guarantees, and radically faster than a database. I happily used it for a project forever ago and was disappointed to see it so thoroughly ignored.

I did a bit of digging after reading this comment and came across its' currently maintained implementation in Clojure by author Klaus Wuestefeld[0] :)

[0] https://github.com/klauswuestefeld/prevayler-clj

Oh, this is interesting. What sort of project did you use it for?