375 lines
17 KiB
HTML
375 lines
17 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
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"http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html">
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<style type="text/css"></style>
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BODY {font-family: Verdana,Arial,Helvetica; margin-top: 2em; margin-left: 0em; margin-right: 0em}
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H1 {font-family: Verdana,Arial,Helvetica}
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H2 {font-family: Verdana,Arial,Helvetica}
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H3 {font-family: Verdana,Arial,Helvetica}
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A:link, A:visited, A:active { text-decoration: underline }
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<title>XML resources publication guidelines</title>
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</head>
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<body bgcolor="#fffacd" text="#000000">
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<h1 align="center">XML resources publication guidelines</h1>
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<p></p>
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<p>The goal of this document is to provide a set of guidelines and tips
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helping the publication and deployment of <a
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href="http://www.w3.org/XML/">XML</a> resources for the <a
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href="http://www.gnome.org/">GNOME project</a>. However it is not tied to
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GNOME and might be helpful more generally. I welcome <a
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href="mailto:veillard@redhat.com">feedback</a> on this document.</p>
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<p>The intended audience is the software developers who started using XML
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for some of the resources of their project, as a storage format, for data
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exchange, checking or transformations. There have been an increasing number
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of new XML formats defined, but not all steps have been taken, possibly because of
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lack of documentation, to truly gain all the benefits of the use of XML.
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These guidelines hope to improve the matter and provide a better overview of
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the overall XML processing and associated steps needed to deploy it
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successfully:</p>
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<p>Table of contents:</p>
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<ol>
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<li><a href="#Design">Design guidelines</a></li>
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<li><a href="#Canonical">Canonical URL</a></li>
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<li><a href="#Catalog">Catalog setup</a></li>
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<li><a href="#Package">Package integration</a></li>
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</ol>
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<h2><a name="Design">Design guidelines</a></h2>
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<p>This part intends to focus on the format itself of XML. It may arrive
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a bit too late since the structure of the document may already be cast in
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existing and deployed code. Still, here are a few rules which might be helpful
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when designing a new XML vocabulary or making the revision of an existing
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format:</p>
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<h3>Reuse existing formats:</h3>
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<p>This may sounds a bit simplistic, but before designing your own format,
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try to lookup existing XML vocabularies on similar data. Ideally this allows
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you to reuse them, in which case a lot of the existing tools like DTD, schemas
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and stylesheets may already be available. If you are looking at a
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documentation format, <a href="http://www.docbook.org/">DocBook</a> should
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handle your needs. If reuse is not possible because some semantic or use case
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aspects are too different this will be helpful avoiding design errors like
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targeting the vocabulary to the wrong abstraction level. In this format
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design phase try to be synthetic and be sure to express the real content of
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your data and use the XML structure to express the semantic and context of
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those data.</p>
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<h3>DTD rules:</h3>
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<p>Building a DTD (Document Type Definition) or a Schema describing the
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structure allowed by instances is the core of the design process of the
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vocabulary. Here are a few tips:</p>
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<ul>
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<li>use significant words for the element and attributes names.</li>
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<li>do not use attributes for general textual content, attributes
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will be modified by the parser before reaching the application,
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spaces and line information will be modified.</li>
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<li>use single elements for every string that might be subject to
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localization. The canonical way to localize XML content is to use
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siblings element carrying different xml:lang attributes like in the
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following:
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<pre><welcome>
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<msg xml:lang="en">hello</msg>
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<msg xml:lang="fr">bonjour</msg>
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</welcome></pre>
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</li>
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<li>use attributes to refine the content of an element but avoid them for
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more complex tasks, attribute parsing is not cheaper than an element and
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it is far easier to make an element content more complex while attribute
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will have to remain very simple.</li>
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</ul>
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<h3>Versioning:</h3>
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<p>As part of the design, make sure the structure you define will be usable
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for future extension that you may not consider for the current version. There
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are two parts to this:</p>
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<ul>
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<li>Make sure the instance contains a version number which will allow to
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make backward compatibility easy. Something as simple as having a
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<code>version="1.0"</code> on the root document of the instance is
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sufficient.</li>
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<li>While designing the code doing the analysis of the data provided by the
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XML parser, make sure you can work with unknown versions, generate a UI
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warning and process only the tags recognized by your version but keep in
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mind that you should not break on unknown elements if the version
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attribute was not in the recognized set.</li>
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</ul>
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<h3>Other design parts:</h3>
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<p>While defining you vocabulary, try to think in term of other usage of your
