Hi Folks,
Michael Kay says
(paraphrasing):
It is unwise to define a syntax without
an
underlying data model.
I hereby take this as best
practice:
Every syntax must have an underlying data model.
That is an incredibly
important statement, with huge ramifications.
Let’s first begin with the
obvious question:
What is a
data model?
[Definition]
Quintessential: representing the most perfect or typical
example of a quality or class.
Arguably, the XQuery and
XPath Data Model (XDM) is the quintessential data model.
Let’s see how it specifies its data model.
The XDM specification
creates and defines terms, specifies how one should view the
data, and specifies operations on the data and the results
of those operations. Phew! What does all that mean? I think
it will become clearer with an example.
Example: The XDM
specification introduces and defines a term “sequence.” This
term is unique to the data model – there is no such concept
in XML. In the XDM view of the world, an element consists of
a sequence of items. Here are some of the operations that
can be performed on a sequence: select the first item of a
sequence, select the last item of a sequence, select the nth
item of a sequence, and so forth.
Let’s recap: the XDM
specifies a way to view the data (as a sequence of items),
it defines terms, and it specifies operations on values.
That’s what a data model is.
Here are two relevant
quotes from the XDM specification:
The
XQuery 1.0 and XPath 2.0 Data Model
(henceforth "data model") serves two purposes.
First, it defines the information contained in
the
input to an XSLT or XQuery processor. Second,
it defines all permissible values of
expressions in
the XSLT, XQuery, and XPath languages.
The
XQuery 1.0 and XPath 2.0 Data Model specifies
what information in the documents is
accessible, but
it does not specify the programming-language
interfaces
or bindings used to represent or access the
data.
The next question to be
addressed is this:
When should a data model be created?
Answer: before you
define the syntax.
XML did it
wrong. Ditto for Namespaces. They created a data
model (the Infoset)
after they had already defined the syntax. Bad, bad,
bad.
Michael Kay gave a
fascinating and illuminating
description
of how XML and the Infoset were created in the wrong order:
The
Infoset spec refers to the XML spec,
but not the other way around. In terms of
layering, the Infoset is an overlay on top
of XML, not an underpinning. So it's not
an "underlying" data model, rather an
"overlying" one.
Yes, the Infoset
can be taken as the data model
for XML. But it is an after-the-event
rationalisation;
it did not influence the design of XML. It also
came too late to influence other specifications;
for example in the XPath data model, namespace
nodes have parents, whereas in the Infoset,
namespace
information items do not.
Wow! That is mind-blowing.
[Why
did this happen?]
Lessons Learned:
1. Create a
data model that underlies your syntax.
2. Create the data model before creating your
syntax.
3. In the data model specify the view of the data,
terminology, and operations on the data.
At the outset I stated that
there are huge ramifications. Now it’s time to see one of
them:
An XML Schema defines a new syntax so
before you
create an XML Schema, create
a data
model.
Any additions, changes,
comments?
/Roger
