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      <title>Harvesting RDF Statements from XLinks</title>
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      <div class="head">
         <p><a href="http://www.w3.org/"><img src="http://www.w3.org/Icons/w3c_home" alt="W3C" height="48" width="72"></a></p>
         <h1>Harvesting RDF Statements from XLinks</h1>
         <h2>W3C Note 29 September 2000</h2>
         <dl>
            <dt>This version:</dt>
            <dd><a href="http://www.w3.org/TR/2000/NOTE-xlink2rdf-20000929/">http://www.w3.org/TR/2000/NOTE-xlink2rdf-20000929/</a> (available as <a href="Overview.xml">XML</a> and <a href="Overview.html">HTML</a>)
            </dd>
            <dt>Latest version:</dt>
            <dd><a href="http://www.w3.org/TR/xlink2rdf">http://www.w3.org/TR/xlink2rdf</a>
            </dd>
            <dt>Editor:</dt>

            <dd>
               Ron Daniel Jr.
                (Metacode Technologies Inc.)
                <a href="mailto:rdaniel@metacode.com">&lt;rdaniel@metacode.com></a>

            </dd>

         </dl>
         <p class="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a>&nbsp;&copy;&nbsp;2000&nbsp;<a href="http://www.w3.org/"><abbr title="World Wide Web Consortium">W3C</abbr></a><sup>&reg;</sup> (<a href="http://www.lcs.mit.edu/"><abbr title="Massachusetts Institute of Technology">MIT</abbr></a>, <a href="http://www.inria.fr/"><abbr lang="fr" title="Institut National de Recherche en Informatique et Automatique">INRIA</abbr></a>, <a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>, <a href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>, <a href="http://www.w3.org/Consortium/Legal/copyright-documents-19990405">document use</a>, and <a href="http://www.w3.org/Consortium/Legal/copyright-software-19980720">software licensing</a> rules apply.
         </p>
      </div>
      <hr>
      <div>
         <h2><a name="abstract">Abstract</a></h2>

         <p>Both XLink <a href="#XLink">[XLink]</a> and RDF <a href="#rdf">[RDF]</a> provide a way
            of asserting relations between resources. RDF is primarily for describing
            resources and their relations, while XLink is primarily for specifying and
            traversing hyperlinks. However, the overlap between the two is sufficient
            that a mapping from XLink links to statements in an RDF model can be defined.
            Such a mapping allows XLink elements to be harvested as a source of RDF statements.
            XLink links (hereafter, "links") thus provide an alternate syntax
            for RDF information that may be useful in some situations.
         </p>

         <p>This Note specifies such a mapping, so that links can be harvested and
            RDF statements generated. The purpose of this harvesting is to create RDF
            models that, in some sense, represent the <em>intent</em> of the XML
            document.
            The purpose is <em>not</em> to represent the XLink structure in enough
            detail that a set of links could be round-tripped through an RDF model.
         </p>

      </div>
      <div>
         <h2><a name="status">Status of This Document</a></h2>

         <p>This Note is made available by the W3C
<a href="http://www.w3.org/XML/Activity#linking-wg">XML Linking Working Group</a>
            for the consideration of the XLink and RDF communities in the hopes that
            it may prove useful. However, it is not a formal product of the XML
            Linking Working Group. Thus, it should not be construed as representing
            the consensus of the XML Linking Working Group. Comments should be sent
	    to the public mailing-list <a href="mailto:www-rdf-interest@w3.org"
	    >www-rdf-interest@w3.org</a>(<a
	    href="http://lists.w3.org/Archives/Public/www-rdf-interest/"
	    >archive</a>). However, readers are advised that the XML Linking Working
            Group has no plans to update this document.

         </p>

         <p>Publication of this Note by W3C indicates no endorsement by
            W3C or the W3C Team, or any W3C Members.
         </p>
	 <p>A list of <a href="http://www.w3.org/TR">current W3C Recommendations and
	 other technical documents</a> is available.</p>

      </div>

      <div class="toc">
         <h2><a name="contents">Table of Contents</a></h2>
         <p class="toc">1 <a href="#introduction">Introduction</a><br>&nbsp;&nbsp;&nbsp;&nbsp;1.1 <a href="#terminology">Terminology</a><br>&nbsp;&nbsp;&nbsp;&nbsp;1.2 <a href="#b2d248b3b1c17">Notation and Document Conventions</a><br>2 <a href="#principles">Principles of the Mapping</a><br>3 <a href="#spec">Mapping Specification</a><br>&nbsp;&nbsp;&nbsp;&nbsp;3.1 <a href="#xptr_synth">Synthesizing XPointers</a><br>&nbsp;&nbsp;&nbsp;&nbsp;3.2 <a href="#predicates">Generating RDF Predicates</a><br>&nbsp;&nbsp;&nbsp;&nbsp;3.3 <a href="#simple_links">Simple Linking Elements</a><br>&nbsp;&nbsp;&nbsp;&nbsp;3.4 <a href="#extended_links">Extended XML Links</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;3.4.1 <a href="#arcs">arc-Type Element</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;3.4.2 <a href="#locators">locator-Type Element</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;3.4.3 <a href="#resources">resource-Type Element</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;3.4.4 <a href="#titles">title-Type Element</a><br>&nbsp;&nbsp;&nbsp;&nbsp;3.5 <a href="#linkbases">Linkbases</a><br>4 <a href="#bibliography">References</a><br></p>
         <h3>Appendix</h3>
         <p class="toc">A <a href="#xslt">Implementing the Harvesting in XSLT</a><br></p>
      </div>
      <hr>
      <div class="body">

