Thanks God for Eric,
Eric Armstrong wrote:
> Several of us had a chance to meet with Ken
> Holman over the weekend. He was brought to
> the party by John Deneen, and he was quite
> happy to meet Doug. He very much wants to
> make whatever contribution he can, which
> pretty much makes him "one of the team".
> Ken is very knowledgable about XML and related
> disciplines. And he is, or has been, very
> active at OASIS (Organization for the Advancement
> of Structured Information Standards). He is
> looking forward to helping us define an interchange
> standard, and shepard it through the various
> committees, and so on.
> He also has a remarkable flair for design.
> He picked up a rough sense of what we were about
> in fairly short order, and began making insightful
> observations based on his past design experience.
> Here are some of the technical points he developed
> during the meeting...
> XML Basics
> * XPATH is a basic structure-identification
> * XPOINTER uses that representation mechanism,
> and builds on it to add concepts like a
> structure-range (from struct X to struct Y)
> * XSL/XSLT also uses XPATH as part of its
> representation mechanisms
> * XSLT is a translation mechanism that can
> generate XML, which can then be parsed.
> * XSL is the format-presentation layer. It defines
> a ton of constructs that can be used to specify
> how material prints, or is displayed.
> * RELAX is a very nice schema definition mechanism
> that defines a theory-based representation
> mechanism that lets you construct DTD *diffs*
> and DTD *unions*. Unions let you modularize
> DTDs, and ensure that a document conforms to
> the result of combining them.)
> * SCHEMATRON is an assertion-based validation
> mechanism. Using that mechanisms makes it possible
> to validate assertions like "mixed content
> containing text and inline elements occurs only
> before substructure elements, never between or
> [For me, this one was worth the price of
> dmission. It totally solves the XML limitation
> described in my paper on XML Editor Design.]
> Design Principles
> * Most application designs define an application-
> specific language, and parse that. They tend
> to consider XSLT as an afterthought. To make
> use of it, a different representatiion is
> parsed, written out as XML, and then reparsed
> into the app.
> * But XSLT can quite easily produce SAX or
> DOM output *directly*. So the kind of design
> Ken recommends, uses XSL and a style sheet
> to process any particular XML data. The result
> becomes SAX events or a DOM in the app, so
> that part of the app doesn't change. But now
> you can process any other variant of the
> XML that encodes the information, simply by
> creating a new stylesheet, without a big
> peformance hit -- the result is roughly
> equivalent to having defined that language (or
> any other variant) as the "reference langauge"
> for the application.
> * Ken declared emphatically that DEFINING THE XML
> EARLY ON IS INAPPROPRIATE. He's seen the mistake
> made dozens of times, and counsels his clients
> against it. His take on the matter is that XML
> IS AN INTERCHANGE STANDARD and that the core of
> the application is the services it provides.
> Therefore, the only sequence that works in the
> real world is to define those services, and *then*
> come up with an XML form for the data that needs
> to be interchanged.
> OHS Design
> * In terms of the OHS, Ken's approach had some
> remarkable implications for the design. Rather
> than attempting to define a DTD for a "normal
> form" OHS document, Ken suggests focusing on
> the services, and building (or at least desiging)
> those services. So for example, we need granular
> addressibility. And we want it to apply to legacy
> documents. Ok, then, the system requires
> mechanisms for adding addresses to a legacy
> document! The orginal document continues its
> existence, unchanged. The OHS contains a pointer
> to it, along with a collection of addresses that
> point into it. The "HyperDocument" you view in
> the "HyperScope" is then the product of those
> addresses applied to that document.
> * Note that we have *not* defined a DTD for a
> HyperDocument. We have defined functionality.
> Now, when it comes to interchange data, how
> does that happen? Well, what do you need to
> send? You need to send a pointer to the original
> document, at a minimum -- or possibly the
> document itself if it is inaccessible. And you
> need to send the additional information (like
> the addresses) that are necessary to carry out
> HyperScope functions!
> * Ken's point here, is that XML definition is
> dictated by functional needs -- by what you
> need to transmit to provide the desired services,
> and the resulting XML definition is far removed
> from any sort of "HyperDocument definition" we
> may construct at the outset.
> [Note: From personal experience, I concur
> wholeheartedly. The orginal stab I took at
> XML syntax for such a document looks nothing
> like the node library I am currently constructing.
> More instructively, none of the last 4 versions
> of that library look very much like any of the
> Topic Maps
> Ken also talked about topic maps for a bit.
> (Although I have yet to "get" them, Ken was very big
> on them, and mentioned Jack Park's advocacy several
> times in this context.)
> What I gleaned from our short forays into the
> subject was:
> * Topic maps provide a way of defining the
> semantic content of a structure or, perhaps
> more accurately, it is a way of specifying the
> syntax that is used to represent different
> semantic constructs. (I believe that is
> accurate, although I didn't quite get how
> it works.
> More info: http://www.topicmaps.org
> * Ken suspects we want to use topic maps to
> define the OHS interchange mechansims.
> (Again, I don't see how that works, exactly,
> but I suspect that he and Jack will be
> able to arrive at a meeting of the minds.)
> * My one little "aha" on the subject is that
> if XSLT + a stylesheet can be used as the
> input to an application, then if the input
> is defined using a topic map, then anyone
> can use any syntax they want to encode the
> data -- the syntax will be transformed by
> XSLT for use by the application anyway, and
> that translation will be governed by the
> topic map. (I think that is somewhere
> within a Silicon-valley commute of being
> correct, but...)
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This archive was generated by hypermail 2b29 : Mon Nov 13 2000 - 14:38:16 PST