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RE: What is the meaning of the import statement?

From: <Simon.Cox@csiro.au>
Date: Wed, 6 Nov 2002 12:31:10 +0800
Message-ID: <116D27C8E12BD411B3AB00B0D022B0B801D7A522@yate.wa.csiro.au>
To: mtrninic@galdosinc.com
Cc: www-xml-schema-comments@w3.org, xmlschema-dev@w3.org

Milan - You state:
"What transportation folks need is another stub schema in the base set that
will include the two they require. But the problem is they do not controll
the base set. So they cannot do it. "  
I think this is where you are going wrong.  

The last statement is incorrect and does not follow from anything in your
previous statement or the specs.  There is nothing stopping you writing a
stub schema targetting someone else's namespace that merely includes two
schema documents targetting the same namespace.  There is no violation of
consistency or integrity in allowing anyone to assemble their own subset of
components from any namespace by creating a stub schema that includes a
subset of the schema documents that define components in that namespace.  I
use this trick all the time in creating GML application schemas - I create
stub documents called featureAndValue.xsd, coverageAndTemporal.xsd etc as I
need them. Then I only have a single <import> for the components from the
GML namespace.    


-----Original Message-----
From: Milan Trninic [mailto:mtrninic@galdosinc.com]
Sent: Wednesday, 6 November 2002 12:58 AM
To: Henry S. Thompson
Cc: www-xml-schema-comments@w3.org; xmlschema-dev@w3.org
Subject: Re: What is the meaning of the import statement?


Henry et al,

I have tried to capture my problem in a gif drawing. I hope it goes through.
The explanation follows:

So there are various schema sets, each controlled by a different authority.
There is base, transportation, environment, location, and presumably large
number of other application specific sets.

Now look at the Transportation - Base dependencies. There are two imports
that import two modules from the base set of schemas. This will not work.
What transportation folks need is another stub schema in the base set that
will include the two they require. But the problem is they do not controll
the base set. So they cannot do it. The only way out is that people that
created the base set created at the very beggining the stub schema that
includes ALL schemas from the set. That would be useful for others. But
importing all definitions when you don't need all si not the best option.

Any thoughts on this?

Regards

Milan
=========================================
Milan Trninic
Senior Software Engineer
tel: 1 604 484-2764, 484-2750
mtrninic@galdosinc.com
Galdos Systems IncT http://www.galdosinc.com
=========================================
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----- Original Message -----
From: "Henry S. Thompson" <ht@cogsci.ed.ac.uk>
To: "Milan Trninic" <mtrninic@galdosinc.com>
Cc: <www-xml-schema-comments@w3.org>; <xmlschema-dev@w3.org>
Sent: Monday, November 04, 2002 2:04 AM
Subject: Re: What is the meaning of the import statement?


> "Milan Trninic" <mtrninic@galdosinc.com> writes:
>
> > I've been pondering on one issue related to the multiple <import>s of
the
> > same namespace. Here is the story in short:
> >
> > I wanted to modularize the definitions in our namespace and to import
them
> > selectivelly from the application schemas. (Application meaning the
schemas
> > built on top of our base schemas). And I've figured I can't since the
> > specification allows processors to ignore all but the first <import>
> > statement of the particular namespace.
> >
> > On the other hand, the specification allows processors to take into the
> > account all of the imports as well. And this is what some of the
processors
> > do. This incostistent behaviour is the first problem.
> >
> > But ok, if applications always use only one <import> statement, that
> > incostistency goes away.
>
> Right, that's the sensible defensive strategy.
>
> > Now, this obviously means that we are importing the namespace (with all
of
> > its definitions), not the definitions themselves. But then, why do we
need
> > two attributes there? Why schemaLocation? I mean if the namespace is
always
> > bound to only one schema location, that attribute is completelly
redundand.
>
> Sorry, how does it get bound to _any_ schema location?  Answer -- the
> spec. provides a range of options for processor and/or user to
> employ/specify, everything from "nothing, because I've got that one
> built in" through "try derefing the NS name" to "use schemaLoc".
>
> > The existence of that attribute and the fact that it is not required
that
> > anything actually exists at the end of the namespace URI produces the
> > conclusion that <import> does not really import the namespace (with all
of
> > its definitions), but imports specific definitions from it.
> >
> > Now which one is correct?
>
> Neither.  The fundamental purpose of <xs:import namespace='nsName'> is
> to allow references (e.g. ref=, base=) to names qualified by nsName.
>
> > Even with this issue resolved, the fact that you cannot modularize the
> > schemas is a real problem. Acheving scalability is affected. Building
> > mutually dependant, "networked" or hierarchical schema sets from
different
> > domain and for different purposes is significantly affected. I mean this
> > almost means that I have to have different namespace for each of my
> > definitions.
>
> There is a tradeoff here, but the common approach to this is to go
> ahead and modularise, and use <xs:include> to manage the
> modularisation (that's what _it_ is meant for).  In the case of
> importing a modularly-defined schema, that does mean you have to
> create a stub schema document which consists entirely of
> <xs:include>s, which you then point to from the schemaLoc of your
> import.
>
> Hope this helps.
>
> ht
> --
>   Henry S. Thompson, HCRC Language Technology Group, University of
Edinburgh
>           W3C Fellow 1999--2002, part-time member of W3C Team
>      2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
>     Fax: (44) 131 650-4587, e-mail: ht@cogsci.ed.ac.uk
>      URL: http://www.ltg.ed.ac.uk/~ht/
>  [mail really from me _always_ has this .sig -- mail without it is forged
spam]
>
Received on Tuesday, 5 November 2002 23:31:19 GMT

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