CL-PREVALENCE is an implementation of Object Prevalence for Common Lisp. It was written by and is being maintained by Sven Van Caekenberghe . CL-PREVALENCE is using S-XML for the XML serialization protocol (there is also a more Lisp-like s-expression-based serialization protocol). Object Prevalence is a simple but interesting concept first proposed by Klaus Wuestefeld in 2001. IBM developerWorks has a reasonable Introduction to Object Prevalence article. The main Java implementation is called Prevayler , with a (chaotic) wiki site with lots of information and discussions. The following academic paper seems to describe the most essential features of Object Prevalence, without naming it as such: A Simple and Efficient Implementation for Small Databases by Birrell, Jones, and Wobber [1987]. Basically, the idea is this: Most databases are only a couple o...