Everything is a virtual filesystem: libferris
Libferris is a user address space virtual (Semantic) filesystem. Over the years it has grown to be able to mount relational databases, XML, and many applications including X Window. Rich support for index and search has been added. Recently the similarities between modern Linux kernel filesystem, Semantic Filesystems and the XML data model has been exploited in libferris. This leads to enabling XSLT filesystems and the evaluation of XQuery directly on a filesystem. XQueries are evaluated using both indexing and shortcut loading to allow things like db4 files or kernel filesystems with directory name caching to be used in XQuery evaluation so that large lookup tables can be efficiently queried. As the result of XQuery is usually XML - As the similarities between XML and filesystems are discussed, the option is there for queries to generate filesystems.
Prior knowledge of the existance of Extended Attributes as well as some familiarity with XML and XQuery will be of help to the reader.