         <div class="div1">

            <h2><a name="introduction">1 Introduction</a></h2>

            <p>The XLink specification <a href="#XLink">[XLink]</a> defines ways for
               XML documents to establish hyperlinks between resources. The Resource
               Description Framework specification <a href="#rdf">[RDF]</a> defines a
               framework for the provision of machine-understandable information
               about web resources.
            </p>

            <p>Both XLink and RDF provide a way of asserting relations between resources.
               RDF is primarily for describing resources and their relations, while XLink
               is primarily for specifying and traversing hyperlinks. However, the overlap
               between the two is sufficient that a mapping from XLink links to statements
               in an RDF model can be defined. Such a mapping allows XLink elements to be <a href="#harvest" title="Harvesting">harvested</a> as a source of RDF statements. XLink links
               (hereafter, "links") thus provide an alternate syntax for RDF
               information that may be useful in some situations.
            </p>

            <p>This Note specifies such a mapping, so that links can be harvested
               and RDF statements generated. The purpose of this harvesting is to
               create RDF models that, in some sense, represent the <em>intent</em>
               of the XML document.  The purpose is <em>not</em> to represent the
               XLink structure in enough detail that a set of links could be
               round-tripped through an RDF model.
            </p>

            <p>Readers of this Note are assumed to be familiar with
               <a href="#XLink">[XLink]</a> and <a href="#rdf">[RDF]</a>. Terms that are defined
               in those specifications will not be defined here. Readers should also
               be familiar with XML Base <a href="#xbase">[XML Base]</a>.
               Familiarity with the RDF Schema Candidate Recommendation
               <a href="#rdfs">[RDFSchema]</a> will be necessary for those who wish to make
               use of the mappings provided here that use RDF Schema Classes.
            </p>


            <div class="div2">

               <h3><a name="terminology">1.1 Terminology</a></h3>

               <p>[Definition: <a name="dt-must" title="Must, May, etc.">The key words</a> <b>must</b>, <b>
                        must not
                     </b>, <b>required</b>, <b>shall</b>, <b>shall
                        not
                     </b>, <b>should</b>, <b>should
                        not
                     </b>, <b>recommended</b>, <b>may</b>, and
                     <b>optional</b> in this specification are to be interpreted
                     as described in <a href="#rfc2119">[IETF RFC 2119]</a>.
                  ]
               </p>

               <p>Some special terms are defined here in order to clarify
                  their relationship to similar terms used in the technologies
                  on which the mapping is based. Refer to <a href="#XLink">[XLink]</a> and
                  <a href="#rdf">[RDF]</a> for definitions of other technical
                  terms used here.
               </p>

               <dl>

                  <dt class="label">[Definition: <a name="harvest" title="Harvesting">harvesting</a>]
                  </dt>

                  <dd>

                     <p>The process of generating RDF statements from XLink elements.</p>

                  </dd>

                  <dt class="label">[Definition: <a name="resource" title="resource">Resource</a>]
                  </dt>

                  <dd>

                     <p>A "resource" is anything identified by a URI.</p>

                  </dd>

                  <dt class="label">[Definition: <a name="participating_resource" title="participating resource">Participating resource</a>]
                  </dt>

                  <dd>

                     <p>A resource that has been identified in a link to serve as a
                        potential starting or ending point of traversal.
                     </p>

                  </dd>

               </dl>

            </div>


            <div class="div2">

               <h3><a name="b2d248b3b1c17">1.2 Notation and Document Conventions</a></h3>

               <p>The <code>xlink:</code> and <code>rdf:</code> prefixes are used throughout
                  to stand for the declaration of the XLink and RDF namespaces, respectively,
                  on elements in whose scope the so-marked element or attribute appears (on
                  the same element or on some ancestor element), whether or not a namespace
                  declaration is present in the example. The use of specific namespace prefixes
                  is an editorial convienience; as dictated by the Names in XML
                  Recommendation <a href="#XML-Names">[XML-Names]</a>, any prefix
                  <a href="#dt-must" title="Must, May, etc.">may</a> be used
                  as long as the URI it maps to is the correct one.
               </p>

            </div>


         </div>

         <div class="div1">

            <h2><a name="principles">2 Principles of the Mapping</a></h2>

            <p>Simple RDF statements are comprised of a subject, a predicate, and an object.
               The subject and predicate are identified by URI references, and
               the object may be a URI reference or a literal string. To map an
               XLink link into an RDF statement, we need
               to be able to determine the URI references of the subject and predicate. We must also
               be able to determine the object, be it a URI reference or a literal.
            </p>

            <p>The general principle behind the mapping specified here is that each arc
               in a link gives rise to one RDF statement. The starting resource of the arc
               is mapped to the subject of the RDF statement. The ending resource of the
               arc is mapped to the object of the RDF statement. The arc role is mapped to
               the predicate of the RDF statement. However, a number of corner cases arise,
               described in <a href="#spec"><b>3 Mapping Specification</b></a>.
            </p>

