Kweelt, the Making-of: Mistakes Made and Lessons Learned

In this paper we report our experience in building Kweelt, an open source Java framework for querying XML based on the recent Quilt proposal. Kweelt is intended to provide a reference implementation for the Quilt language but also to offer a framework for all kinds of experiments related to XML including storage, optimization, query language features, etc. And we report in this paper on the differences entailed by the use of two different storage managers, based respectively on character files and relational databases. An important design decision was to do a "direct" implementation of Quilt. Instead of relying on preconceptions (and misconceptions!) inherited from our database query processing background, we wanted this reference implementation to expose exactly what is easy and what is hard both in terms of expressiveness and of efficiency. The process has lead naturally to what may in hindsight be called mistakes, and to formulate lessons that will hopefully be used in future implementations to mix-and-match pieces of existing technology in databases and programming languages for optimal results.