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data, for example how using XSLT stylesheets could be used to make an HTML
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view of your data, or to convert it into a different format. Checking XML
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Schemas and looking at defining an XML Schema with a more complete
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validation and datatyping of your data structures is important, this helps
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avoiding some mistakes in the design phase.</p>
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<h3>Namespace:</h3>
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<p>If you expect your XML vocabulary to be used or recognized outside of your
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application (for example binding a specific processing from a graphic shell
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like Nautilus to an instance of your data) then you should really define an <a
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href="http://www.w3.org/TR/REC-xml-names/">XML namespace</a> for your
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vocabulary. A namespace name is an URL (absolute URI more precisely). It is
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generally recommended to anchor it as an HTTP resource to a server associated
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with the software project. See the next section about this. In practice this
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will mean that XML parsers will not handle your element names as-is but as a
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couple based on the namespace name and the element name. This allows it to
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recognize and disambiguate processing. Unicity of the namespace name can be
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for the most part guaranteed by the use of the DNS registry. Namespace can
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also be used to carry versioning information like:</p>
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<p><code>"http://www.gnome.org/project/projectname/1.0/"</code></p>
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<p>An easy way to use them is to make them the default namespace on the
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root element of the XML instance like:</p>
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<pre><structure xmlns="http://www.gnome.org/project/projectname/1.0/">
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<data>
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...
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</data>
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</structure></pre>
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<p>In that document, structure and all descendant elements like data are in
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the given namespace.</p>
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<h2><a name="Canonical">Canonical URL</a></h2>
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<p>As seen in the previous namespace section, while XML processing is not
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tied to the Web there is a natural synergy between both. XML was designed to
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be available on the Web, and keeping the infrastructure that way helps
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deploying the XML resources. The core of this issue is the notion of
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"Canonical URL" of an XML resource. The resource can be an XML document, a
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DTD, a stylesheet, a schema, or even non-XML data associated with an XML
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resource, the canonical URL is the URL where the "master" copy of that
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resource is expected to be present on the Web. Usually when processing XML a
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copy of the resource will be present on the local disk, maybe in
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/usr/share/xml or /usr/share/sgml maybe in /opt or even on C:\projectname\
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(horror !). The key point is that the way to name that resource should be
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independent of the actual place where it resides on disk if it is available,
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and the fact that the processing will still work if there is no local copy
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(and that the machine where the processing is connected to the Internet).</p>
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<p>What this really means is that one should never use the local name of a
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resource to reference it but always use the canonical URL. For example in a
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DocBook instance the following should not be used:</p>
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<pre><!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"<br>
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"/usr/share/xml/docbook/4.2/docbookx.dtd"></pre>
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<p>But always reference the canonical URL for the DTD:</p>
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<pre><!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"<br>
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> </pre>
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<p>Similarly, the document instance may reference the <a
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href="http://www.w3.org/TR/xslt">XSLT</a> stylesheets needed to process it to
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generate HTML, and the canonical URL should be used:</p>
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<pre><?xml-stylesheet
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href="http://docbook.sourceforge.net/release/xsl/current/html/docbook.xsl"
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type="text/xsl"?></pre>
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<p>Defining the canonical URL for the resources needed should obey a few
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simple rules similar to those used to design namespace names:</p>
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<ul>
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<li>use a DNS name you know is associated to the project and will be
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available on the long term</li>
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<li>within that server space, reserve the right to the subtree where you
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intend to keep those data</li>
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<li>version the URL so that multiple concurrent versions of the resources
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can be hosted simultaneously</li>
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</ul>
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<h2><a name="Catalog">Catalog setup</a></h2>
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<h3>How catalogs work:</h3>
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<p>The catalogs are the technical mechanism which allow the XML processing
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tools to use a local copy of the resources if it is available even if the
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instance document references the canonical URL. <a
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href="http://www.oasis-open.org/committees/entity/">XML Catalogs</a> are
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anchored in the root catalog (usually <code>/etc/xml/catalog</code> or
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defined by the user). They are a tree of XML documents defining the mappings
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between the canonical naming space and the local installed ones, this can be
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seen as a static cache structure.</p>
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<p>When the XML processor is asked to process a resource it will
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automatically test for a locally available version in the catalog, starting
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from the root catalog, and possibly fetching sub-catalog resources until it
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finds that the catalog has that resource or not. If not the default
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processing of fetching the resource from the Web is done, allowing in most
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case to recover from a catalog miss. The key point is that the document
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instances are totally independent of the availability of a catalog or from
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the actual place where the local resource they reference may be installed.
