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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"> 6: <style type="text/css"></style> 7:  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 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: guidelines.html,v 1.1.1.1 2012/02/21 23:37:59 misho Exp $</p> 371: 372: <p></p> 373: </body> 374: </html>