OK, I've read some books, tons of articles, and...

[John Cowan]

moliphan at footprint.com wrote:

> By creating an XML file and a DTD, you have a defined, portable set of
> business rules - portable, because if I send you my XML file which contains
> a reference to a DTD I have created and which is located on the Internet,
> you are presumably unable to alter the contents if I impose a validity
> check.

Well, that's a little strong.  You can't alter the structure, we might
say, or at least only in predetermined ways.  The content (not just
actual #PCDATA content, but the values of attributes) can be altered
arbitrarily.

> The structure of XML allows parties interested in my data to do searches
> with filters specific to the structure of the data. For example, knowing
> that a "Grommet" element exists with a "Magical" attribute, interested
> parties could search for Grommets with a Magical attribute of "Yes".

Just so.

> A common storage area for DTDs would possibly allow those unfamiliar with my
> data structure to view it.

The whole Web serves as the common storage area, since references to DTD
are by URL, a local file name being a degenerate case of an URL.
 
> A question I have is, how does my behaviour travel with the data (as
> structure does not define behaviour)? I have seen how Java parsers can
> traverse document elements, and given elements I can now associate actions
> with them using Java, but how does that help you, my interested party
> unless you can use my code with the data?

It doesn't.  That's the meaning of the buzzphrase "XML gives Java something
to do/chew on."  
 
> Also, is an XML file going to act as a database in some circumstances?
> Although I have seen examples of this, I wonder how the heck that is
> supposed to work with the portability idea as multiple database instances
> would be difficult to reconcile.

Depends what you mean by "database".  An XML document residing on a server
someplace can represent a database, particularly if it has a DTD of the
form:  top-level element contains zero or more row elements, each of which
can contain various column elements.  That makes it look like a relational
table.  Of course, it does not provide the ACID properties that real
databases have!

> The paradox as I see it is that XML provides an open definition of
> structuring data, but there is difficulty then in providing a generic (low
> cost) method of using the data. My data will be (and, hopefully act)
> different from yours and everybody else's, therefore no generic agent is
> going to know what to do with it.

XML does not make this problem any worse, though, and moving around behavior
is not specifically an XML problem, although behavior-specifying languages
could be written as XML applications: the not-yet-fully-defined XSL does this
for the behavior of rendering on paper or screen; Java can provide generalized
mobile behavior.

-- 
John Cowan	http://www.ccil.org/~cowan		cowan at ccil.org
	You tollerday donsk?  N.  You tolkatiff scowegian?  Nn.
	You spigotty anglease?  Nnn.  You phonio saxo?  Nnnn.
		Clear all so!  'Tis a Jute.... (Finnegans Wake 16.5)

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