doc/guidelines.html - platform/external/libxml2 - Gitiles

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   <title>XML resources publication guidelines</title>
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 <body bgcolor="#fffacd" text="#000000">
 <h1 align="center">XML resources publication guidelines</h1>

 <p></p>

 <p>The goal of this document is to provide a set of guidelines and tips
 helping the publication and deployment of <a
 href="http://www.w3.org/XML/">XML</a> resources for the <a
 href="http://www.gnome.org/">GNOME project</a>. However it is not tied to
 GNOME and might be helpful more generally. I welcome <a
 href="mailto:veillard@redhat.com">feedback</a> on this document.</p>

 <p>The intended audience is the software developers who started using XML
 for some of the resources of their project, as a storage format, for data
 exchange, checking or transformations. There have been an increasing number
 of new XML formats defined, but not all steps have been taken, possibly because of
 lack of documentation, to truly gain all the benefits of the use of XML.
 These guidelines hope to improve the matter and provide a better overview of
 the overall XML processing and associated steps needed to deploy it
 successfully:</p>

 <p>Table of contents:</p>
 <ol>
   <li><a href="#Design">Design guidelines</a></li>
   <li><a href="#Canonical">Canonical URL</a></li>
   <li><a href="#Catalog">Catalog setup</a></li>
   <li><a href="#Package">Package integration</a></li>
 </ol>

 <h2><a name="Design">Design guidelines</a></h2>

 <p>This part intends to focus on the format itself of XML. It may  arrive
 a bit too late since the structure of the document may already be cast in
 existing and deployed code. Still, here are a few rules which might be helpful
 when designing a new XML vocabulary or making the revision of an existing
 format:</p>

 <h3>Reuse existing formats:</h3>

 <p>This may sounds a bit simplistic, but before designing your own format,
 try to lookup existing XML vocabularies on similar data. Ideally this allows
 you to reuse them, in which case a lot of the existing tools like DTD, schemas
 and stylesheets may already be available. If you are looking at a
 documentation format, <a href="http://www.docbook.org/">DocBook</a> should
 handle your needs. If reuse is not possible because some semantic or use case
 aspects are too different this will be helpful avoiding design errors like
 targeting the vocabulary to the wrong abstraction level. In this format
 design phase try to be synthetic and be sure to express the real content of
 your data and use the XML structure to express the semantic and context of
 those data.</p>

 <h3>DTD rules:</h3>

 <p>Building a DTD (Document Type Definition) or a Schema describing the
 structure allowed by instances is the core of the design process of the
 vocabulary. Here are a few tips:</p>
 <ul>
   <li>use significant words for the element and attributes names.</li>
   <li>do not use attributes for general textual content, attributes
     will be modified by the parser before reaching the application,
     spaces and line informations will be modified.</li>
   <li>use single elements for every string that might be subject to
     localization. The canonical way to localize XML content is to use
     siblings element carrying different xml:lang attributes like in the
     following:
     <pre>&lt;welcome&gt;
   &lt;msg xml:lang="en"&gt;hello&lt;/msg&gt;
   &lt;msg xml:lang="fr"&gt;bonjour&lt;/msg&gt;
 &lt;/welcome&gt;</pre>
   </li>
   <li>use attributes to refine the content of an element but avoid them for
     more complex tasks, attribute parsing is not cheaper than an element and
     it is far easier to make an element content more complex while attribute
     will have to remain very simple.</li>
 </ul>

 <h3>Versioning:</h3>

 <p>As part of the design, make sure the structure you define will be usable
 for future extension that you may not consider for the current version. There
 are two parts to this:</p>
 <ul>
   <li>Make sure the instance contains a version number which will allow to
     make backward compatibility easy. Something as simple as having a
     <code>version="1.0"</code> on the root document of the instance is
     sufficient.</li>
   <li>While designing the code doing the analysis of the data provided by the
     XML parser, make sure you can work with unknown versions, generate a UI
     warning and process only the tags recognized by your version but keep in
     mind that you should not break on unknown elements if the version
     attribute was not in the recognized set.</li>
 </ul>

