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16: <title>XML resources publication guidelines</title>
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18:
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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
25: helping the publication and deployment of <a
26: href="http://www.w3.org/XML/">XML</a> resources for the <a
27: href="http://www.gnome.org/">GNOME project</a>. However it is not tied to
28: GNOME and might be helpful more generally. I welcome <a
29: href="mailto:veillard@redhat.com">feedback</a> on this document.</p>
30:
31: <p>The intended audience is the software developers who started using XML
32: for some of the resources of their project, as a storage format, for data
33: exchange, checking or transformations. There have been an increasing number
34: of new XML formats defined, but not all steps have been taken, possibly because of
35: lack of documentation, to truly gain all the benefits of the use of XML.
36: These guidelines hope to improve the matter and provide a better overview of
37: the overall XML processing and associated steps needed to deploy it
38: successfully:</p>
39:
40: <p>Table of contents:</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 intends to focus on the format itself of XML. It may arrive
51: a bit too late since the structure of the document may already be cast in
52: existing and deployed code. Still, here are a few rules which might be helpful
53: when designing a new XML vocabulary or making the revision of an existing
54: format:</p>
55:
56: <h3>Reuse existing formats:</h3>
57:
58: <p>This may sounds a bit simplistic, but before designing your own format,
59: try to lookup existing XML vocabularies on similar data. Ideally this allows
60: you to reuse them, in which case a lot of the existing tools like DTD, schemas
61: and stylesheets may already be available. If you are looking at a
62: documentation format, <a href="http://www.docbook.org/">DocBook</a> should
63: handle your needs. If reuse is not possible because some semantic or use case
64: aspects are too different this will be helpful avoiding design errors like
65: targeting the vocabulary to the wrong abstraction level. In this format
66: design phase try to be synthetic and be sure to express the real content of
67: your data and use the XML structure to express the semantic and context of
68: those data.</p>
69:
70: <h3>DTD rules:</h3>
71:
72: <p>Building a DTD (Document Type Definition) or a Schema describing the
73: structure allowed by instances is the core of the design process of the
74: vocabulary. Here are a few tips:</p>
75: <ul>
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>
80: <li>use single elements for every string that 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><welcome>
85: <msg xml:lang="en">hello</msg>
86: <msg xml:lang="fr">bonjour</msg>
87: </welcome></pre>
88: </li>
89: <li>use attributes 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
98: for future extension that you may not consider for the current version. There
99: are two parts to 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:
112: <h3>Other design parts:</h3>
113:
114: <p>While defining you vocabulary, try to think in term of other usage of your
115: data, for example how using XSLT stylesheets could be used to make an HTML
116: view of your data, or to convert it into a different format. Checking XML
117: Schemas and looking at defining an XML Schema with a more complete
118: validation and datatyping of your data structures is important, this helps
119: avoiding 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
124: application (for example binding a specific processing from a graphic shell
125: like Nautilus to an instance of your data) then you should really define an <a
126: href="http://www.w3.org/TR/REC-xml-names/">XML namespace</a> for your
127: vocabulary. A namespace name is an URL (absolute URI more precisely). It is
128: generally recommended to anchor it as an HTTP resource to a server associated
129: with the software project. See the next section about this. In practice this
130: will mean that XML parsers will not handle your element names as-is but as a
131: couple based on the namespace name and the element name. This allows it to
132: recognize and disambiguate processing. Unicity of the namespace name can be
133: for the most part guaranteed by the use of the DNS registry. Namespace can
134: also be used to carry versioning information like:</p>
135:
136: <p><code>"http://www.gnome.org/project/projectname/1.0/"</code></p>
137:
138: <p>An easy way to use them is to make them the default namespace on the
139: root element of the XML instance like:</p>
140: <pre><structure xmlns="http://www.gnome.org/project/projectname/1.0/">
141: <data>
142: ...
