mirror of
https://gitlab.gnome.org/GNOME/libxml2.git
synced 2024-12-27 03:21:26 +03:00
375 lines
17 KiB
HTML
375 lines
17 KiB
HTML
<!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>
|