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Re: RE: Caution using XML Schema backward- or forward-compatib

  • From: Thomas Lord <lord@e...>
  • To: "Costello, Roger L." <costello@m...>
  • Date: Sun, 30 Dec 2007 18:13:14 -0800

Re:  RE: Caution using XML Schema backward- or forward-compatib
I think you are making a strikingly nice "rosetta stone" in some ways.
You're helping sort out confusion, definitely, and I do risk working
against that by adding to confusion but... I hope not.

I think people should (and probably do) understand it when you name
some technology, like "Schematron" or "DTD" as two things at once:
both a practical definition (here's the pretty reliable technology available
for use today) and as an abstraction (stuff "like this" fills a particular
conceptual role in the overall flow of data and computation).

I didn't quite see that that is the effect of what you are writing until
you responded to me as you did, so thank you.

It's a nice bright-lines picture you're drawing and I still do wonder
how some of the stuff from modern programming language type
theory can be sketched in, at least to convey the concepts.   But,
I'm out of ideas for that personally, for the moment, so....


Costello, Roger L. wrote:
Hi Tom,

A colleague just sent me something that I find helpful:

A client may perform the following steps on  
the data it retrieves from a web service: 

1. Validate you get what you expect
2. Understand what you get
3. Use what you get 

A web service may make the following artifacts available to clients to
assist them with "validating that they get what they expect":

(a) A grammar-based schema for validating that the retrieved data
contains the expected tags and they are arranged in the expected order.
(b) A rule-based schema for validating that the relationships of the
data are as expected (co-constraints, cardinality constraints, and
algorithmic constraints are fulfilled). 

The technologies for these artifacts are:

(a) XML Schema, RELAX NG, DTD
(b) Schematron

[To tie this back to an earlier email, I assert that these "validation
artifacts" are one thing and a "versioning strategy" is another, and
the two should be separate.]

A web service may also make artifacts available to clients to assist
them with "understanding what they get":

(a) A document (or documents) to help clients understand the data they

There are many technologies to achieve this, including prose (i.e.
create a web page that client developers can read), data dictionary,


-----Original Message-----
From: Thomas Lord [mailto:lord@e...] 
Sent: Saturday, December 29, 2007 9:30 PM
To: Costello, Roger L.
Cc: xml-dev@l...
Subject: Re:  RE: Caution using XML Schema backward- or
forward-compatibility as a versioning strategy for data exchange

Costello, Roger L. wrote:
I think that for a client to be able to utilize a web service, the
service must specify three things:

(1) Syntax of the data that the web service makes available to
use a grammar-based language such as XML Schemas, or RELAX NG, or


(2) Relationship constraints (e.g. co-constraints) on the data; use


Seems a bit arbitrary.   Why "relationship constraints" of that 
particular form?
What's your theory, here?   Your claim wasn't that Schematron can be
but that "[in order] for a client to be able to utilize a web service 
[....]" which is
a remarkably strong claim.


(3) Semantics of the data; use a data dictionary, or English prose,
RDF/S, or OWL, some combination thereof.


Again, what's your theory?   Some notation that usefully indicates
seems a good idea, I grant you.   Obviously, also, service has to be 
documented somewhere.
How did you get from there to "English prose, RDF/S, or OWL, some
combination thereof"?

(2) and (3) suggest investments, presumably with some return.   They 
also suggest
suggestions competitive with a lot of well developed theory in program 
typing and
in modeling the semantics of programs.   So, why are the technologies 
and approaches
you suggest the right choice here?



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