 <h3>Other design parts:</h3>

 <p>While defining you vocabulary, try to think in term of other usage of your
 data, for example how using XSLT stylesheets could be used to make an HTML
 view of your data, or to convert it into a different format. Checking XML
 Schemas and looking at defining an XML Schema with a more complete
 validation and datatyping of your data structures is important, this helps
 avoiding some mistakes in the design phase.</p>

 <h3>Namespace:</h3>

 <p>If you expect your XML vocabulary to be used or recognized outside of your
 application (for example binding a specific processing from a graphic shell
 like Nautilus to an instance of your data) then you should really define an <a
 href="http://www.w3.org/TR/REC-xml-names/">XML namespace</a> for your
 vocabulary. A namespace name is an URL (absolute URI more precisely). It is
 generally recommended to anchor it as an HTTP resource to a server associated
 with the software project. See the next section about this. In practice this
 will mean that XML parsers will not handle your element names as-is but as a
 couple based on the namespace name and the element name. This allows it to
 recognize and disambiguate processing. Unicity of the namespace name can be
 for the most part guaranteed by the use of the DNS registry. Namespace can
 also be used to carry versioning information like:</p>

 <p><code>"http://www.gnome.org/project/projectname/1.0/"</code></p>

 <p>An easy way to use them is to make them the default namespace on the
 root element of the XML instance like:</p>
 <pre>&lt;structure xmlns="http://www.gnome.org/project/projectname/1.0/"&gt;
   &lt;data&gt;
   ...
   &lt;/data&gt;
 &lt;/structure&gt;</pre>

 <p>In that document, structure and all descendant elements like data are in
 the given namespace.</p>

 <h2><a name="Canonical">Canonical URL</a></h2>

 <p>As seen in the previous namespace section, while XML processing is not
 tied to the Web there is a natural synergy between both. XML was designed to
 be available on the Web, and keeping the infrastructure that way helps
 deploying the XML resources. The core of this issue is the notion of
 "Canonical URL" of an XML resource. The resource can be an XML document, a
 DTD, a stylesheet, a schema, or even non-XML data associated with an XML
 resource, the canonical URL is the URL where the "master" copy of that
 resource is expected to be present on the Web. Usually when processing XML a
 copy of the resource will be present on the local disk, maybe in
 /usr/share/xml or /usr/share/sgml maybe in /opt or even on C:\projectname\
 (horror !). The key point is that the way to name that resource should be
 independent of the actual place where it resides on disk if it is available,
 and the fact that the processing will still work if there is no local copy
 (and that the machine where the processing is connected to the Internet).</p>

 <p>What this really means is that one should never use the local name of a
 resource to reference it but always use the canonical URL. For example in a
 DocBook instance the following should not be used:</p>
 <pre>&lt;!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"<br>


                          "/usr/share/xml/docbook/4.2/docbookx.dtd"&gt;</pre>

 <p>But always reference the canonical URL for the DTD:</p>
 <pre>&lt;!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"<br>


                          "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"&gt;   </pre>

 <p>Similarly, the document instance may reference the <a
 href="http://www.w3.org/TR/xslt">XSLT</a> stylesheets needed to process it to
 generate HTML, and the canonical URL should be used:</p>
 <pre>&lt;?xml-stylesheet
   href="http://docbook.sourceforge.net/release/xsl/current/html/docbook.xsl"
   type="text/xsl"?&gt;</pre>

 <p>Defining the canonical URL for the resources needed should obey a few
 simple rules similar to those used to design namespace names:</p>
 <ul>
   <li>use a DNS name you know is associated to the project and will be
     available on the long term</li>
   <li>within that server space, reserve the right to the subtree where you
     intend to keep those data</li>
   <li>version the URL so that multiple concurrent versions of the resources
     can be hosted simultaneously</li>
 </ul>

 <h2><a name="Catalog">Catalog setup</a></h2>

 <h3>How catalogs work:</h3>

 <p>The catalogs are the technical mechanism which allow the XML processing
 tools to use a local copy of the resources if it is available even if the
 instance document references the canonical URL. <a
 href="http://www.oasis-open.org/committees/entity/">XML Catalogs</a> are
 anchored in the root catalog (usually <code>/etc/xml/catalog</code> or
 defined by the user). They are a tree of XML documents defining the mappings
 between the canonical naming space and the local installed ones, this can be
 seen as a static cache structure.</p>

