Kudos to Kendall Clark for stating XML is Not Self-Describing, as I did last month.

He writes;

Well, I’ve read too much Wittgenstein (not to mention too much Aquinas, Meister Eckhart, and Julian of Norwich) to think that a name is necessarily a self-description

I haven’t read them at all (8-), but I think I have a pretty good understanding of self-description that I developed “Bottom up” during my study of Web architecture over the past few years. As Kendall brought this up again, I’d thought I’d write a few more words about it.

As I see it, description is always with respect to some context. For example, “The sky is blue” is not a self-descriptive statement unless you know;

  • English
  • Which sky I mean
  • Which colour blue I mean

For any bag-o-bits, it seems to me that there exists a finite amount of contextual knowledge which is necessary in order to be able to understand it. “Self-describing” then, should mean that the bag itself contains sufficient information to identify the required contextual knowledge.

Tim Berners-Lee likes to talk a lot about this. Last year in Honolulu at WWW2002, his keynote was Specs Count, and much of it was about the value in the ability to be able to perform successive application of public specifications in order to understand a message. That’s contextual knowledge, and as you can see in his talk, it doesn’t begin with the HTTP message, it goes all the way down to the IP segment and Ethernet frame; even those bits must be considered (see an example of where this issue can show up in practice).

Where the Web fits in here, is with its contribution of an enormously valuable piece of contextual knowledge; RFC 2396 aka URIs. With respect to the example above, I can use URIs instead of strings, where those URIs can be used to provide the specifics of which blue I meant, by relating it to other colours.

There’s lots to be said about XML, RDF, and why SOA based Web services can never be self-descriptive (hint; too many methods). But I’ll leave it at that for now.


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  1. […] fully honest, Self Descriptiveness can hardly be achieved in practice as explained by Mark Baker in his blog, but your project deserves as much Self Descriptiveness as you can possibly […]

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