143: </data>
144: </structure></pre>
145:
146: <p>In that document, structure and all descendant elements like data are in
147: the 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
152: tied to the Web there is a natural synergy between both. XML was designed to
153: be available on the Web, and keeping the infrastructure that way helps
154: deploying 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
156: DTD, a stylesheet, a schema, or even non-XML data associated with an XML
157: resource, the canonical URL is the URL where the "master" copy of that
158: resource is expected to be present on the Web. Usually when processing XML a
159: copy 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
162: independent of the actual place where it resides on disk if it is available,
163: and 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 means is that one should never use the local name of a
167: resource to reference it but always use the canonical URL. For example in a
168: DocBook instance the following should not be used:</p>
169: <pre><!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"<br>
170:
171:
172: "/usr/share/xml/docbook/4.2/docbookx.dtd"></pre>
173:
174: <p>But always reference the canonical URL for the DTD:</p>
175: <pre><!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"<br>
176:
177:
178: "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> </pre>
179:
180: <p>Similarly, the document instance may reference the <a
181: href="http://www.w3.org/TR/xslt">XSLT</a> stylesheets needed to process it to
182: generate HTML, and the canonical URL should be used:</p>
183: <pre><?xml-stylesheet
184: href="http://docbook.sourceforge.net/release/xsl/current/html/docbook.xsl"
185: type="text/xsl"?></pre>
186:
187: <p>Defining the canonical URL for the resources needed should obey a few
188: simple 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>within 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 concurrent 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 catalogs work:</h3>
201:
202: <p>The catalogs are the technical mechanism which allow the XML processing
203: tools to use a local copy of the resources if it is available even if the
204: instance document references the canonical URL. <a
205: href="http://www.oasis-open.org/committees/entity/">XML Catalogs</a> are
206: anchored in the root catalog (usually <code>/etc/xml/catalog</code> or
207: defined by the user). They are a tree of XML documents defining the mappings
208: between the canonical naming space and the local installed ones, this can be
209: seen as a static cache structure.</p>
210:
211: <p>When the XML processor is asked to process a resource it will
212: automatically test for a locally available version in the catalog, starting
213: from the root catalog, and possibly fetching sub-catalog resources until it
214: finds that the catalog has that resource or not. If not the default
215: processing of fetching the resource from the Web is done, allowing in most
216: case to recover from a catalog miss. The key point is that the document
217: instances are totally independent of the availability of a catalog or from
218: the actual place where the local resource they reference may be installed.
219: This greatly improves the management of the documents in the long run, making
220: them independent of the platform or toolchain used to process them. The
221: figure below tries to express that mechanism:<img src="catalog.gif"
222: alt="Picture describing the catalog "></p>
223:
224: <h3>Usual catalog setup:</h3>
225:
226: <p>Usually catalogs for a project are setup as a 2 level hierarchical cache,
227: the root catalog containing only "delegates" indicating a separate subcatalog
228: dedicated to the project. The goal is to keep the root catalog clean and
229: simplify the maintenance of the catalog by using separate catalogs per
230: project. For example when creating a catalog for the <a
231: href="http://www.w3.org/TR/xhtml1">XHTML1</a> DTDs, only 3 items are added to
232: the root catalog:</p>
233: <pre> <delegatePublic publicIdStartString="-//W3C//DTD XHTML 1.0"
234: catalog="file:///usr/share/sgml/xhtml1/xmlcatalog"/>
235: <delegateSystem systemIdStartString="http://www.w3.org/TR/xhtml1/DTD"
236: catalog="file:///usr/share/sgml/xhtml1/xmlcatalog"/>
237: <delegateURI uriStartString="http://www.w3.org/TR/xhtml1/DTD"
238: catalog="file:///usr/share/sgml/xhtml1/xmlcatalog"/></pre>
239:
240: <p>They are all "delegates" meaning that if the catalog system is asked to
241: resolve a reference corresponding to them, it has to lookup a sub catalog.
