Gitiles
Code Review Sign In
gerrit-public.fairphone.software / platform / external / libxml2 / d105eb6fb89ce4b450272a07433510a4eed9c7a1 / . / doc / guidelines.html
blob: 807d2521244d86390cf6e0d77bb8610c5903a91f [file] [log] [blame]
Daniel Veillard83298842002-12-28 15:12:33 +0000[diff] [blame]1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
2 "http://www.w3.org/TR/html4/loose.dtd">
3<html>
4<head>
5 <meta http-equiv="Content-Type" content="text/html">
Daniel Veillard6943a4d2002-12-28 18:07:59 +0000[diff] [blame]6 <style type="text/css">
7<!--
Daniel Veillard83298842002-12-28 15:12:33 +0000[diff] [blame]8TD {font-family: Verdana,Arial,Helvetica}
9BODY {font-family: Verdana,Arial,Helvetica; margin-top: 2em; margin-left: 0em; margin-right: 0em}
10H1 {font-family: Verdana,Arial,Helvetica}
11H2 {font-family: Verdana,Arial,Helvetica}
12H3 {font-family: Verdana,Arial,Helvetica}
Daniel Veillard6943a4d2002-12-28 18:07:59 +0000[diff] [blame]13A:link, A:visited, A:active { text-decoration: underline }-->
14
15 </style>
Daniel Veillard83298842002-12-28 15:12:33 +0000[diff] [blame]16 <title>XML resources publication guidelines</title>
17</head>
18
19<body bgcolor="#fffacd" text="#000000">
20<h1 align="center">XML resources publication guidelines</h1>
21
22<p></p>
23
24<p>The goal of this document is to provide a set of guidelines and tips
25helping the publication and deployment of <a
26href="http://www.w3.org.XML/">XML</a> resources for the <a
27href="http://www.gnome.org/">GNOME project</a>. However it is not tied to
28GNOME and might be helpful more generally, I welcome <a
29href="mailto:veillard@redhat.com">feedback</a> on this document.</p>
30
31<p>The intended audience are the software developpers who started using XML
32for some of the resources of their project, as a storage format, for data
33exchange, checking or transformations. There have been an increasing number
34of new XML format defined, but not all steps have been taken, possibly by
35lack of documentation, to truely gain all the benefits of the use of XML.
36Those guidelines hopes to improve the matter and provide a better overview of
37the overall XML processing and associated steps needed deploy it
Daniel Veillard6943a4d2002-12-28 18:07:59 +0000[diff] [blame]38successfully:</p>
Daniel Veillard83298842002-12-28 15:12:33 +0000[diff] [blame]39
40<p>Table of content:</p>
41<ol>
42 <li><a href="#Design">Design guidelines</a></li>
43 <li><a href="#Canonical">Canonical URL</a></li>
44 <li><a href="#Catalog">Catalog setup</a></li>
45 <li><a href="#Package">Package integration</a></li>
46</ol>
47
48<h2><a name="Design">Design guidelines</a></h2>
49
50<p>This part intend to focuse on the format itself of XML, those may arrive
51a bit too late since the structure of the document may already be cast in
52existing and deployed code. Still here are a few rules which might be helpful
53when designing a new XML vocabulary or making the revision of an existing
54format:</p>
55
56<h3>Reuse existing formats:</h3>
57
58<p>This may sounds a bit simplistic, but before designing your own format,
59try to lookup existing XML vocabularies on similar data. Ideally this allows
60to reuse them, in which case a lot of the existing tools like DTD, schemas
61and stylesheets may already be available. If you are looking at a
62documentation format, <a href="http://www.docbook.org/">DocBook</a> should
63handle your needs. If reuse is not possible because some semantic or use case
64aspects are too differents this will be helpful avoiding design errors like
65targetting the vocabulary to the wrong abstraction level. In this format
66design phase try to be synthetic and be sure to express the real content of
67your data and use the XML structure to express the semantic and context of
68those data.</p>
69
Daniel Veillard6943a4d2002-12-28 18:07:59 +0000[diff] [blame]70<h3>DTD rules:</h3>
Daniel Veillard83298842002-12-28 15:12:33 +0000[diff] [blame]71