            <p>RDF statements are typically collected together into "models."
               The details of how models are structured are implementation dependent. This
               Note assumes that harvested statements are added to "the current
               model," which is the model being constructed when the
               statement was harvested.
               But this Note, like <a href="#rdfs">[RDFSchema]</a>, does not specify exactly how models
               must be structured.
            </p>

         </div>


         <div class="div1">

            <h2><a name="spec">3 Mapping Specification</a></h2>

            <p>The following sections describe the mapping in detail.</p>

            <div class="div2">

               <h3><a name="xptr_synth">3.1 Synthesizing XPointers</a></h3>

               <p>RDF is based on the use of URIs for identifying resources. In XLink, the
                  linking element itself (in the case of a simple link) or a subelement of the
                  linking element (in the case of an extended link) often serves as one of the
                  participating resources in the link. This requires that we be able to
                  define URI references that identify those linking elements.
                  Those URI references
                  <a href="#dt-must" title="Must, May, etc.">must</a> follow the XPointer specification.

               </p>

               <p>Any legal XPointer that identifies the proper element is allowed.
                  However, in order that
                  different implementations harvest equivalent RDF statements from
                  an XLink, the procedure in this section <a href="#dt-must" title="Must, May, etc.">
                     should
                  </a> be used when synthesizing XPointers for such
                  linking elements.
                  The general approach recommended is for the synthesized XPointer
                  to do element-wise navigation down the tree to reach the linking
                  element. The navigation begins at the nearest identified point
                  in the tree.
               </p>

               <p>More formally, the base of the synthesized URI reference
                  <a href="#dt-must" title="Must, May, etc.">shall</a> be specified as defined
                  in <a href="#xbase">[XML Base]</a>.
               </p>

               <p>The fragment identifier of the synthesized URI reference
                  <a href="#dt-must" title="Must, May, etc.">shall</a> be delimited from the
                  URI by the '#' character, as
                  required by RFC 2396<a href="#rfc2396">[RFC 2396]</a>. The fragment identifier
                  of the synthesized URI reference <a href="#dt-must" title="Must, May, etc.">shall</a>
                  be an XPointer<a href="#xptr">[XPTR]</a>.
               </p>

               <p>The XPointer <a href="#dt-must" title="Must, May, etc.">should</a> follow the
                  production:

                  <h5>Recommended Syntax for Synthesized XPointers</h5>
                  <table class="scrap">
                     <tbody>
                        <tr>
                           <td><pre>
     XPointer    ::= Name
                   | ChildSeq

     ChildSeq    ::=  '/' [1-9] [0-9]* ('/' [1-9] [0-9]*)*
                   |  Name ('/' [1-9] [0-9]*)+
</pre></td>
                        </tr>
                     </tbody>
                  </table>

                  <div class="note">
                     <p class="prefix"><b>Note:</b></p>
                     <p>This is an edited version of the
                        <a href="http://www.w3.org/TR/xptr#NT-ChildSeq">ChildSeq</a>
                        production, assuming
                        that the production is updated in line with recent working group
                        discussions. In the case of any differences between this document and
                        the final XPointer specification, the XPointer specification's definition
                        of ChildSeq should be followed.
                     </p>

                  </div>


               <p>The initial locator term
                  of the XPointer <a href="#dt-must" title="Must, May, etc.">should</a> be an ID
                  reference to the nearest ancestor of the linking element, including
                  the linking element itself, that bears an attribute of type
                  <code>ID</code>. If no such attribute exists on any ancestor of
                  the linking element, the '/' character
                  <a href="#dt-must" title="Must, May, etc.">should</a> be the first linking term,
                  indicating that navigation proceeds from the document element.

               </p>

               <p>As an example, consider a document that contains the following
                  simple link:
               </p>

               <table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%">
                  <tr>
                     <td><pre>In heavy trading, &lt;org
  xlink:type='simple'
  xlink:href="http://www.foo.com/"
  xml:base="http://www.bar.com/report1"
  ID="com231"
>Foo Manufacturing&lt;/org> closed sharply lower...</pre></td>
                  </tr>
               </table>

               <p>The synthesized XPointer for this linking element is:</p>

               <table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%">
                  <tr>
                     <td><pre>http://www.bar.com/report1#com231</pre></td>
                  </tr>
               </table>

            </div>


            <div class="div2">

               <h3><a name="predicates">3.2 Generating RDF Predicates</a></h3>

               <p>Unless stated otherwise, RDF statements are generated to represent the
                  information for the arcs in a link. The value of
                  the <code>xlink:arcrole</code> attribute, if one
                  is given on an "arc"-type element or
                  "simple"-type element,
                  <a href="#dt-must" title="Must, May, etc.">must</a> be mapped to
                  the predicate of the RDF statement. Note that the value of
                  the <code>xlink:arcrole</code> attribute is already required,
                  by the XLink specification, to be a URI reference.

