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XML-L  October 2000

XML-L October 2000

Subject:

Re: referencing an external DTD

From:

Paul Kelly <[log in to unmask]>

Reply-To:

General discussion of Extensible Markup Language <[log in to unmask]>

Date:

Wed, 11 Oct 2000 10:30:36 +0100

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (74 lines)

My two'pennorth - public identifiers are also used for cataloging DTDs by
content management systems like Astoria from Chrystal Software. The system
id of a DTD is going to change as you move documents from the database in
the CMS to a local file system and back again. The public ID remains
invariant and allows the CMS to match documents against stored DTDs.

Of course, a lot of these systems have an SGML pedigree which might be why
they adopted the public identifier mechanism for handling XML documents as
well...

It's no bad idea to have a consistent naming mechanism for creating public
identifiers  for your DTDs:

1) They provide documentary info (as another poster has pointed out).

2) Some XML software will actually use the public identifier.

3) It doesn't break anything else.



Paul Kelly
MERANT International



-----Original Message-----
From: John E. Simpson [mailto:[log in to unmask]]
Sent: 10 October 2000 18:32
To: [log in to unmask]
Subject: Re: referencing an external DTD


At 12:26 PM 10/10/2000 -0400, Garg, Sanjeev wrote:
>Also, please explain me significance of '-//W3C/DTD XHTML 1.0
>Strict//EN' parameter. I have specified it after looking at the previous
>mails.
>What I can figure out is: the language is English. :-)

Ha!

That "-//W3C [etc.]//EN" thing is called a public identifier. Here's what
Tim Bray's Annotated XML Spec (http://www.xml.com/axml/axml.html) says
about public IDs:

    Public Identifiers Are Non-Portable
    ...public identifiers are a trick inherited from SGML that
    are probably only useful to people who already have working
    SGML software installed. Remember that if you use public
    identifiers within your own organization, that's perfectly
    OK, but if you want to interchange XML documents with anybody
    external, they have the right to demand, and you have the
    obligation to provide, a working system identifier (URI) for
    each external entity.

In general, the purpose of a public ID (as opposed the system ID, like your
"http://xxx/xxx/xxx/sample.dtd" example) is to say "what a thing is" (as
opposed to "where to find a thing"). If your software doesn't know how to
map public IDs to some useful purpose, including a public ID really serves
only as a form of documentation. The system ID, on the other hand, is
(almost?) universally understood by XML processing applications.

The difference between public and system IDs is like the difference
between, say, A4 paper size and US letter. In a complete universe, free of
ambiguities, laser printers would only need a single paper size. In this
universe, which paper size you use depends on where you happen to be
sitting. XML processors are required to handle system IDs; they may or may
not be "smart" enough (or sufficiently backwards-compatible) to handle
public IDs.

===============================================================
John E. Simpson               | "He asked me if I knew what
http://www.flixml.org         | time it was. I said, 'Yes, but
XML Q&A: http://www.xml.com   | not right now.'" (Steven Wright)

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