72<p>Building a DTD (Document Type Definition) or a Schema describing the
73structure allowed by instances is the core of the design process of the
74vocabulary. Here are a few tips:</p>
75<ul>
Daniel Veillardd105eb62003-01-13 16:21:45 +0000[diff] [blame^]76 <li>use significant words for the element and attributes names.</li>
77 <li>do not use attributes for general textual content, attributes
78 will be modified by the parser before reaching the application,
79 spaces and line informations will be modified.</li>
Daniel Veillard83298842002-12-28 15:12:33 +0000[diff] [blame]80 <li>use single elements for every strings which might be subject to
81 localization, the canonical way to localize XML content is to use
82 siblings element carrying different xml:lang attributes like in the
83 following:
84 <pre>&lt;welcome&gt;
85 &lt;msg xml:lang="en"&gt;hello&lt;/msg&gt;
86 &lt;msg xml:lang="fr"&gt;bonjour&lt;/msg&gt;
87&lt;/welcome&gt;</pre>
88 </li>
89 <li>use attribute to refine the content of an element but avoid them for
90 more complex tasks, attribute parsing is not cheaper than an element and
91 it is far easier to make an element content more complex while attribute
92 will have to remain very simple.</li>
93</ul>
94
95<h3>Versioning:</h3>
96
97<p>As part of the design, make sure the structure you define will be usable
98for future extension that you may not consider for the current version, there
99is 2 parts for this:</p>
100<ul>
101 <li>make sure the instance contains a version number which will allow to
102 make backward compatibility easy, something as simple as having a
103 <code>version="1.0"</code> on the root document of the instance is
104 sufficient</li>
105 <li>while designing the code doing the analysis of the data provided by the
106 XML parser, make sure you can work with unknown versions, generate a UI
107 warning and process only the tags recognized by your version but keep in
108 mind that you should not break on unknown elements if the version
109 attribute was not in the recognized set.</li>
110</ul>
111
Daniel Veillard6943a4d2002-12-28 18:07:59 +0000[diff] [blame]112<h3>Other design parts:</h3>
Daniel Veillard83298842002-12-28 15:12:33 +0000[diff] [blame]113
114<p>While defining you vocabulary, try to think in term of other usage to your
115data, for example how using XSLT stylesheets could be used to make an HTML
116view of your data, or to convert it into a different format. Checking XML
117Schemas and looking at defining an XML Schemas with a more complete
118validation and datatyping of your data structures are important, this helps
119avoiding some mistakes in the design phase.</p>
120
121<h3>Namespace:</h3>
122
123<p>If you expect your XML vocabulary to be used or recognized outside of your
124application (for example binding a specific processing from a graphic shell
125like Nautilus to instance of your data) then you should really define an <a
126href="http://www.w3.org/TR/REC-xml-names/">XML namespace</a> for your
127vocabulary. A namespace name is an URL (absolute URI more precisely), it is
128generally recommended to anchor it as an HTTP resource to a server associated
129with the software project, see the next section about this. In practice this
130will mean that XML parsers will not handle your element names as-is but as a
131couple based on the namespace name and the element name. This allow to
132recognize and disambiguate processing. Unicity of the namespace name can be
133for the most part garanteed by the use of the DNS registry. Namespace can
134also be used to carry versionning informations like:</p>
135
136<p><code>"http://www.gnome.org/project/projectname/1.0/"</code></p>
137
138<p>an an easy way to use them is to make them the default namespace on the
139root element of the XML instance like:</p>
140<pre>&lt;structure xmlns="http://www.gnome.org/project/projectname/1.0/"&gt;
141 &lt;data&gt;
142 ...
