public identifiers [Re: Referencing multiple DTD in an XML document]

Liam R. E. Quin liamquin at
Thu Nov 4 06:25:48 GMT 1999

On Wed, 3 Nov 1999, james anderson wrote:
> David Brownell wrote:
> > I'd encourage you to use PUBLIC identifiers as well, so that
> > local cached copies of those declarations can easily be used.
> Could someone expound further on this statement?

I would *discourage* you from using PUBLIC identifiers.
Any system that bases caches on PUBLIC rather then SYSTEM identifiers
for XML is broken.

The PUBILC keyword is there for backwards compatibility with SGML;
its meaning in XML is not formally defined, and it's usually ignored.
Therefore, if you want interoperability, simply forget it's there.


> I've yet to see a conclusive discussion of whether
> dtd's (or for that matter schema) in the namespace age are stable. That is,
> whether the same system uri will always identify an identical dtd/schema.
They are not and it won't.

<!NOTATION sh SYSTEM "/bin/sh -c">
<!ENITY systemdate SYSTEM "/bin/date" NDATA sh>
now systemdate returns a different value every tiem you use it (maybe
evenwithin the same document, depending on the processor).

This use of NOTATION is implied by the XML spec and by the SGML handbook,
but since a docuemnt you recieve could just as easily use "format" or "rm",
it's not a good idea, by the way.

> For example, would a processor be conforming if it were to, upon not finding a
> resource "under" a public identifier, retrieve and expand the resource located
> through the system identifier and cache the expanded (or even decoded result)
> "under" the public identifier?
You can do whatever you like with a public identifier -- it's undefined.
No amount of wishful thinking will make it otherwise, for those who
want to use it :-) sorry.


Liam Quin, Barefoot Computing, Toronto;  The barefoot agitator
l i a m q u i n     at    i n t e r l o g    dot   c o m
Ankh on, ankle5/Ankle{MD} on DALnet

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