180 Personal Content Experience: Managing Digital Life in the Mobile Age every vertex, i.e., link in the graph, there is another vertex in the opposite direc- tion. The result is still a valid RDF, but RDF itself does not make such a requirement. 26 For more information about RDF, see W3C RDF pages or Berners-Lee et al. (2001). Indeed, RDF is a tool for creating the Semantic Web. Quoting W3C, “The Semantic Web provides a common framework that allows data to be shared and re-used across application, enterprise, and community boundaries. It is a collab- orative effort led by W3C with participation from a large number of researchers and industrial partners. It is based on the RDF, which integrates a variety of appli- 27 cations using XML for syntax and URIs for naming.” That defi nition is at the heart of what we propose in this book. Our key contri- bution is not advancing the Semantic Web per se, but we are using it as a corner- stone for all mobile content management. We are also extending it and adding components to it to create full management solution for all layers in the stack. The problem with this option is more practical than theoretical. There are no ready and available RDF databases for mobile environments. At the time of shifting from framework design to implementation, there were several more or less academic test RDF databases available, while none of them were designed to work in mobile devices with limited RAM and stringent power consumption requirements. We are optimis- tic that future implementations of our framework can also use inter- nally RDF and RDF query languages. One prospective source for 28 mobile RDF query language in the future is Wilbur, an open source project. COMMENT: FOOD FOR THOUGHT I think it would be important to emphasize the fuzzy boundaries between “metadata” and “data” (or, to para- phrase an old saying: “One man’s metadata is another man’s data” :-). With the emergence or more and more (seemingly useful) metadata we fi nd that metadata in itself (separate from the content it describes) can become useful. And immediately having made this observation we should recognize the paramount importance of (meta)data *interoperability*. Clearly, there are a large number of different metadata standards, and through them some degree of interoperabil- ity is attained. I fear, though, that most of these standards having been developed for a particular domain or purpose, interoperability is predominantly achieved 26 http://www.w3.org/RDF/ 27 www.w3.org/2001/sw/ 28 http://wilbur-rdf.sourceforge.net/