 <p>When the XML processor is asked to process a resource it will
 automatically test for a locally available version in the catalog, starting
 from the root catalog, and possibly fetching sub-catalog resources until it
 finds that the catalog has that resource or not. If not the default
 processing of fetching the resource from the Web is done, allowing in most
 case to recover from a catalog miss. The key point is that the document
 instances are totally independent of the availability of a catalog or from
 the actual place where the local resource they reference may be installed.
 This greatly improves the management of the documents in the long run, making
 them independent of the platform or toolchain used to process them. The
 figure below tries to express that  mechanism:<img src="catalog.gif"
 alt="Picture describing the catalog "></p>

 <h3>Usual catalog setup:</h3>

 <p>Usually catalogs for a project are setup as a 2 level hierarchical cache,
 the root catalog containing only "delegates" indicating a separate subcatalog
 dedicated to the project. The goal is to keep the root catalog clean and
 simplify the maintenance of the catalog by using separate catalogs per
 project. For example when creating a catalog for the <a
 href="http://www.w3.org/TR/xhtml1">XHTML1</a> DTDs, only 3 items are added to
 the root catalog:</p>
 <pre>  &lt;delegatePublic publicIdStartString="-//W3C//DTD XHTML 1.0"
                   catalog="file:///usr/share/sgml/xhtml1/xmlcatalog"/&gt;
   &lt;delegateSystem systemIdStartString="http://www.w3.org/TR/xhtml1/DTD"
                   catalog="file:///usr/share/sgml/xhtml1/xmlcatalog"/&gt;
   &lt;delegateURI uriStartString="http://www.w3.org/TR/xhtml1/DTD"
                   catalog="file:///usr/share/sgml/xhtml1/xmlcatalog"/&gt;</pre>

 <p>They are all "delegates" meaning that if the catalog system is asked to
 resolve a reference corresponding to them, it has to lookup a sub catalog.
 Here the subcatalog was installed as
 <code>/usr/share/sgml/xhtml1/xmlcatalog</code> in the local tree. That
 decision is left to the sysadmin or the packager for that system and may
 obey different rules, but the actual place on the filesystem (or on a
 resource cache on the local network) will not influence the processing as
 long as it is available. The first rule indicate that if the reference uses a
 PUBLIC identifier beginning with the</p>

 <p><code>"-//W3C//DTD XHTML 1.0"</code></p>

 <p>substring, then the catalog lookup should be limited to the specific given
 lookup catalog. Similarly the second and third entries indicate those
 delegation rules for SYSTEM, DOCTYPE or normal URI references when the URL
 starts with the <code>"http://www.w3.org/TR/xhtml1/DTD"</code> substring
 which indicates the location on the W3C server where the XHTML1 resources are
 stored. Those are the beginning of all Canonical URLs for XHTML1 resources.
 Those three rules are sufficient in practice to capture all references to XHTML1
 resources and direct the processing tools to the right subcatalog.</p>

 <h3>A subcatalog example:</h3>

 <p>Here is the complete subcatalog used for XHTML1:</p>
 <pre>&lt;?xml version="1.0"?&gt;
 &lt;!DOCTYPE catalog PUBLIC "-//OASIS//DTD Entity Resolution XML Catalog V1.0//EN"
           "http://www.oasis-open.org/committees/entity/release/1.0/catalog.dtd"&gt;
 &lt;catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog"&gt;
   &lt;public publicId="-//W3C//DTD XHTML 1.0 Strict//EN"
           uri="xhtml1-20020801/DTD/xhtml1-strict.dtd"/&gt;
   &lt;public publicId="-//W3C//DTD XHTML 1.0 Transitional//EN"
           uri="xhtml1-20020801/DTD/xhtml1-transitional.dtd"/&gt;
   &lt;public publicId="-//W3C//DTD XHTML 1.0 Frameset//EN"
           uri="xhtml1-20020801/DTD/xhtml1-frameset.dtd"/&gt;
   &lt;rewriteSystem systemIdStartString="http://www.w3.org/TR/xhtml1/DTD"
           rewritePrefix="xhtml1-20020801/DTD"/&gt;
   &lt;rewriteURI uriStartString="http://www.w3.org/TR/xhtml1/DTD"
           rewritePrefix="xhtml1-20020801/DTD"/&gt;
 &lt;/catalog&gt;</pre>