143 &lt;/data&gt;
144&lt;/structure&gt;</pre>
145
146<p>In that document, structure and all descendant elements like data are in
147the given namespace.</p>
148
149<h2><a name="Canonical">Canonical URL</a></h2>
150
151<p>As seen in the previous namespace section, while XML processing is not
152tied to the Web there is a natural synergy between both, XML was designed to
153be available on the Web, and keeping the infrastructure that way helps
154deploying the XML resources. The core of this issue is the notion of
155"Canonical URL" of an XML resource, the resource can be an XML document, a
156DTD, a stylesheet, a schemas, or even non-XML data associated to an XML
157resource, the canonical URL is the URL where the "master" copy of that
158resource is expected to be present on the Web. Usually when processing XML a
159copy of the resource will be present on the local disk, maybe in
160/usr/share/xml or /usr/share/sgml maybe in /opt or even on C:\projectname\
161(horror !), the key point is that the way to name that resource should be
162independant of the actual place where it reside on disk if it is available,
163and the fact that the processing will still work if there is no local copy
164(and that the machine where the processing is connected to the Internet).</p>
165
166<p>What this really mean is that one should never use the local name of a
167resource to reference it but always use the canonical URL. For example in a
168DocBook instance the following should not be used:</p>
169<pre>&lt;!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"<br>
Daniel Veillard6943a4d2002-12-28 18:07:59 +0000[diff] [blame]170
171
Daniel Veillard83298842002-12-28 15:12:33 +0000[diff] [blame]172 "/usr/share/xml/docbook/4.2/docbookx.dtd"&gt;</pre>
173
174<p>But always reference the canonical URL for the DTD:</p>
175<pre>&lt;!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"<br>
Daniel Veillard6943a4d2002-12-28 18:07:59 +0000[diff] [blame]176
177
Daniel Veillard83298842002-12-28 15:12:33 +0000[diff] [blame]178 "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"&gt; </pre>
179
180<p>Similary, the document instance may reference the <a
181href="http://www.w3.org/TR/xslt">XSLT</a> stylesheets needed to process it to
182generate HTML, and the canonical URL should be used:</p>
183<pre>&lt;?xml-stylesheet
184 href="http://docbook.sourceforge.net/release/xsl/current/html/docbook.xsl"
185 type="text/xsl"?&gt;</pre>
186
Daniel Veillard6943a4d2002-12-28 18:07:59 +0000[diff] [blame]187<p>Defining the canonical URL for the resources needed should obey a few
Daniel Veillard83298842002-12-28 15:12:33 +0000[diff] [blame]188simple rules similar to those used to design namespace names:</p>
189<ul>
190 <li>use a DNS name you know is associated to the project and will be
191 available on the long term</li>
192 <li>whithin that server space, reserve the right to the subtree where you
193 intend to keep those data</li>
194 <li>version the URL so that multiple concurent versions of the resources
195 can be hosted simultaneously</li>
196</ul>
197
198<h2><a name="Catalog">Catalog setup</a></h2>
199
200<h3>How catalog works:</h3>
201
202<p>The catalogs are the technical mechanism which allow the XML processing
203tools to use a local copy of the resources if it is available even if the
204instance document references the canonical URL. <a
205href="http://www.oasis-open.org/committees/entity/">XML Catalogs</a> are
206anchored in the root catalog (usually <code>/etc/xml/catalog</code> or
207defined by the user). They are a tree of XML documents defining the mappings
208between the canonical naming space and the local installed ones, this can be
209seen as a static cache structure.</p>
210
211<p>When the XML processor is asked to process a resource it will
212automatically test for a locally available version in the catalog, starting
213from the root catalog, and possibly fetching sub-catalog resources until it
214finds that the catalog has that resource or not. If not the default
215processing of fetching the resource from the Web is done, allowing in most
216case to recover from a catalog miss. The key point is that the document
217instances are totally independant of the availability of a catalog or from
218the actual place where the loacl resource they reference may be installed.