               </p>

               <p>If no <code>xlink:arcrole</code> attribute is specified,
                  harvesting software <a href="#dt-must" title="Must, May, etc.">should not</a> generate
                  an RDF statement. That is certainly the safest course. However,
                  many XML files may not have arcrole attributes. Implementations that
                  wish to attempt to harvest RDF statements from such files
                  <a href="#dt-must" title="Must, May, etc.">may</a> map
                  the element type of the linking element to the predicate
                  of the RDF statement, as long as the element type is namespace
                  qualified. This ensures that an an absolute URI reference can
                  be constructed from the namespace
                  URI and the local part. In this case the namespace name and the
                  local part are concatenated to synthesize the absolute URI
                  reference for the predicate. Implementations
                  <a href="#dt-must" title="Must, May, etc.">should</a> examine the namespace
                  URI to test if it ends in one of the URI characters '#', '?', or
                  '/'. If it does then the namespace URI and the local part
                  <a href="#dt-must" title="Must, May, etc.">shall</a> be concatenated using the
                  simple approach documented in <a href="#rdf">[RDF]</a>. If the namespace
                  URI does not end in such a character, implementations
                  <a href="#dt-must" title="Must, May, etc.">may</a> create a URI reference by
                  inserting a '#' character. Note however that such URI references
                  <a href="#dt-must" title="Must, May, etc.">must not</a> be exchanged with external
                  parties, as they are not guaranteed to actually exist.

               </p>

            </div>


            <div class="div2">

               <h3><a name="simple_links">3.3 Simple Linking Elements</a></h3>

               <p>If a simple link's <code>xlink:arcrole</code> attribute has the
                  value "http://www.w3.org/1999/xlink/properties/linkbase",
                  the link <a href="#dt-must" title="Must, May, etc.">shall</a> be harvested according
                  to section <a href="#linkbases"><b>3.5 Linkbases</b></a>.
                  Otherwise the mapping defined in this section
                  <a href="#dt-must" title="Must, May, etc.">shall</a> be used.
               </p>

               <p>All simple links define zero or one traversal arcs. No traversal
                  arc is specified if the <code>xlink:href</code> attribute is not specified.
                  Therefore, harvesting software <a href="#dt-must" title="Must, May, etc.">shall not</a>
                  generate an RDF statements if there is no <code>xlink:href</code> attribute
                  in the link.
               </p>

               <p>The starting resource of the simple link
                  <a href="#dt-must" title="Must, May, etc.">shall</a> be mapped to the
                  subject of the RDF statement. Note that the starting resource of a
                  simple link is the linking element itself. Therefore, the harvesting
                  software <a href="#dt-must" title="Must, May, etc.">must</a> create a URI
                  reference that identifies the linking element, as defined in
                  section <a href="#xptr_synth"><b>3.1 Synthesizing XPointers</b></a>.
               </p>

               <p>The predicate of the RDF statement is obtained from the simple
                  Link as defined in <a href="#predicates"><b>3.2 Generating RDF Predicates</b></a>.

               </p>

               <p>The ending resource of the simple link
                  <a href="#dt-must" title="Must, May, etc.">shall</a> be mapped to the object of
                  the RDF statement. Note that the ending resource
                  of a simple link is always a URI reference, provided as the value of
                  the <code>xlink:href</code> attribute.
               </p>


               <p>If an <code>xlink:role</code> attribute is specified on the simple
                  link, it <a href="#dt-must" title="Must, May, etc.">shall</a> result in an
                  additional statement being added to the model. The subject of
                  the statement is the ending resource of the simple link,
                  its predicate is "<code>rdf:type</code>", and its
                  object is the resource identified by the role attribute.

               </p>

               <p>If an implementation wishes to use facilities defined in the
                  RDF Schema specification <a href="#rdfs">[RDFSchema]</a>, it
                  <a href="#dt-must" title="Must, May, etc.">may</a> add a second statement to the
                  RDF model when an <code>xlink:role</code> attribute is specified.
                  The subject of the second statement is the resource identified by
                  the <code>role</code> attribute, its predicate is
                  "<code>rdf:type</code>", and its object is the
                  resource "rdfs:Class". The second statement
                  <a href="#dt-must" title="Must, May, etc.">should</a> only be added to
                  the model if an equivalent statement is not already part of the
                  model.
               </p>

               <p>An example of a simple linking element is:</p>

               <table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%">
                  <tr>
                     <td><pre>... In a &lt;x:extRef
  xlink:type="simple"
  xlink:href="http://www.foo.com/papers/crops.txt"
  xlink:arcrole="http://links.org/namespace/cite"
  xlink:role="http://links.org/namespace/screed"
>recent paper&lt;/x:extRef>, Dr. Taylor assumes that ...</pre></td>
                  </tr>
               </table>

               <p>Mapping that link according to this specification (and assuming it was
                  the fourth child element within the third child element of the document)
                  results in the RDF model shown below:
               </p>
               <img src="xlink2rdf-fig1.gif" alt="RDF model with 3 arcs">

               <p>If the <code>xlink:role</code> attribute had not been specified, then
                  the result would have been the RDF model shown below:
               </p>
               <img src="xlink2rdf-fig2.gif" alt="RDF model with 1 arc">

            </div>


            <div class="div2">

               <h3><a name="extended_links">3.4 Extended XML Links</a></h3>


               <p>We first describe the rules for harvesting the components of an
                  extended link (arcs, locators, and resources). Then we describe the
                  rules for the extended link as a whole.
               </p>


               <div class="div3">

                  <h4><a name="arcs">3.4.1 <code>arc</code>-Type Element
                     </a>
                  </h4>

                  <p>If an arc contains an <code>xlink:arcrole</code> attribute whose
                     value is "http://www.w3.org/1999/xlink/properties/linkbase",
                     it <a href="#dt-must" title="Must, May, etc.">shall</a> be harvested according to the
                     procedure in section <a href="#linkbases"><b>3.5 Linkbases</b></a>. Otherwise the procedures
                     in this section <a href="#dt-must" title="Must, May, etc.">shall</a> be used.