 <p>There are a few things to notice:</p>
 <ul>
   <li>this is an XML resource, it points to the DTD using Canonical URLs, the
     root element defines a namespace (but based on an URN not an HTTP
   URL).</li>
   <li>it contains 5 rules, the 3 first ones are direct mapping for the 3
     PUBLIC identifiers defined by the XHTML1 specification and associating
     them with the local resource containing the DTD, the 2 last ones are
     rewrite rules allowing to build the local filename for any URL based on
     "http://www.w3.org/TR/xhtml1/DTD", the local cache simplifies the rules by
     keeping the same structure as the on-line server at the Canonical URL</li>
   <li>the local resources are designated using URI references (the uri or
     rewritePrefix attributes), the base being the containing sub-catalog URL,
     which means that in practice the copy of the XHTML1 strict DTD is stored
     locally in
     <code>/usr/share/sgml/xhtml1/xmlcatalog/xhtml1-20020801/DTD/xhtml1-strict.dtd</code></li>
 </ul>

 <p>Those 5 rules are sufficient to cover all references to the resources held
 at the Canonical URL for the XHTML1 DTDs.</p>

 <h2><a name="Package">Package integration</a></h2>

 <p>Creating and removing catalogs should be handled as part of the process of
 (un)installing the local copy of the resources. The catalog files being XML
 resources should be processed with XML based tools to avoid problems with the
 generated files, the xmlcatalog command coming with libxml2 allows you to create
 catalogs, and add or remove rules at that time. Here is a complete example
 coming from the RPM for the XHTML1 DTDs post install script. While this example
 is platform and packaging specific, this can be useful as a an example in
 other contexts:</p>
 <pre>%post
 CATALOG=/usr/share/sgml/xhtml1/xmlcatalog
 #
 # Register it in the super catalog with the appropriate delegates
 #
 ROOTCATALOG=/etc/xml/catalog

 if [ ! -r $ROOTCATALOG ]
 then
     /usr/bin/xmlcatalog --noout --create $ROOTCATALOG
 fi

 if [ -w $ROOTCATALOG ]
 then
         /usr/bin/xmlcatalog --noout --add "delegatePublic" \
                 "-//W3C//DTD XHTML 1.0" \
                 "file://$CATALOG" $ROOTCATALOG
         /usr/bin/xmlcatalog --noout --add "delegateSystem" \
                 "http://www.w3.org/TR/xhtml1/DTD" \
                 "file://$CATALOG" $ROOTCATALOG
         /usr/bin/xmlcatalog --noout --add "delegateURI" \
                 "http://www.w3.org/TR/xhtml1/DTD" \
                 "file://$CATALOG" $ROOTCATALOG
 fi</pre>

 <p>The XHTML1 subcatalog is not created on-the-fly in that case, it is
 installed as part of the files of the packages. So the only work needed is to
 make sure the root catalog exists and register the delegate rules.</p>

 <p>Similarly, the script for the post-uninstall just remove the rules from the
 catalog:</p>
 <pre>%postun
 #
 # On removal, unregister the xmlcatalog from the supercatalog
 #
 if [ "$1" = 0 ]; then
     CATALOG=/usr/share/sgml/xhtml1/xmlcatalog
     ROOTCATALOG=/etc/xml/catalog

     if [ -w $ROOTCATALOG ]
     then
             /usr/bin/xmlcatalog --noout --del \
                     "-//W3C//DTD XHTML 1.0" $ROOTCATALOG
             /usr/bin/xmlcatalog --noout --del \
                     "http://www.w3.org/TR/xhtml1/DTD" $ROOTCATALOG
             /usr/bin/xmlcatalog --noout --del \
                     "http://www.w3.org/TR/xhtml1/DTD" $ROOTCATALOG
     fi
 fi</pre>

 <p>Note the test against $1, this is needed to not remove the delegate rules
 in case of upgrade of the package.</p>

