PDOM and XQL Engine, the XML Data Server

XML-based applications often blur the borders between Internet-, document- and database technology. This is reflected in the architecture of XML-based information systems. Monolithic systems are superseded by systems composed of lightweight, reuseable Java components which are loosely coupled via standardized interfaces. Using the XQL-processor and the PDOM, XML component systems can easily and transparently be augmented with functionality characteristic for database systems: declarative queries and transaction-safe persistence.

http://xml.ipsi.fhg.de/XMLCC/english/XML%20Competence%20Center/noside_description/projects/pdom/pdome.html