I have been evaluating AllegroCache for a customer for one of their tasks. AllegroCache provides a metaclass that allows programmers to make CLOS classes persistent with very little work. AllegroCache is similar to Berkeley DB B-Tree but is implemented in Lisp and scales to very large problem sets. Individual class slots can be tagged to be indexed for fast lookup and search.
It is possible to write something like AllegroCache in a dynamic language like Common Lisp. The obvious idea for me is: do this in other dynamic languages like Ruby! The job would be easier in Ruby assuming that Ruby is already linked with the open source licensed Berkeley DB libraries. I already use Ruby's DBM disk-based hash libraries a lot, but being able to search on multiple class attributes would be great. AllegroCache supports great query functionality, so implementing something close to AllegroCache's functionality in Ruby would probably be at least a couple month task. One problem with this idea is that Franz Lisp with AllegroCache is really targeted at high end, high performance systems - Ruby is simply much slower that natively compiled Common Lisp, so an AllegroCache port to Ruby would not be as performant.