242: Here the subcatalog was installed as
243: <code>/usr/share/sgml/xhtml1/xmlcatalog</code> in the local tree. That
244: decision is left to the sysadmin or the packager for that system and may
245: obey different rules, but the actual place on the filesystem (or on a
246: resource cache on the local network) will not influence the processing as
247: long as it is available. The first rule indicate that if the reference uses a
248: PUBLIC identifier beginning with the</p>
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
253: lookup catalog. Similarly the second and third entries indicate those
254: delegation rules for SYSTEM, DOCTYPE or normal URI references when the URL
255: starts with the <code>"http://www.w3.org/TR/xhtml1/DTD"</code> substring
256: which indicates the location on the W3C server where the XHTML1 resources are
257: stored. Those are the beginning of all Canonical URLs for XHTML1 resources.
258: Those three rules are sufficient in practice to capture all references to XHTML1
259: resources 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><?xml version="1.0"?>
265: <!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">
267: <catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog">
268: <public publicId="-//W3C//DTD XHTML 1.0 Strict//EN"
269: uri="xhtml1-20020801/DTD/xhtml1-strict.dtd"/>
270: <public publicId="-//W3C//DTD XHTML 1.0 Transitional//EN"
271: uri="xhtml1-20020801/DTD/xhtml1-transitional.dtd"/>
272: <public publicId="-//W3C//DTD XHTML 1.0 Frameset//EN"
273: uri="xhtml1-20020801/DTD/xhtml1-frameset.dtd"/>
274: <rewriteSystem systemIdStartString="http://www.w3.org/TR/xhtml1/DTD"
275: rewritePrefix="xhtml1-20020801/DTD"/>
276: <rewriteURI uriStartString="http://www.w3.org/TR/xhtml1/DTD"
277: rewritePrefix="xhtml1-20020801/DTD"/>
278: </catalog></pre>
279:
280: <p>There are 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 simplifies 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
299: at 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
305: resources should be processed with XML based tools to avoid problems with the
306: generated files, the xmlcatalog command coming with libxml2 allows you to create
307: catalogs, and add or remove rules at that time. Here is a complete example
308: coming from the RPM for the XHTML1 DTDs post install script. While this example
309: is platform and packaging specific, this can be useful as a an example in
310: other contexts:</p>
311: <pre>%post
312: CATALOG=/usr/share/sgml/xhtml1/xmlcatalog
313: #
314: # Register it in the super catalog with the appropriate delegates
315: #
316: ROOTCATALOG=/etc/xml/catalog
317:
318: if [ ! -r $ROOTCATALOG ]
319: then
320: /usr/bin/xmlcatalog --noout --create $ROOTCATALOG
321: fi
322:
323: if [ -w $ROOTCATALOG ]
324: then
325: /usr/bin/xmlcatalog --noout --add "delegatePublic" \
326: "-//W3C//DTD XHTML 1.0" \
327: "file://$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
334: fi</pre>
335:
336: <p>The XHTML1 subcatalog is not created on-the-fly in that case, it is
337: installed as part of the files of the packages. So the only work needed is to
338: make sure the root catalog exists and register the delegate rules.</p>
339:
340: <p>Similarly, the script for the post-uninstall just remove the rules from the
341: catalog:</p>
342: <pre>%postun
343: #
344: # On removal, unregister the xmlcatalog from the supercatalog
345: #
346: if [ "$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
359: fi</pre>
360:
361: <p>Note the test against $1, this is needed to not remove the delegate rules
362: in case of upgrade of the package.</p>
363:
364: <p>Following the set of guidelines and tips provided in this document should
365: help deploy the XML resources in the GNOME framework without much pain and
366: ensure a smooth evolution of the resource and instances.</p>
367:
368: <p><a href="mailto:veillard@redhat.com">Daniel Veillard</a></p>
369:
370: <p>$Id$</p>
371:
372: <p></p>
373: </body>
374: </html>
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