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RE: A Systematic Approach to using Simple XML Vocabularies to

• To: "'Roger L. Costello'" <costello@m...>, "'xml-dev@l...'" <xml-dev@l...>
• Subject: RE: A Systematic Approach to using Simple XML Vocabularies to Implement Large (Complex) Systems
• From: "Bullard, Claude L (Len)" <len.bullard@i...>
• Date: Thu, 16 Dec 2004 10:21:20 -0600

BTW, that's also called a 'stored procedure'.

Yes, links/relationships that aren't hardwired to particular 
instance values can be functions or queries.  
In many applications, that can be what the URI identifies.
A table schema IS a simple vocabulary.  That's what normalization 
is all about.

It isn't that magic isn't useful; it is that it is so 
unreliable and seldom necessary.

len


From: Roger L. Costello [mailto:costello@m...]

So properties are things that can be aggregated, typed, and related to 
other objects ... dynamically.  In a large, dynamic system this agility 
is crucial (I assert).  This is called "opportunistic composition and 
relationships".

In my last message I issued a challenge: how can simple vocabularies be used
to construct an Invoice for the purchase of a Book.  Here's how to implement
the Invoice using Naz's approach:

<bag id="RLC">
    <is-a instanceOf="PostalAddress"/>
    <has property="Addressee">Roger L. Costello</has>
    <has property="Street">38 Boylston St.</has>
    <has property="City">Boston</has>
    <has property="State">MA</has>
    <has property="Zipcode">01320</has>
    <refersTo xlink:role="purchased"
		  xlink:href="#Bach"/>
</bag>

<bag id="Bach">
    <is-a instanceOf="Book"/>
    <has property="Title">Illusions</has>
    <has property="Author">Richard Bach</has>
    <has property="Date">1977</has>
    <has property="ISBN">0-440-34319-4</has>
    <has property="Publisher">Dell Publishing Co.</has>
    <refersTo xlink:role="purchased-by"
		  xlink:href="#RLC"/>
</bag>

<bag>
    <is-a instanceOf="Invoice"/>
    <refersTo xlink:role="contains"
              xlink:href="#RLC"/>
    <refersTo xlink:role="contains"
              xlink:href="#RLC"/>
</bag>

The advantages of the Bag approach are:

1. Opportunistic composition and relationships.
2. Highly scalable.
3. Well-suited in cross-domains.
4. Well-suited in evolving systems.
5. A flexible, dynamic Web of data is created.

The disadvantages of the Bag approach are:

1. It requires more skill to build applications because
   properties and relationships are not fixed.  We are 
   all comfortable with writing applications that can 
   process this:

   <Person>
      <Name>Roger L. Costello</Bag>
      <Age>39</Age>
      <SSN>310-60-1234</SSN>
   </Person>

   But it is much more challenging to write an application 
   that can process a {Name, Age, SSN} collection which in 
   one part of the system represents a Person, in another 
   part of the system represents an Employee, etc.
   Further, the relationships between this object and other
   objects may dynamically change (see observation #6 above).

2. A flexible query and navigation language is required.  The
   language must allow navigation based upon dynamically 
   established relationships.  For example,

   "[From the Person object] Navigate to the "driving" object."

   See Observation #6 above.  Unfortunately, XPath provides a
   fixed (static) set of navigation capabilities - child, ancestor,
   preceding-sibling, etc.  [I assert] These are inadequate in a 
   large, dynamic system.

Comments?  /Roger

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