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diff --git a/doc/guidelines.html b/doc/guidelines.html new file mode 100644 index 0000000..af4a7b3 --- /dev/null +++ b/doc/guidelines.html @@ -0,0 +1,374 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" + "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<head> + <meta http-equiv="Content-Type" content="text/html"> + <style type="text/css"></style> +<!-- +TD {font-family: Verdana,Arial,Helvetica} +BODY {font-family: Verdana,Arial,Helvetica; margin-top: 2em; margin-left: 0em; margin-right: 0em} +H1 {font-family: Verdana,Arial,Helvetica} +H2 {font-family: Verdana,Arial,Helvetica} +H3 {font-family: Verdana,Arial,Helvetica} +A:link, A:visited, A:active { text-decoration: underline } + </style> +--> + <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> +</body> +</html> |