                  </p>

                  <p>
                     XLink elements of the <code>arc</code> type use the <code>xlink:to</code>
                     and <code> xlink:from</code> attributes to specify the endpoints of
                     zero or more possible traversals by referencing, not URIs, but
                     rather labels that have been defined in the <code>xlink:label</code>
                     attributes of <code>locator</code>-type and <code>resource</code>-type elements.

                  </p>

                  <p>
                     The number of RDF statements harvested from a single <code>arc</code>-type
                     element is equal to the number of possible traversals specified by that
                     element.
                     That quantity is the multiplicative product of the number of resource and/or
                     locator elements identified by the <code>xlink:to</code> and <code>xlink:from

                     </code> attributes. Each RDF statement will correspond to one and only one
                     of the traversals.

                  </p>

                  <p>
                     The starting resources of the traversals
                     <a href="#dt-must" title="Must, May, etc.">shall</a> be mapped to the subject of
                     the RDF statement(s). The ending resources of
                     the traversals <a href="#dt-must" title="Must, May, etc.">shall</a> be mapped to
                     the object of the RDF statement(s). The predicate of the RDF statement
                     is obtained as specified in <a href="#predicates"><b>3.2 Generating RDF Predicates</b></a>.

                  </p>

                  <p>Note that any element content of an arc is not harvested.</p>

               </div>


               <div class="div3">

                  <h4><a name="locators">3.4.2 <code>locator</code>-Type Element
                     </a>
                  </h4>

                  <p>Each XLink <code>locator</code>-type element gives rise to zero or
                     more statements in the RDF model. The subject of all of those
                     statements is the value of the <code>xlink:href</code> attribute of
                     the locator, except as noted below.

                  </p>

                  <p>
                     If the locator element provides an <code>xlink:role</code> attribute,
                     one additional statement <a href="#dt-must" title="Must, May, etc.">shall</a> be
                     added to the model. The value of the locator's <code>xlink:href</code>
                     attribute <a href="#dt-must" title="Must, May, etc.">shall</a> be mapped to the
                     subject of the statement. The value of the <code>xlink:role</code>
                     attribute <a href="#dt-must" title="Must, May, etc.">shall </a> be mapped to the
                     object, and the predicate <a href="#dt-must" title="Must, May, etc."> shall</a>
                     be "rdf:type". Harvesting software that uses the
                     facilities of the RDF Schema specification <a href="#dt-must" title="Must, May, etc.">
                        may
                     </a> generate an additional statement whose subject is the
                     value of the <code>xlink:role</code> attribute, whose predicate is
                     "rdf:type" and whose object is "rdfs:Class".
                     The second statement <a href="#dt-must" title="Must, May, etc.">should not</a> be
                     added to the RDF model if an equivalent statement already exists in
                     the model.

                  </p>


                  <p>
                     If the locator element provides an <code>xlink:label</code> attribute, an
                     RDF statement is added to the model. The value of the href attribute <a href="#dt-must" title="Must, May, etc.">shall</a> be mapped to the subject of the statement. The
                     predicate of the statement <a href="#dt-must" title="Must, May, etc.">shall</a> be <code>
                        xlink:label
                     </code>. The object of the statement <a href="#dt-must" title="Must, May, etc.">shall

                     </a> be the value of the <code>xlink:label</code> attribute.

                  </p>

                  <p>If the locator element provides an <code>xlink:title</code> attribute, an
                     RDF statement <a href="#dt-must" title="Must, May, etc.">shall</a> be added to the model.
                     The value of the <code>xlink:href</code> attribute <a href="#dt-must" title="Must, May, etc.">shall

                     </a> be mapped to the subject of the statement. The predicate of the
                     statement <a href="#dt-must" title="Must, May, etc.">shall</a> be
                     "<code>xlink:title</code>".
                     The object of the statement <a href="#dt-must" title="Must, May, etc.">shall</a> be the
                     value of the title attribute.