219This greatly improve the management of the document in the long run, making
Daniel Veillard6943a4d2002-12-28 18:07:59 +0000[diff] [blame]220them independant of the platform or toolchain used to process them. The
221figure below tries to express that mechanism:<img src="catalog.gif"
222alt="Picture describing the catalog "></p>
Daniel Veillard83298842002-12-28 15:12:33 +0000[diff] [blame]223
224<h3>Usual catalog setup:</h3>
225
226<p>Usually catalogs for a project are setup as a 2 level hierarchical cache,
227the root catalog containing only "delegates" indicating a separate subcatalog
228dedicated to the project. The goal is to keep the root catalog clean and
229simplify the maintainance of the catalog by using separate catalogs per
230project. For example when creating a catalog for the <a
231href="http://www.w3.org/TR/xhtml1">XHTML1</a> DTDs, only 3 items are added to
232the root catalog:</p>
233<pre> &lt;delegatePublic publicIdStartString="-//W3C//DTD XHTML 1.0"
234 catalog="file:///usr/share/sgml/xhtml1/xmlcatalog"/&gt;
235 &lt;delegateSystem systemIdStartString="http://www.w3.org/TR/xhtml1/DTD"
236 catalog="file:///usr/share/sgml/xhtml1/xmlcatalog"/&gt;
237 &lt;delegateURI uriStartString="http://www.w3.org/TR/xhtml1/DTD"
238 catalog="file:///usr/share/sgml/xhtml1/xmlcatalog"/&gt;</pre>
239
240<p>They are all "delegates" meaning that if the catalog system is asked to
241resolve a reference corresponding to them, it has to lookup a sub catalog.
242Here the subcatalog was installed as
243<code>/usr/share/sgml/xhtml1/xmlcatalog</code> in the local tree, that
244decision is left to the sysadmin or the packager for that system and may
245obbey different rules, but the actual place on the filesystem (or on a
246resource cache on the local network) will not influence the processing as
247long as it is available. The first rule indicate that if the reference uses a
Daniel Veillard6943a4d2002-12-28 18:07:59 +0000[diff] [blame]248PUBLIC identifier beginning with the</p>
Daniel Veillard83298842002-12-28 15:12:33 +0000[diff] [blame]249
250<p><code>"-//W3C//DTD XHTML 1.0"</code></p>
251
252<p>substring, then the catalog lookup should be limited to the specific given
253lookup catalog. Similary the second and third entries indicate those
254delegation rules for SYSTEM, DOCTYPE or normal URI references when the URL
255starts with the <code>"http://www.w3.org/TR/xhtml1/DTD"</code> subscting
256which indicates the location on the W3C server where the XHTML1 resources are
257stored, those are the beginning of all Canonical URLs for XHTML1 resources.
258Those 3 rules are sufficient in practice to capture all references to XHTML1
259resources and direct the processing tools to the right subcatalog.</p>
260
261<h3>A subcatalog example:</h3>
262
263<p>Here is the complete subcatalog used for XHTML1:</p>
264<pre>&lt;?xml version="1.0"?&gt;
265&lt;!DOCTYPE catalog PUBLIC "-//OASIS//DTD Entity Resolution XML Catalog V1.0//EN"
266 "http://www.oasis-open.org/committees/entity/release/1.0/catalog.dtd"&gt;
267&lt;catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog"&gt;
268 &lt;public publicId="-//W3C//DTD XHTML 1.0 Strict//EN"
269 uri="xhtml1-20020801/DTD/xhtml1-strict.dtd"/&gt;
270 &lt;public publicId="-//W3C//DTD XHTML 1.0 Transitional//EN"
271 uri="xhtml1-20020801/DTD/xhtml1-transitional.dtd"/&gt;
272 &lt;public publicId="-//W3C//DTD XHTML 1.0 Frameset//EN"
273 uri="xhtml1-20020801/DTD/xhtml1-frameset.dtd"/&gt;
274 &lt;rewriteSystem systemIdStartString="http://www.w3.org/TR/xhtml1/DTD"
275 rewritePrefix="xhtml1-20020801/DTD"/&gt;
276 &lt;rewriteURI uriStartString="http://www.w3.org/TR/xhtml1/DTD"
277 rewritePrefix="xhtml1-20020801/DTD"/&gt;
Daniel Veillard6943a4d2002-12-28 18:07:59 +0000[diff] [blame]278&lt;/catalog&gt;</pre>
Daniel Veillard83298842002-12-28 15:12:33 +0000[diff] [blame]279
280<p>There is a few things to notice:</p>
281<ul>
282 <li>this is an XML resource, it points to the DTD using Canonical URLs, the
