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July 23, 2002
Short notes points out an
Short notes points out an article on XML and Semantic Transparency by Robin Cover that says just what I've been saying on XML and semantics. Should be required reading for anyone using XML.
10:14 PM | Recommend This | Print This
Comments on Staying Sane
I just read Clemens Vasters Staying sane in an XML Web Services World. While there are some good things in there that I do agree with (like the immutability of XML Schema) I disagree strongly with his philosophical statements on semantics. In particular, he says:
Using XML, we express semantics in a well defined way.
and
XML Schema is based on semantics. If the underlying semantics change so that they are largely incompatible with the previous semantics, the Schema changes and becomes a new one, even if it would be technically sufficient to express the new semantics.
This is just not true. XML has nothing to do with semantics and the semantics could change drastically without any change to the schema. You can't express semantics using a context free grammar---only syntax. Certainly most XML (unless its nonsense) has a semantics associated with it, but the semantics is not expressed by the XML or its schema.
Don't let someone tell you that they're going to express semantics in XML. It can't be done. The semantics will live somewhere else. Here's a simple example from a programming language. What does the following expression mean?
a + b
You can't tell: it might be simple addition, string concatenation, matrix addition, or even something I just made up. Its meaning is context sensitive, for starters, since we'd need to know the types of a and b and even then, we can only guess what + means without a definition. Yet, all of those semantic realities exist on the same piece of syntax: a + b. The same is true of XML. The schema, tells you the right form, but it doesn't tell you the meaning.
This is junior level computer science. You can express semantics in a number of ways:
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Operational semantics
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Translational semantics
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Denotational semantics
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Predicate-based (Hoare) semantics
None of these methods will work with a context free grammar like XML. You need something at least as powerful as recursive functions (turing machines) to make any of these methods work. (For a good introduction, you might look at Formal Semantics by Glynn Winskel. There are lots of other good books on the subject that I'd be happy to point out--I've got a whole bookshelf of them.)
08:58 PM | Recommend This | Print This
Wal-Mart CIO Interview
Wal-Mart is the world's largest company and one which has consistently used technology as a competitive advantage. In this article, their CIO talks about how they manage IT systems. He lists three key philosophies behind his IT strategy:
- The first philosophy is to run a centralized information system for our operations all over the world, and we run that from Arkansas.
- The second is to have common systems and common platforms.
- The third is to be merchants first and technologists second.
In this day of XML standards, one might question why someone cares about (1) and (2) until you start asking about cost. I like to say that an engineer is someone who can do for a dollar what any fool can do for two. I've also learned, the hard way, that XML is great for interoperability when you can't control both ends of the system (or don't expect to in the future). When you do control all the pieces, however, XML is a whole lot of parsing for nothing.
Its hard to get people to see the wisdom in the first two philosophies. There's a natural tendency to autonomy, decentralization, and disparate systems. Many confuse these statements with geographic or system centralization, which is not the case. I'm sure, for example, that Wal-Mart has both people and systems spread out around the globe. What they don't have is fiefdoms. I've written a white paper on IT organization that speaks to how I'd like to see IT functions organized in Utah.
04:06 PM | Recommend This | Print This
Doonesbury on WiFi
This Doonesbury strip appeared Sunday. All you need is an iPAQ (or a laptop, although its hard to walk with) and NetStumbler. I just went to the NetStumbler site to get the URL to link here and see that they have the same cartoon on their homepage. As an aside, the NetStumber homepage appears to be a Slash site being operated as a blog.