                  </p>

                  <p>If the resource element contains one or more title elements, they
                     are harvested as described in section <a href="#titles"><b>3.4.4 title-Type Element</b></a>.
                  </p>

               </div>


               <div class="div3">

                  <h4><a name="resources">3.4.3 <code>resource</code>-Type Element
                     </a>
                  </h4>

                  <p>Each XLink <code>resource</code>-type element gives rise to zero or
                     more statements in the RDF model. Unless noted
                     otherwise, the subject of all of those statements is the resource element
                     itself, identified by an XPointer synthesized according to the procedure described
                     in section <a href="#xptr_synth"><b>3.1 Synthesizing XPointers</b></a>.
                  </p>

                  <p>If the resource element provides an <code>xlink:role</code> attribute, one
                     RDF statement <a href="#dt-must" title="Must, May, etc.">shall</a> be added to the model,
                     and a second RDF statement <a href="#dt-must" title="Must, May, etc.">may</a> be added to
                     the model. The subject of the first statement is the synthesized URI reference
                     for the resource. The value of the xlink:role attribute is mapped to the object
                     of the statement. The predicate of the statement is '<code>rdf:type</code>'.
                     A second statement <a href="#dt-must" title="Must, May, etc.">may</a> be added to the model
                     if the software supports the RDF Schema specification <a href="#rdfs">[RDFSchema]</a>.
                     The value of the <code>xlink:role</code> attribute is mapped to the subject
                     of the optional statement. The predicate of the statement is "rdf:type
                     " and the object is "rdfs:Class". The second
                     statement <a href="#dt-must" title="Must, May, etc.">should not</a> be added to the
                     model if an identical statement already exists in the model.

                  </p>

                  <p>If the resource element provides an <code>xlink:label</code> attribute,
                     another RDF statement <a href="#dt-must" title="Must, May, etc.">shall</a> be added to
                     the model. The subject of the statement is the synthesized URI reference
                     for the resource. The predicate of the statement is
                     "xlink:label". The object of the statement is the value of
                     the label attribute.

                  </p>

                  <p>If the resource element provides an <code>xlink:title</code> attribute, another
                     RDF statement <a href="#dt-must" title="Must, May, etc.">shall</a> be added to the model.
                     The subject of the statement is the synthesized URI reference for the resource.
                     The predicate of the statement is "xlink:title". The object of
                     the statement is the value of the title attribute.
                  </p>

                  <p>If the resource element contains one or more title elements, they are harvested
                     as described in section <a href="#titles"><b>3.4.4 title-Type Element</b></a>.
                  </p>

               </div>


               <div class="div3">

                  <h4><a name="titles">3.4.4 <code>title</code>-Type Element
                     </a>
                  </h4>

                  <p>XLink <code>title</code>-type elements have an XLink-defined meaning
                     only if they appear as a child element within an extended, locator,
                     or resource element.


                  </p>

                  <p>If an XLink <code>extended</code>-, <code>locator</code>-, or
                     <code>resource</code>-type element contains one or more
                     <code>title</code>-type elements, one RDF statement <a href="#dt-must" title="Must, May, etc.">shall</a> be added to the model for each
                     title element.
                     The subject of the statement <a href="#dt-must" title="Must, May, etc.">shall</a>
                     be either the value of the <code>xlink:href</code> attribute (in the
                     case of a <code>locator</code> element) or a synthesized XPointer identifying
                     the <code>extended</code> or <code>resource</code> element. The predicate
                     of the statement
                     <a href="#dt-must" title="Must, May, etc.">shall</a> be <code>xlink:title</code>.
                     The object of the statement
                     <a href="#dt-must" title="Must, May, etc.">shall</a> be a synthesized XPointer
                     identifying the title element. (Identifying the title element, rather
                     than just its content, allows attributes such as <code>xml:lang</code>
                     to be captured along with the title.)

                  </p>

                  <div class="note">
                     <p class="prefix"><b>Note:</b></p>

                     <p>
                        Implementations <a href="#dt-must" title="Must, May, etc.">may</a> add a second
                        RDF statement to the model for each <code>title</code>-type element.
                        The object of the second statement shall be a synthesized XPointer
                        identifying the title element. The predicate of the second statement
                        shall be <code>rdf:value</code>. The object of the second statement
                        shall be the content of the title element. (If the title element
                        contains mixed content, the object is a string containing XML markup.
                        The implementation's facilities for dealing with situations
                        where the <code>rdf:parseType</code> attribute has the value
                        "literal" will be needed.)

                     </p>

                  </div>

                  <p>As an example, consider the following fragment of an extended
                     link:
                  </p>

                  <table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%">
                     <tr>
                        <td><pre>&lt;annotation xlink:type='extended' ID='genid22'>
  &lt;caption xlink:type='title' ID='genid23'>&lt;i>Recent&lt;/i> comments&lt;/caption>
  &lt;link xlink:type='arc' ...</pre></td>
                     </tr>
                  </table>

                  <p>The RDF statements harvested from the title are shown below:</p>
                  <img src="xlink2rdf-fig3.gif" alt="RDF model with 2 arcs, second one pointing at &lt;i>Recent&lt;/i>"></div>


            </div>


            <div class="div2">

               <h3><a name="linkbases">3.5 Linkbases</a></h3>

               <p>A linkbase is an XML document which functions like a database of
                  links. A linkbase arc is an XLink element (<code>simple</code>- or
                  <code>arc</code>-type) whose <code>xlink:arcrole</code> attribute takes the value
                  of "http://www.w3.org/1999/xlink/properties/linkbase".
                  The ending resource of a linkbase arc is a linkbase.
               </p>

               <p>When harvesting software encounters a linkbase arc, it
                  <a href="#dt-must" title="Must, May, etc.">shall not</a> generate an RDF statement for
                  the arc. It <a href="#dt-must" title="Must, May, etc.">should</a> traverse the arc to
                  retrieve the linkbase, and harvest the links from the linkbase to add to
                  the current model using the methods specified in this Note.
               </p>

               <div class="note">
                  <p class="prefix"><b>Note:</b></p>

                  <p>Different applications might make different tradeoffs on depth of
                     traversal in light of varying network conditions. This Note does not
                     mandate specific behavior, but does <a href="#dt-must" title="Must, May, etc.">recommend

                     </a> that all havesting applications attempt to obtain at
                     least the immediately referenced linkbase.

