Ed Summers posed an interesting question in reply to my assertion that the Dublin Core Abstract Model (DCAM) is the central jewel in the Dublin Core effort.
'It's funny--as a "library-technology-person" who has recently started dabbling in RDF and semweb technologies DublinCore seems pretty successful. It's a nice vocabulary to be able to invoke when describing resources, and it turns up in specs for FOAF, OAI-(ORE|PMH), RSS, Atom, RDFa, SKOS. The vocabulary I get--the DCAM is a tougher nut for me to crack. It hasn't been abundantly clear to me why it is needed when you have RDF already. I've summed it up to myself as the result of parallel evolution--but perhaps you could characterize it better. Maybe you already have? :-)'
It is always nice to see independent endorsement of the roughly-rightness of DC as a vocabulary, and I hope my earlier remarks in no way impugn the value of the global consensus these vocabulary terms represent. They are valuable to a great many, but from the first workshop almost 15 years ago, we recognized (even in the name) that DC needed to be extensible and interoperable. This is where the abstract model is important.
The evolution of RDF and DCAM are not parallel in any exclusive way, but rather intimately intertwined. Indeed, DC was the prototypical client for RDF, and DC mavens have from the beginning been an integral part of the RDF and Semantic Web development community. RDF was born at a meeting of four people (Bill Arms, then of CNRI, Jim Miller, then of the W3C, Dan Connally, then and now of the W3C, and myself, representing the DC community). The W3C folks recognized that the PICS effort then underway was inadequate to the larger needs for expressing general metadata, and thought the time was ripe for the development of something more broadly useful.
PICS (Platform for Internet Content Selection) was an effort hastily conceived to fend off assertions that porn would infect every classroom unless the gubmint stepped in to protect us. Someone (TimBL? Dan? Jim?) realized that there was benefit in building a general purpose architecture to support the declaration of reusable semantic assertions. Bill knew of this, and of the DC effort, and brought us together in a meeting at the CNRI offices in Reston, Virginia. My only contribution to the meeting was to say... 'gee, that sounds swell!' Or something like that.
So, some of the Web techies in the DC community jumped in enthusiastically and soon we had an RDF camp as an alternative to simple HTML META tag attribute-value pairs. DC fed functional requirements to the RDF folks, and we figured in a year or two the whole world would be declaring metadata using RDF. Our tender naiveté makes me laugh and shake my head now. We really thought we had this one by the tail.
It didn't quite work out that way, of course. Ten years later, and RDF still struggles in the technology marketplace (hoping for lots of shocked comments to this assertion). Why is that? Basically, because RDF fulfills a second order requirement: interoperability. It is fairly straight forward to build a closed system where everyone knows what they need. This is the way most systems used to be built, of course, and one of the wondrous things about the Web is it introduces global scope as an intrinsic technological attribute. Not to say we always take full advantage.
In the metadata realm we're trying to achieve global semantic scope as well as technological scope. And we want it to be extensible. And we hoped that applications would be built independently of one another on a technological platform that would make possible interoperability without pre-coordination. If you believe TimBL, this is the future of the Web. I've wanted to believe, and still want to. If it is to happen, it requires more than RDF. It requires conventions about how we structure our metadata assertions. This is where DCAM comes in. The abstract model provides a syntax-independent (hence the abstract bit) set of conventions for expressing metadata on the web. RDF is the natural idiom for the expression of the DCAM, but it is NOT essential. You can build any arbitrary syntactical representation of the metadata according to DCAM, and a lossless transformation to any other arbitrary syntactical representation should be possible between two machines that grok both syntaxes.
So, staying 'on the tracks' is a matter of adopting those conventions (not an intrinsic part of RDF, but naturally expressible in RDF). If you happen to be using RDF, all the better, but we make no assumptions that RDF is the only appropriate syntactic rendition. If you've reached the end of this post (I'm guessing the world-wide audience for this is post is... say... 9), and want more (we're down to 3 now), you should talk to Andy Powell or Mikael Nilsson or their co-authors, who did the heavy lifting on getting this thing done. The Metadata world owes them a substantial debt.
Three dragons flying: Ok, the production values of the image in this post aren't exactly great... the iPhone will never win awards as a camera. My OCLC Programs and Research colleague, Karen Yoshimura, scribbled this out on a paper restaurant table cloth faster (and far more beautifully) than I can write my name. Man, I wish I could do that.