 <p>Following the set of guidelines and tips provided in this document should
 help deploy the XML resources in the GNOME framework without much pain and
 ensure a smooth evolution of the resource and instances.</p>

 <p><a href="mailto:veillard@redhat.com">Daniel Veillard</a></p>

 <p>$Id$</p>

 <p></p>
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	<title>XML resources publication guidelines</title>
	</head>

	<body bgcolor="#fffacd" text="#000000">
	<h1 align="center">XML resources publication guidelines</h1>

	<p></p>

	<p>The goal of this document is to provide a set of guidelines and tips
	helping the publication and deployment of <a
	href="http://www.w3.org/XML/">XML</a> resources for the <a
	href="http://www.gnome.org/">GNOME project</a>. However it is not tied to
	GNOME and might be helpful more generally. I welcome <a
	href="mailto:veillard@redhat.com">feedback</a> on this document.</p>

	<p>The intended audience is the software developers who started using XML
	for some of the resources of their project, as a storage format, for data
	exchange, checking or transformations. There have been an increasing number
	of new XML formats defined, but not all steps have been taken, possibly because of
	lack of documentation, to truly gain all the benefits of the use of XML.
	These guidelines hope to improve the matter and provide a better overview of
	the overall XML processing and associated steps needed to deploy it
	successfully:</p>

	<p>Table of contents:</p>
	<ol>
	<li><a href="#Design">Design guidelines</a></li>
	<li><a href="#Canonical">Canonical URL</a></li>
	<li><a href="#Catalog">Catalog setup</a></li>
	<li><a href="#Package">Package integration</a></li>
	</ol>

	<h2><a name="Design">Design guidelines</a></h2>

	<p>This part intends to focus on the format itself of XML. It may arrive
	a bit too late since the structure of the document may already be cast in
	existing and deployed code. Still, here are a few rules which might be helpful
	when designing a new XML vocabulary or making the revision of an existing
	format:</p>

	<h3>Reuse existing formats:</h3>

	<p>This may sounds a bit simplistic, but before designing your own format,
	try to lookup existing XML vocabularies on similar data. Ideally this allows
	you to reuse them, in which case a lot of the existing tools like DTD, schemas
	and stylesheets may already be available. If you are looking at a
	documentation format, <a href="http://www.docbook.org/">DocBook</a> should
	handle your needs. If reuse is not possible because some semantic or use case
	aspects are too different this will be helpful avoiding design errors like
	targeting the vocabulary to the wrong abstraction level. In this format
	design phase try to be synthetic and be sure to express the real content of
	your data and use the XML structure to express the semantic and context of
	those data.</p>

	<h3>DTD rules:</h3>

	<p>Building a DTD (Document Type Definition) or a Schema describing the
	structure allowed by instances is the core of the design process of the
	vocabulary. Here are a few tips:</p>
	<ul>
	<li>use significant words for the element and attributes names.</li>
	<li>do not use attributes for general textual content, attributes
	will be modified by the parser before reaching the application,
	spaces and line informations will be modified.</li>
	<li>use single elements for every string that might be subject to
	localization. The canonical way to localize XML content is to use
	siblings element carrying different xml:lang attributes like in the
	following:
	<pre><welcome>
	<msg xml:lang="en">hello</msg>
	<msg xml:lang="fr">bonjour</msg>
	</welcome></pre>
	</li>
	<li>use attributes to refine the content of an element but avoid them for
	more complex tasks, attribute parsing is not cheaper than an element and
	it is far easier to make an element content more complex while attribute
	will have to remain very simple.</li>
	</ul>

	<h3>Versioning:</h3>

	<p>As part of the design, make sure the structure you define will be usable
	for future extension that you may not consider for the current version. There
	are two parts to this:</p>
	<ul>
	<li>Make sure the instance contains a version number which will allow to
	make backward compatibility easy. Something as simple as having a
	<code>version="1.0"</code> on the root document of the instance is
	sufficient.</li>
	<li>While designing the code doing the analysis of the data provided by the
	XML parser, make sure you can work with unknown versions, generate a UI
	warning and process only the tags recognized by your version but keep in
	mind that you should not break on unknown elements if the version
	attribute was not in the recognized set.</li>
	</ul>