                  </p>

               </div>

            </div>


         </div>


         <div class="div1">

            <h2><a name="bibliography">4 References</a></h2>

            <dl>

               <dt class="label"><a name="XLink">XLink</a></dt>
               <dd>
                  Steve DeRose, Eve Maler, David Orchard, and Ben Trafford, editors. <cite>
                     XML Linking Language (XLink)
                  </cite>. World Wide Web Consortium, 2000.
                    (See <a href="http://www.w3.org/TR/xlink">http://www.w3.org/TR/xlink</a>.)
               </dd>


               <dt class="label"><a name="XML-Names">XML-Names</a></dt>
               <dd>Tim Bray, Dave Hollander, and Andrew Layman, editors. <cite>
                     Namespaces in XML
                  </cite>. Textuality, Hewlett-Packard, and Microsoft.
                  World Wide Web Consortium, 1999.  (See <a href="http://www.w3.org/TR/REC-xml-names">http://www.w3.org/TR/REC-xml-names</a>.)
               </dd>


               <dt class="label"><a name="rfc2119">IETF RFC 2119</a></dt>
               <dd>S. Bradner, editor. <cite>
                     Key words for use in RFCs to Indicate Requirement Levels
                  </cite>. March
                  1997. (See <a href="http://www.ietf.org/rfc/rfc2119.txt">http://www.ietf.org/rfc/rfc2119.txt

                  </a>.)
               </dd>


               <dt class="label"><a name="rdf">RDF</a></dt>
               <dd>Ora Lassila and Ralph Swick, editors. <cite>Resource Description
                     Framework (RDF) Model and Syntax Specification
                  </cite>. World Wide Web
                  Consortium, 1999.  (See <a href="http://www.w3.org/TR/REC-rdf-syntax">http://www.w3.org/TR/REC-rdf-syntax</a>.)
               </dd>


               <dt class="label"><a name="xptr">XPTR</a></dt>
               <dd>Ron
                  Daniel, Steve DeRose, and Eve Maler, editors.<cite> XML Pointer Language
                     (XPointer) V1.0
                  </cite>. World Wide Web Consortium, 1998.
                    (See <a href="http://www.w3.org/TR/xptr">http://www.w3.org/TR/xptr</a>.)
               </dd>


               <dt class="label"><a name="rfc2396">RFC 2396</a></dt>
               <dd>
                  RFC 2396: Uniform Resource Identifiers. Internet Engineering Task Force,
                  1995.  (See <a href="http://www.ietf.org/rfc/rfc2396.txt">http://www.ietf.org/rfc/rfc2396.txt</a>.)
               </dd>


               <dt class="label"><a name="rdfs">RDFSchema</a></dt>
               <dd>
                  Dan Brickley, R.V. Guha, editors. <cite>Resource Description
                     Framework (RDF) Schema Specification 1.0
                  </cite>. World Wide Web
                  Consortium, 2000.
                    (See <a href="http://www.w3.org/TR/rdf-schema">http://www.w3.org/TR/rdf-schema</a>.)
               </dd>


               <dt class="label"><a name="xbase">XML Base</a></dt>
               <dd>Jonathan Marsh, editor. <cite>XML Base (XBase)

                  </cite>. World Wide Web Consortium, 1999.  (See <a href="http://www.w3.org/TR/xmlbase">http://www.w3.org/TR/xmlbase</a>.)
               </dd>

            </dl>

         </div>

      </div>


      <div class="back">

         <div class="div1">

            <h2><a name="xslt">A Implementing the Harvesting in XSLT</a></h2>

            <p>One way of harvesting RDF statements from XML documents that contain
               XLinks is through the use of XSLT. This appendix presents a simple
               example of such a harvester. It has a number of limitations. It does
               not generate the synthesized XPointers which are preferred for reasons
               of interoperability. Second, it uses a Java extension function for
               adding statements to an RDF storage manager. There is no standard API
               for such a storage manager. Accordingly,
               this appendix is provided as an example only. It does not
               specify any normative behavior.
            </p>

            <p>
               The stylesheet itself is given in listing 1. Its operation is
               very simple. It looks for simple XLinks, stores the subject, object,
               and predicate in variables, then calls an extension function to add
               the RDF statement to some storage mechanism.

            </p>

            <p>

               <table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%">
                  <tr>
                     <td><pre>

&lt;!--

  Simple XSLT stylesheet to harvest RDF statements from simple
  XLinks. XLinks are detected and an extension function called
  to add RDF statements to an RDF repository. The extension function
  is a very simple mockup that just prints its arguments to stdout.

  Note that the repository is updated as a side effect of
  examining the document. While practical, this is somewhat at odds
  with the philosophy behind XSLT. Any application that actually
  cares about the order in which RDF statements are made is
  cautioned about using this approach.

