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| <title>XML resources publication guidelines</title> |
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| |
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| <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 information 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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