This is about a feature of the Datomic database called Ions, which appear to be analogous to stored procedures (e.g. running Clojure functions in the db). From reading https://docs.datomic.com/cloud/ions/ions.html, that's an oversimplification, but I didn't know what it was so figured I'd save someone a search.
Transaction functions were always part of datomic, even when running it outside of AWS. I think of Ion as a tool that wraps all the complex insanity of AWS so you can write an app as closely as possible to how you'd write a traditional Clojure app meant to run on a single server. In that sense it's not even specifically about datomic... You can even choose not to use it as your database, but of course then you'll be on your own as far as scaling your db.
