https://github.com/permazen/permazen/wiki/FAQ#architecture
It provides a Java view of the data, and handles translating operations in that later into operations on the underlying stores.
That's exactly the kind of thing Codd was arguing against 50 years ago.
Information does not fit in Java objects. It doesn't want to be stuck (and does not work well) in hierarchical property & inheritance relationships. That's not how knowledge works.
Information (and the human mind) does tend towards working better when formulated in terms of first order / predicate logic. Which is what the relational data model is based upon. SQL is a rather crude approximation of that.
The OO stuff I think has gotten in the way of our jobs. When I worked in Java it was a mountain of pointless data/transfer object transformations. Microservices has made this worse. It's a really bad paradigm.