Compelling use case for XML Catalogs?

["Costello, Roger L." <costello@mitre.org>]

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