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This greatly improves the management of the documents in the long run, making
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them independent of the platform or toolchain used to process them. The
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figure below tries to express that mechanism:<img src="catalog.gif"
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alt="Picture describing the catalog "></p>
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<h3>Usual catalog setup:</h3>
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<p>Usually catalogs for a project are setup as a 2 level hierarchical cache,
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the root catalog containing only "delegates" indicating a separate subcatalog
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dedicated to the project. The goal is to keep the root catalog clean and
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simplify the maintenance of the catalog by using separate catalogs per
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project. For example when creating a catalog for the <a
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href="http://www.w3.org/TR/xhtml1">XHTML1</a> DTDs, only 3 items are added to
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the root catalog:</p>
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<pre> <delegatePublic publicIdStartString="-//W3C//DTD XHTML 1.0"
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catalog="file:///usr/share/sgml/xhtml1/xmlcatalog"/>
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<delegateSystem systemIdStartString="http://www.w3.org/TR/xhtml1/DTD"
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catalog="file:///usr/share/sgml/xhtml1/xmlcatalog"/>
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<delegateURI uriStartString="http://www.w3.org/TR/xhtml1/DTD"
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catalog="file:///usr/share/sgml/xhtml1/xmlcatalog"/></pre>
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<p>They are all "delegates" meaning that if the catalog system is asked to
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resolve a reference corresponding to them, it has to lookup a sub catalog.
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Here the subcatalog was installed as
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<code>/usr/share/sgml/xhtml1/xmlcatalog</code> in the local tree. That
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decision is left to the sysadmin or the packager for that system and may
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obey different rules, but the actual place on the filesystem (or on a
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resource cache on the local network) will not influence the processing as
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long as it is available. The first rule indicate that if the reference uses a
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PUBLIC identifier beginning with the</p>
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<p><code>"-//W3C//DTD XHTML 1.0"</code></p>
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<p>substring, then the catalog lookup should be limited to the specific given
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lookup catalog. Similarly the second and third entries indicate those
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delegation rules for SYSTEM, DOCTYPE or normal URI references when the URL
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starts with the <code>"http://www.w3.org/TR/xhtml1/DTD"</code> substring
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which indicates the location on the W3C server where the XHTML1 resources are
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stored. Those are the beginning of all Canonical URLs for XHTML1 resources.
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Those three rules are sufficient in practice to capture all references to XHTML1
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resources and direct the processing tools to the right subcatalog.</p>
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<h3>A subcatalog example:</h3>
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<p>Here is the complete subcatalog used for XHTML1:</p>
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<pre><?xml version="1.0"?>
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<!DOCTYPE catalog PUBLIC "-//OASIS//DTD Entity Resolution XML Catalog V1.0//EN"
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"http://www.oasis-open.org/committees/entity/release/1.0/catalog.dtd">
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<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog">
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<public publicId="-//W3C//DTD XHTML 1.0 Strict//EN"
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uri="xhtml1-20020801/DTD/xhtml1-strict.dtd"/>
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<public publicId="-//W3C//DTD XHTML 1.0 Transitional//EN"
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uri="xhtml1-20020801/DTD/xhtml1-transitional.dtd"/>
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<public publicId="-//W3C//DTD XHTML 1.0 Frameset//EN"
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uri="xhtml1-20020801/DTD/xhtml1-frameset.dtd"/>
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<rewriteSystem systemIdStartString="http://www.w3.org/TR/xhtml1/DTD"
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rewritePrefix="xhtml1-20020801/DTD"/>
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<rewriteURI uriStartString="http://www.w3.org/TR/xhtml1/DTD"
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rewritePrefix="xhtml1-20020801/DTD"/>
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</catalog></pre>
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<p>There are a few things to notice:</p>
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<ul>
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<li>this is an XML resource, it points to the DTD using Canonical URLs, the
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root element defines a namespace (but based on an URN not an HTTP
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URL).</li>
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<li>it contains 5 rules, the 3 first ones are direct mapping for the 3
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PUBLIC identifiers defined by the XHTML1 specification and associating
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them with the local resource containing the DTD, the 2 last ones are
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rewrite rules allowing to build the local filename for any URL based on