	<h3>Other design parts:</h3>

	<p>While defining you vocabulary, try to think in term of other usage of your
	data, for example how using XSLT stylesheets could be used to make an HTML
	view of your data, or to convert it into a different format. Checking XML
	Schemas and looking at defining an XML Schema with a more complete
	validation and datatyping of your data structures is important, this helps
	avoiding some mistakes in the design phase.</p>

	<h3>Namespace:</h3>

	<p>If you expect your XML vocabulary to be used or recognized outside of your
	application (for example binding a specific processing from a graphic shell
	like Nautilus to an instance of your data) then you should really define an <a
	href="http://www.w3.org/TR/REC-xml-names/">XML namespace</a> for your
	vocabulary. A namespace name is an URL (absolute URI more precisely). It is
	generally recommended to anchor it as an HTTP resource to a server associated
	with the software project. See the next section about this. In practice this
	will mean that XML parsers will not handle your element names as-is but as a
	couple based on the namespace name and the element name. This allows it to
	recognize and disambiguate processing. Unicity of the namespace name can be
	for the most part guaranteed by the use of the DNS registry. Namespace can
	also be used to carry versioning information like:</p>

	<p><code>"http://www.gnome.org/project/projectname/1.0/"</code></p>

	<p>An easy way to use them is to make them the default namespace on the
	root element of the XML instance like:</p>
	<pre><structure xmlns="http://www.gnome.org/project/projectname/1.0/">
	<data>
	...
	</data>
	</structure></pre>

	<p>In that document, structure and all descendant elements like data are in
	the given namespace.</p>

	<h2><a name="Canonical">Canonical URL</a></h2>

	<p>As seen in the previous namespace section, while XML processing is not
	tied to the Web there is a natural synergy between both. XML was designed to
	be available on the Web, and keeping the infrastructure that way helps
	deploying the XML resources. The core of this issue is the notion of
	"Canonical URL" of an XML resource. The resource can be an XML document, a
	DTD, a stylesheet, a schema, or even non-XML data associated with an XML
	resource, the canonical URL is the URL where the "master" copy of that
	resource is expected to be present on the Web. Usually when processing XML a
	copy of the resource will be present on the local disk, maybe in
	/usr/share/xml or /usr/share/sgml maybe in /opt or even on C:\projectname\
	(horror !). The key point is that the way to name that resource should be
	independent of the actual place where it resides on disk if it is available,
	and the fact that the processing will still work if there is no local copy
	(and that the machine where the processing is connected to the Internet).</p>

	<p>What this really means is that one should never use the local name of a
	resource to reference it but always use the canonical URL. For example in a
	DocBook instance the following should not be used:</p>
	<pre><!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"<br>


	"/usr/share/xml/docbook/4.2/docbookx.dtd"></pre>

	<p>But always reference the canonical URL for the DTD:</p>
	<pre><!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"<br>


	"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> </pre>

	<p>Similarly, the document instance may reference the <a
	href="http://www.w3.org/TR/xslt">XSLT</a> stylesheets needed to process it to
	generate HTML, and the canonical URL should be used:</p>
	<pre><?xml-stylesheet
	href="http://docbook.sourceforge.net/release/xsl/current/html/docbook.xsl"
	type="text/xsl"?></pre>

	<p>Defining the canonical URL for the resources needed should obey a few
	simple rules similar to those used to design namespace names:</p>
	<ul>
	<li>use a DNS name you know is associated to the project and will be
	available on the long term</li>
	<li>within that server space, reserve the right to the subtree where you
	intend to keep those data</li>
	<li>version the URL so that multiple concurrent versions of the resources
	can be hosted simultaneously</li>
	</ul>

	<h2><a name="Catalog">Catalog setup</a></h2>

	<h3>How catalogs work:</h3>

	<p>The catalogs are the technical mechanism which allow the XML processing
	tools to use a local copy of the resources if it is available even if the
	instance document references the canonical URL. <a
	href="http://www.oasis-open.org/committees/entity/">XML Catalogs</a> are
	anchored in the root catalog (usually <code>/etc/xml/catalog</code> or
	defined by the user). They are a tree of XML documents defining the mappings
	between the canonical naming space and the local installed ones, this can be
	seen as a static cache structure.</p>

