Decentralizing Gensyms

Stefan: "Maybe there’s something immediately, obviously wrong with this idea — but if so, I can’t see it."

Stefan is tallking about the 'application/data-format' media type proposal - it takes a required parameter uri that links to an RDDL document that tells you about the media type. In effect trying to use indirection to solve a layer 8 problem - what does a representation mean?

I can think of at least two problems. Proliferation for one - a new problem is created wherby lots of of custom media formats are created - anyine who's worked in the content or XML business will know that people will define ill-conceived and badly designed formats at the drop of a hat. Mixed content for another - an old problem is not solved. The latter is significant and the proposal doesn't address it all, because it solves a non-problem. Specifically, the problem as descibed by Stefan,

"What’s wrong with application/xml? Nothing, really, except that it doesn’t tell you anything more than that what is being sent is XML: You don’t have any way to tell what XML it is unless you actually parse and e.g. look at the outer element’s XML namespace."

isn't the problem. The problem, limited to XML for the moment, is namespace mixing. The extreme of the problem appears most clearly in documents that are actually machine processable, such as RDF/XML ones, but have other semantics, such as OWL. Put another way, knowing "what XML is",  isn't really the point. The core problem, if you're looking for one, is that XML itself doesn't have a semantics. Atom gets around this in a clever way by not allowing extensions to redefine what entries and feeds signify, but even there you still have to deal with the embedded content. For an example that doesn't require an understanding of formal interlingua, consider an XHTML document laced with microformats - what does the "application/data-format" mechanism get you there?

Another problem. Why tie the media type description mechanism to one particular format - RDDL? I think if the parameter were called 'about' instead of 'uri', that issue might be clearer.

 

Sorry, but 'applicaiton/data-format' seems to be the same approach to representations as overloaded POST is for protocol actions. It conflates the least meaningful signal with the most general one. It's no surprise that a few former WS-* fans are mooting this. If you want to indicate meaning, use intrinsically meaningful interlingua, not gensyms or links to gensyms. For The Web (FTW!), RDF/OWL is the best game in town as far as interlingua go.

"I saw that one enquiry only gave occasion to another, that book referred to book, that to search was not always to find, and to find was not always to be informed"
- Samuel Johnson, circa 1753

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3 Comments


    If I follow you correctly, you advocate the use of either Microformats or RDF instead of XML, thereby erasing the problem. Right? But most people, including standards developers such as those Atom folks you're sort of familiar with, as well as most developers of internal systems, invent their own XML vocabulary. While I'm aware of all of the problems that exist with this approach, a standard justifies a new MIME type, while an internally invented format surely doesn't.

    So *if* I decide to invent my own XML vocabulary, what's wrong with this approach?


    "If I follow you correctly, you advocate the use of either Microformats or RDF instead of XML"

    Not entirely. I gave the uF example as one where the mechanism gets you nothing. I think if you want formats to be understood, either use public formats or shared interlingua, and not private ones.

    "But most people, including standards developers such as those Atom folks you're sort of familiar with, as well as most developers of internal systems, invent their own XML vocabulary."

    Atom's not a good example, as far as a supporting argument for this media type goes.

    "While I'm aware of all of the problems that exist with this approach, a standard justifies a new MIME type, while an internally invented format surely doesn't. So *if* I decide to invent my own XML vocabulary, what's wrong with this approach?"

    At the risk of repeating myself, proliferation and not solving one of the real problem with XML+XMLNS. Why would I support a technique that's only valuable for private formats, will probably encourage people to think that using them is ok, and doesn't seem to make systems using them any better? This really is sounding like WS-*, one layer up.


    OK, point taken. I don't agree, but past experience has taught me to qualify this with a cautionary "at least not yet".