Re: [xml-dev] Compelling use case for XML Catalogs?

[Stephen D Green <stephengreenubl@gmail.com>]

To elaborate on what I meant by 'earlier standards' I was thinking
of DTDs and the reliance earlier XML standardised markups had
on them. DTDs had several ways that they could use URLs to point
to remote resources so it was especially necessary to allow these
to be redirected locally. I'm guessing the main use was to allow a
docbook document referring to a standard OASIS Docbook DTD
published on the OASIS site to have a local copy of the DTD to
avoid problems that probably existed in the early days of XML
(when it depended on DTDs) e.g. with offline parsing. Not to say
there aren't still occasions where such problems occur but parsers I
think that once XML standard formats get widespread use the
software can be written to expect them and have built in ways of
recognising a namespace and domain-specific parsing requirements
and not need XML Catalogs so much.

----

Stephen D Green

On 11 June 2011 08:38, Stephen D Green <stephengreenubl@gmail.com> wrote:

I suggest that XML Catalogs may be solving a problem

which can occur when the XML is standardised by a remote

standards body which publishes a standard URL for a

schema. There is a risk that conforming documents will

include a schema location linking to the remote, published

schema: The inclusion of such a URL combined with any

software which tries to resolve that link when it parses the

XML documents might result in excessive requests to the

remote resource, tying both the remote servers and the

local network and Internet connections. I think this scenario

might be less of a concern as software is being designed

with regard for domain requirements but once upon a time

software handling XML possibly tended to stick to generic

patterns (earlier standards?) in managing the XML and this

might have meant always trying to resolve such URLs using

the Internet by default. I imagine today most software can

be configured others ways than with XML Catalogs alone

as it has become more sophisticated and adapted to user

requirements.

----

Stephen D Green

On 10 June 2011 19:02, Costello, Roger L. <costello@mitre.org> wrote:

Hi Folks,

XML Catalogs are used to map URLs in XML files to local resources.

EXAMPLE #1

This XML Schema document <xsd:include>s a file that is at a URL:

-------------------------------------------------------------------------------------
<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

   <xsd:include schemaLocation="http://www.xfront.com/catalog-test/Author.xsd" />

   ...

</xsd:schema>
-------------------------------------------------------------------------------------

An XML Catalog is used to specify a mapping from that URL to a local copy:

-------------------------------------------------------------------------------------
<?xml version="1.0"?>
<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog">

   <uri name="http://www.xfront.com/catalog-test/Author.xsd"
          uri="Author.xsd"/>

</catalog>
-------------------------------------------------------------------------------------

When the XML Schema is executed, an XML Catalog tool will dynamically perform the mapping and the XML Schema will use the local copy of Author.xsd rather than the version at the URL.

Let's take an XSLT example.

EXAMPLE #2

This XSLT document uses the document() function to access a file that is at a URL:

-------------------------------------------------------------------------------------
<?xml version="1.0"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                       version="1.0">

   <xsl:value-of select="document('http://www.xfront.com/FitnessCenter.xml')"/>
   ...
</xsl:stylesheet>
-------------------------------------------------------------------------------------

An XML Catalog is used to specify a mapping from that URL to a local copy:

-------------------------------------------------------------------------------------
<?xml version="1.0"?>
<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog">

   <uri name=" http://www.xfront.com/FitnessCenter.xml "
          uri="FitnessCenter.xsl"/>

</catalog>
-------------------------------------------------------------------------------------

When the XSLT is executed, an XML Catalog tool will dynamically perform the mapping and the XSLT will use the local copy of FitnessCenter.xsl rather than the version at the URL.

QUESTIONS

In a real operational environment, wouldn't you always have your own local copies of XML Schemas, XSLT programs, Schematron schemas? And wouldn't they all be configured to reference local resources? Further, when an XML document arrives from the network, you may not know what URLs it contains and you don't want processing to dereference them, so how can you possibly create, in advance, an XML Catalog? Given that, there seems to be no non-trivial use for XML Catalogs.

Would you provide a compelling use case for XML Catalogs please?

/Roger

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