	<p>When the XML processor is asked to process a resource it will
	automatically test for a locally available version in the catalog, starting
	from the root catalog, and possibly fetching sub-catalog resources until it
	finds that the catalog has that resource or not. If not the default
	processing of fetching the resource from the Web is done, allowing in most
	case to recover from a catalog miss. The key point is that the document
	instances are totally independent of the availability of a catalog or from
	the actual place where the local resource they reference may be installed.
	This greatly improves the management of the documents in the long run, making
	them independent of the platform or toolchain used to process them. The
	figure below tries to express that mechanism:<img src="catalog.gif"
	alt="Picture describing the catalog "></p>

	<h3>Usual catalog setup:</h3>

	<p>Usually catalogs for a project are setup as a 2 level hierarchical cache,
	the root catalog containing only "delegates" indicating a separate subcatalog
	dedicated to the project. The goal is to keep the root catalog clean and
	simplify the maintenance of the catalog by using separate catalogs per
	project. For example when creating a catalog for the <a
	href="http://www.w3.org/TR/xhtml1">XHTML1</a> DTDs, only 3 items are added to
	the root catalog:</p>
	<pre> <delegatePublic publicIdStartString="-//W3C//DTD XHTML 1.0"
	catalog="file:///usr/share/sgml/xhtml1/xmlcatalog"/>
	<delegateSystem systemIdStartString="http://www.w3.org/TR/xhtml1/DTD"
	catalog="file:///usr/share/sgml/xhtml1/xmlcatalog"/>
	<delegateURI uriStartString="http://www.w3.org/TR/xhtml1/DTD"
	catalog="file:///usr/share/sgml/xhtml1/xmlcatalog"/></pre>

	<p>They are all "delegates" meaning that if the catalog system is asked to
	resolve a reference corresponding to them, it has to lookup a sub catalog.
	Here the subcatalog was installed as
	<code>/usr/share/sgml/xhtml1/xmlcatalog</code> in the local tree. That
	decision is left to the sysadmin or the packager for that system and may
	obey different rules, but the actual place on the filesystem (or on a
	resource cache on the local network) will not influence the processing as
	long as it is available. The first rule indicate that if the reference uses a
	PUBLIC identifier beginning with the</p>

	<p><code>"-//W3C//DTD XHTML 1.0"</code></p>

	<p>substring, then the catalog lookup should be limited to the specific given
	lookup catalog. Similarly the second and third entries indicate those
	delegation rules for SYSTEM, DOCTYPE or normal URI references when the URL
	starts with the <code>"http://www.w3.org/TR/xhtml1/DTD"</code> substring
	which indicates the location on the W3C server where the XHTML1 resources are
	stored. Those are the beginning of all Canonical URLs for XHTML1 resources.
	Those three rules are sufficient in practice to capture all references to XHTML1
	resources and direct the processing tools to the right subcatalog.</p>

	<h3>A subcatalog example:</h3>

	<p>Here is the complete subcatalog used for XHTML1:</p>
	<pre><?xml version="1.0"?>
	<!DOCTYPE catalog PUBLIC "-//OASIS//DTD Entity Resolution XML Catalog V1.0//EN"
	"http://www.oasis-open.org/committees/entity/release/1.0/catalog.dtd">
	<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog">
	<public publicId="-//W3C//DTD XHTML 1.0 Strict//EN"
	uri="xhtml1-20020801/DTD/xhtml1-strict.dtd"/>
	<public publicId="-//W3C//DTD XHTML 1.0 Transitional//EN"
	uri="xhtml1-20020801/DTD/xhtml1-transitional.dtd"/>
	<public publicId="-//W3C//DTD XHTML 1.0 Frameset//EN"
	uri="xhtml1-20020801/DTD/xhtml1-frameset.dtd"/>
	<rewriteSystem systemIdStartString="http://www.w3.org/TR/xhtml1/DTD"
	rewritePrefix="xhtml1-20020801/DTD"/>
	<rewriteURI uriStartString="http://www.w3.org/TR/xhtml1/DTD"
	rewritePrefix="xhtml1-20020801/DTD"/>
	</catalog></pre>