283 root element defines a namespace (but based on an URN not an HTTP
284 URL).</li>
285 <li>it contains 5 rules, the 3 first ones are direct mapping for the 3
286 PUBLIC identifiers defined by the XHTML1 specification and associating
287 them with the local resource containing the DTD, the 2 last ones are
288 rewrite rules allowing to build the local filename for any URL based on
289 "http://www.w3.org/TR/xhtml1/DTD", the local cache simplify the rules by
290 keeping the same structure as the on-line server at the Canonical URL</li>
291 <li>the local resources are designated using URI references (the uri or
292 rewritePrefix attributes), the base being the containing sub-catalog URL,
293 which means that in practice the copy of the XHTML1 strict DTD is stored
294 locally in
295 <code>/usr/share/sgml/xhtml1/xmlcatalog/xhtml1-20020801/DTD/xhtml1-strict.dtd</code></li>
296</ul>
297
298<p>Those 5 rules are sufficient to cover all references to the resources held
299at the Canonical URL for the XHTML1 DTDs.</p>
300
301<h2><a name="Package">Package integration</a></h2>
302
303<p>Creating and removing catalogs should be handled as part of the process of
304(un)installing the local copy of the resources. The catalog files being XML
305resources should be processed with XML based tools to avoid problems with the
306generated files, the xmlcatalog command coming with libxml2 allows to create
307catalogs, and add or remove rules at that time. Here is a complete example
Daniel Veillard6943a4d2002-12-28 18:07:59 +0000[diff] [blame]308coming from RPM for the XHTML1 DTDs post install script. While this example
309is platform and packaging specific, this can be useful as a an example in
310other contexts:</p>
Daniel Veillard83298842002-12-28 15:12:33 +0000[diff] [blame]311<pre>%post
312CATALOG=/usr/share/sgml/xhtml1/xmlcatalog
313#
314# Register it in the super catalog with the appropriate delegates
315#
316ROOTCATALOG=/etc/xml/catalog
317
318if [ ! -r $ROOTCATALOG ]
319then
320 /usr/bin/xmlcatalog --noout --create $ROOTCATALOG
321fi
322
323if [ -w $ROOTCATALOG ]
324then
325 /usr/bin/xmlcatalog --noout --add "delegatePublic" \
326 "-//W3C//DTD XHTML 1.0" \
327 "file://$CATALOG" $ROOTCATALOG
328 /usr/bin/xmlcatalog --noout --add "delegateSystem" \
329 "http://www.w3.org/TR/xhtml1/DTD" \
330 "file://$CATALOG" $ROOTCATALOG
331 /usr/bin/xmlcatalog --noout --add "delegateURI" \
332 "http://www.w3.org/TR/xhtml1/DTD" \
333 "file://$CATALOG" $ROOTCATALOG
334fi</pre>
335
336<p>The XHTML1 subcatalog is not created on-the-fly in that case, it is
337installed as part of the files of the packages. So the only work needed is to
338make sure the root catalog exists and register the delegate rules.</p>
339
340<p>Similary, the script for the post-uninstall just remove the rules from the
341catalog:</p>
342<pre>%postun
343#
344# On removal, unregister the xmlcatalog from the supercatalog
345#
346if [ "$1" = 0 ]; then
347 CATALOG=/usr/share/sgml/xhtml1/xmlcatalog
348 ROOTCATALOG=/etc/xml/catalog
349
350 if [ -w $ROOTCATALOG ]
351 then
352 /usr/bin/xmlcatalog --noout --del \
353 "-//W3C//DTD XHTML 1.0" $ROOTCATALOG
354 /usr/bin/xmlcatalog --noout --del \
355 "http://www.w3.org/TR/xhtml1/DTD" $ROOTCATALOG
356 /usr/bin/xmlcatalog --noout --del \
357 "http://www.w3.org/TR/xhtml1/DTD" $ROOTCATALOG
358 fi
359fi</pre>
360
361<p>Note the test against $1, this is needed to not remove the delegate rules
362in case of upgrade of the package.</p>
363
364<p>Following the set of guidelines and tips provided in this document should
365help deploy the XML resources in the GNOME framework without much pain and
366ensure a smooth evolution of the resource and instances.</p>
367
368<p><a href="mailto:veillard@redhat.com">Daniel Veillard</a></p>
369
370<p>$Id$</p>
371
Daniel Veillard6943a4d2002-12-28 18:07:59 +0000[diff] [blame]372<p></p>
Daniel Veillard83298842002-12-28 15:12:33 +0000[diff] [blame]373</body>
374</html>