  Credit where due: I got a head start on this by using Dan Connolly's
  stylesheet that tried to turn XLinks into RDF's XML syntax.

  Ron Daniel Jr.
  rdaniel@metacode.com   2000-09-15
-->

&lt;xsl:stylesheet
    version="1.0"
    xmlns:xsl  ="http://www.w3.org/1999/XSL/Transform"
    xmlns:rdf  ="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:xlink="http://www.w3.org/1999/xlink"
    xmlns:rdfs ="http://www.w3.org/TR/rdf-schema/"

    xmlns:Radix="http://www.example.com/com.example.Radix"
>

  &lt;!-- Some 'useful' declarations. -->
  &lt;xsl:variable name='rdf-type'
	      select='"http://www.w3.org/1999/02/22-rdf-syntax-ns#type"'/>
  &lt;xsl:variable name='rdfs-class'
	      select='"http://www.w3.org/TR/rdf-schema#Class"'/>

  &lt;xsl:output method="text"/>

  &lt;!-- Look for simple links that are linkbase references, ignore them.
       (This rule is explicitly given a higher priority
        than the system-assigned priorities of the other rules so
        that it will match and discard linkbase references.)
  -->
  &lt;xsl:template priority="2"
       match='*[@xlink:type="simple"][@xlink:arcrole=
               "http://www.w3.org/1999/xlink/properties/linkbase"]'>
      &lt;!-- For now, do nothing. A more complete example could pull in
           the linkbase and harvest its content.
      -->
  &lt;/xsl:template>


  &lt;!-- Process the simple links that are not linkbase references.
       Pull the various bits of info into variables and then call the
       extension function.
       (Make sure that we have an href specified. If not, there is no
       arc to add to the RDF model.)
  -->
  &lt;xsl:template match='*[@xlink:type="simple"][@xlink:href]'>

    &lt;!-- Subject - Synthesizing the XPointer in an interoperable way is
         left for more ambitious examples. For simplicity, just generate
         a unique ID.
    -->
    &lt;xsl:variable name='subject'
		  select='concat("#xpointer-for-", generate-id())'/>

    &lt;!-- Predicate name comes from arcrole (preferred) or element type
	 (allowed). Look for arcrole, but if it doesn't exist, use
	 element name. (Note that we have to concatenate the namespace
         URI and the local name according to the RDF spec to make a URI
         reference. This should really test to see if namespace URI
         ends with #, /, or ?)
     -->
    &lt;xsl:variable name='predicate'>
      &lt;xsl:choose>
        &lt;!-- Get arcrole attribute if possible. -->
	&lt;xsl:when test='@xlink:arcrole'>
	  &lt;xsl:value-of select='@xlink:arcrole'/>
	&lt;/xsl:when>
        &lt;!-- If no arcrole, use element type as long as there is a
             namespace URI so it can be made into a URI reference.
        -->
	&lt;xsl:when test='namespace-uri()'>
	  &lt;xsl:value-of select='concat(namespace-uri() , name())'/>
	&lt;/xsl:when>
      &lt;/xsl:choose>
    &lt;/xsl:variable>

    &lt;xsl:variable name='object' select='string(@xlink:href)'/>
    &lt;xsl:variable name='objType' select='string(@xlink:role)'/>

    &lt;!--  Here it is - the main call to add a statement to the
          RDF database.
    -->
    &lt;xsl:if test='$predicate'>
      &lt;xsl:value-of select='Radix:addStatement($subject,
	       string($predicate), $object)'/>
    &lt;/xsl:if>

    &lt;!-- Additional call if xlink:role specified. (We rely on the
         underlying RDF storage implementation to deal with possible
         multiple additions of the  rdf:type(objType, rdfs:Class)
         statement).
    -->
    &lt;xsl:if test='$objType'>
      &lt;xsl:value-of select='Radix:addStatement($object,
	       $rdf-type, $objType)'/>
      &lt;xsl:value-of select='Radix:addStatement($objType,
	       $rdf-type, $rdfs-class)'/>
    &lt;/xsl:if>
  &lt;/xsl:template>


  &lt;!-- don't pass text thru -->
  &lt;xsl:template match="text()|@*">
  &lt;/xsl:template>

&lt;/xsl:stylesheet>

</pre></td>
                  </tr>
               </table>

            <p>The stylesheet makes use of an extension function,
               <code>Radix:addStatement(subject, predicate, object)</code>, which
               would actually add the statement to an RDF storage manager. For
               demonstration purposes, a dummy implementation of that extension
               function is given in listing 2. This was tested using the Saxon
               implementation of XSLT. Other implementations may have different
               conventions for the use of extension functions.

            </p>

            <p>

               <table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%">
                  <tr>
                     <td><pre>

package com.example;
/** Simple demo of a RDF interface. This one is trivial, it has
  * one call that lets statements be added.
  */
public class Radix
{

  /** Mockup of a routine to add RDF statements to a model being
    * constructed. Prints the subject, predicate, and object on
    * different lines with progressive indention to make it easy
    * to read.
    */
  public static String
  addStatement(String subject, String predicate, String object)
  {
    System.out.println(predicate + "\n  " + subject + "\n    " + object);
    return "";
  }
}

</pre></td>
                  </tr>
               </table>


         </div>

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