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"http://www.w3.org/TR/xhtml1/DTD", the local cache simplifies the rules by
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keeping the same structure as the on-line server at the Canonical URL</li>
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<li>the local resources are designated using URI references (the uri or
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rewritePrefix attributes), the base being the containing sub-catalog URL,
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which means that in practice the copy of the XHTML1 strict DTD is stored
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locally in
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<code>/usr/share/sgml/xhtml1/xmlcatalog/xhtml1-20020801/DTD/xhtml1-strict.dtd</code></li>
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</ul>
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<p>Those 5 rules are sufficient to cover all references to the resources held
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at the Canonical URL for the XHTML1 DTDs.</p>
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<h2><a name="Package">Package integration</a></h2>
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<p>Creating and removing catalogs should be handled as part of the process of
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(un)installing the local copy of the resources. The catalog files being XML
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resources should be processed with XML based tools to avoid problems with the
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generated files, the xmlcatalog command coming with libxml2 allows you to create
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catalogs, and add or remove rules at that time. Here is a complete example
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coming from the RPM for the XHTML1 DTDs post install script. While this example
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is platform and packaging specific, this can be useful as a an example in
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other contexts:</p>
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<pre>%post
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CATALOG=/usr/share/sgml/xhtml1/xmlcatalog
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#
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# Register it in the super catalog with the appropriate delegates
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#
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ROOTCATALOG=/etc/xml/catalog
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if [ ! -r $ROOTCATALOG ]
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then
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/usr/bin/xmlcatalog --noout --create $ROOTCATALOG
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fi
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if [ -w $ROOTCATALOG ]
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then
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/usr/bin/xmlcatalog --noout --add "delegatePublic" \
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"-//W3C//DTD XHTML 1.0" \
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"file://$CATALOG" $ROOTCATALOG
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/usr/bin/xmlcatalog --noout --add "delegateSystem" \
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"http://www.w3.org/TR/xhtml1/DTD" \
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"file://$CATALOG" $ROOTCATALOG
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/usr/bin/xmlcatalog --noout --add "delegateURI" \
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"http://www.w3.org/TR/xhtml1/DTD" \
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"file://$CATALOG" $ROOTCATALOG
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fi</pre>
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<p>The XHTML1 subcatalog is not created on-the-fly in that case, it is
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installed as part of the files of the packages. So the only work needed is to
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make sure the root catalog exists and register the delegate rules.</p>
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<p>Similarly, the script for the post-uninstall just remove the rules from the
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catalog:</p>
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<pre>%postun
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#
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# On removal, unregister the xmlcatalog from the supercatalog
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#
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if [ "$1" = 0 ]; then
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CATALOG=/usr/share/sgml/xhtml1/xmlcatalog
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ROOTCATALOG=/etc/xml/catalog
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if [ -w $ROOTCATALOG ]
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then
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/usr/bin/xmlcatalog --noout --del \
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"-//W3C//DTD XHTML 1.0" $ROOTCATALOG
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/usr/bin/xmlcatalog --noout --del \
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"http://www.w3.org/TR/xhtml1/DTD" $ROOTCATALOG
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/usr/bin/xmlcatalog --noout --del \
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"http://www.w3.org/TR/xhtml1/DTD" $ROOTCATALOG
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fi
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fi</pre>
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<p>Note the test against $1, this is needed to not remove the delegate rules
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in case of upgrade of the package.</p>
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<p>Following the set of guidelines and tips provided in this document should
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help deploy the XML resources in the GNOME framework without much pain and
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ensure a smooth evolution of the resource and instances.</p>
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<p><a href="mailto:veillard@redhat.com">Daniel Veillard</a></p>
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<p>$Id$</p>
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<p></p>
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</body>
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</html>
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