	<p>There are a few things to notice:</p>
	<ul>
	<li>this is an XML resource, it points to the DTD using Canonical URLs, the
	root element defines a namespace (but based on an URN not an HTTP
	URL).</li>
	<li>it contains 5 rules, the 3 first ones are direct mapping for the 3
	PUBLIC identifiers defined by the XHTML1 specification and associating
	them with the local resource containing the DTD, the 2 last ones are
	rewrite rules allowing to build the local filename for any URL based on
	"http://www.w3.org/TR/xhtml1/DTD", the local cache simplifies the rules by
	keeping the same structure as the on-line server at the Canonical URL</li>
	<li>the local resources are designated using URI references (the uri or
	rewritePrefix attributes), the base being the containing sub-catalog URL,
	which means that in practice the copy of the XHTML1 strict DTD is stored
	locally in
	<code>/usr/share/sgml/xhtml1/xmlcatalog/xhtml1-20020801/DTD/xhtml1-strict.dtd</code></li>
	</ul>

	<p>Those 5 rules are sufficient to cover all references to the resources held
	at the Canonical URL for the XHTML1 DTDs.</p>

	<h2><a name="Package">Package integration</a></h2>

	<p>Creating and removing catalogs should be handled as part of the process of
	(un)installing the local copy of the resources. The catalog files being XML
	resources should be processed with XML based tools to avoid problems with the
	generated files, the xmlcatalog command coming with libxml2 allows you to create
	catalogs, and add or remove rules at that time. Here is a complete example
	coming from the RPM for the XHTML1 DTDs post install script. While this example
	is platform and packaging specific, this can be useful as a an example in
	other contexts:</p>
	<pre>%post
	CATALOG=/usr/share/sgml/xhtml1/xmlcatalog
	#
	# Register it in the super catalog with the appropriate delegates
	#
	ROOTCATALOG=/etc/xml/catalog

	if [ ! -r $ROOTCATALOG ]
	then
	/usr/bin/xmlcatalog --noout --create $ROOTCATALOG
	fi

	if [ -w $ROOTCATALOG ]
	then
	/usr/bin/xmlcatalog --noout --add "delegatePublic" \
	"-//W3C//DTD XHTML 1.0" \
	"file://$CATALOG" $ROOTCATALOG
	/usr/bin/xmlcatalog --noout --add "delegateSystem" \
	"http://www.w3.org/TR/xhtml1/DTD" \
	"file://$CATALOG" $ROOTCATALOG
	/usr/bin/xmlcatalog --noout --add "delegateURI" \
	"http://www.w3.org/TR/xhtml1/DTD" \
	"file://$CATALOG" $ROOTCATALOG
	fi</pre>

	<p>The XHTML1 subcatalog is not created on-the-fly in that case, it is
	installed as part of the files of the packages. So the only work needed is to
	make sure the root catalog exists and register the delegate rules.</p>

	<p>Similarly, the script for the post-uninstall just remove the rules from the
	catalog:</p>
	<pre>%postun
	#
	# On removal, unregister the xmlcatalog from the supercatalog
	#
	if [ "$1" = 0 ]; then
	CATALOG=/usr/share/sgml/xhtml1/xmlcatalog
	ROOTCATALOG=/etc/xml/catalog

	if [ -w $ROOTCATALOG ]
	then
	/usr/bin/xmlcatalog --noout --del \
	"-//W3C//DTD XHTML 1.0" $ROOTCATALOG
	/usr/bin/xmlcatalog --noout --del \
	"http://www.w3.org/TR/xhtml1/DTD" $ROOTCATALOG
	/usr/bin/xmlcatalog --noout --del \
	"http://www.w3.org/TR/xhtml1/DTD" $ROOTCATALOG
	fi
	fi</pre>

	<p>Note the test against $1, this is needed to not remove the delegate rules
	in case of upgrade of the package.</p>

	<p>Following the set of guidelines and tips provided in this document should
	help deploy the XML resources in the GNOME framework without much pain and
	ensure a smooth evolution of the resource and instances.</p>

	<p><a href="mailto:veillard@redhat.com">Daniel Veillard</a></p>

	<p>$Id$</p>

	<p></p>
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