Goal (Mission ?)

From the charter of the Working Group:

The XML Linking WG continues the effort begun in the XML WG in January 1997.

The objective of the XML Linking WG is to design advanced, scalable, and maintainable hyperlinking and addressing functionality for XML. Such functionality includes the hyperlinking abilities of HTML, but extends them to include at least the following:

Linking elements in XML documents that allow arbitrary names and semantics
Linking elements in XML documents not co-located with any of the resources they associate
Addressing into logical sub-structures of XML documents in a robust way
Allowing link authors to influence the behavior of traversed links in basic ways

Principle

XLink defines the following:

A namespace (usually prefixed as "xlink") http://wwww.w3.org/namespace/xlink/1999/
A set of elements and attributes anchored in that namespace allowing to define Linking Elements i.e. elements having a given linking semantic
Three classes of Linking elements:
- Simple link elements (HTML like)
- Extended link elements
- Extended link sets

An XLink processor expected to detects those contructs so they are handled at the application level.

However XLink does not enforce behaviour, this is application dependant with hints defined on the link elements.

XLink Core properties

The core properties (attributes):

xlink:type attribute indicates the type of the link:
- value simple simple link
- value extended extended link
xlink:href this is the only locator attribute, and its value is an URI-Reference [RFC 2396].
xlink:locator are elements contained in extended links and defining resources paritcipating in the link. Each locator holds an xlink:role attribute allwoing to reference it.
xlink:arc are elements contained in extended links and defining traversal behaviour using the following attributes:
- xlink:from the source of the arc, the attribute value target a locator referenced by its xlink:role name
- xlink:to the destination of the arc

XLink Behaviour properties

Those are attribute holding behaviour related informations:

xlink:show indicate expected behaviour on traversal effect:
- value embed the link target should be included like <IMG>
- value new the link target should create a new rendering, like opening a new window.
- value replace the link target should replace the resource containing the link, like <A> in most cases.
xlink:actuate indicate expected triggering of link:
- value auto indicate at link detection time, like <IMG> if autoloading of images is turned on.
- value onRequest indicate activation at user's request, like <A>

XLink Semantic properties

Those are placeholders for application semantic:

xlink:title is designed to provide human-readable text describing the links, simple links must use an attribute while extended links must use an element.
xlink:role is an attribute containing a qualified name used to describe the function of the link's or locator's content.
for example XLink uses role="xlink:external-linkset" to distinguish Links Sets from Extended links.

Simple Links

Here are the limitations on simple links:

The source of the link is the link element itself
There is only one target
The title is carried by a single attribute (unicity)

But they benefit from a lighter syntax

Simple Links View

Simple Link Example

this can be as simple as:


<students xlink:href="students.xml"> The list of students.</students>

assuming the following element declaration:

<!ELEMENT student ANY>
<!ATTLIST student
 xmlns:xlink   CDATA #FIXED "http://www.w3.org/1999/xlink/namespace/"
 xlink:type    CDATA #FIXED "simple"
 xlink:href    CDATA #REQUIRED
 xlink:role    CDATA #IMPLIED
 xlink:title   CDATA #IMPLIED
 xlink:show    (new|embed|replace) "replace"
 xlink:actuate (onRequest|auto)    "onRequest"
>

Extended Links

Here is a set of "advanced" features:

Multiple sources and targets using xlink:locator elements
Multiple actuation possible using xlink:arc elements
Multiple xlink:title elements (I18N)

XLink Extended Links Examples

<element xmlns:xlink="http://www.w3.org/1999/namespace/xlink/"
	 xlink:type="extended">
 <xlink:locator href="Source" role="role1"/>	 
 <xlink:locator href="Target" role="role2"/>
 <xlink:arc from="role1" to="role2"
	       show="embed" actuate="auto"/>
 <xlink:title>The link title<xlink:title/>
 <xlink:title xml:lang="fr">Description du lien<xlink:title/>
    ...
</element>

Extended Link Sets

This construct allows to reference an external set of links

the classic example of its use is a set of annotations
is of type xlink:extended but with xlink:role="xlink:external-linkset"
all embedded locators are pointers to sets of links

Extended Link Set example

<xlink:extended xmlns:xlink="http://www.w3.org/1999/xlink/namespace/"
         role="xlink:external-linkset">
     <xlink:title>DV's Links</xlink:title>
     <xlink:locator href="mylinkset.xml"/>
</xlink:extended>

Issue List

There is still some issues left like:

Nested links
Conformance testing
Resource media specification

But the core is now stable.

Related efforts

XPointer since URI-References to XML resources are needed for Xlink
A base mechanism similar to HTML <BASE>, expect something soon
A better include mechanism (c.f. the old "embed" value for show), please have a look at the XInclude Note

Current status

A new public working draft is on the way
We don't expect it to be a Last Call one yet
Expected Last call this Winter, REC a few month later

May the mission complete sucessfully...

Ap: Simple Link mapping to RDF

<element xmlns:xlink="http://www.w3.org/namespace/xlink/1999/"
         xlink:href="target"
	 xlink:type="simple"
	 xlink:show="embed"
	 xlink:actuate="auto"
	 xlink:role="The link role"
	 xlink:title="The link title">
	 ...
</element>

translates to
  	 
<element>	 
  <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
           xmlns:xlink="http://www.w3.org/namespace/xlink/1999/">
    <rdf:Description about="#xptr(../..)">
       <xlink:href rdf:resource="target"/>
       <xlink:type rdf:resource="xlink:simple"/>
       <xlink:show rdf:resource="xlink:embed"/>
       <xlink:actuate rdf:resource="xlink:auto"/>
       <xlink:role>The link role</xlink:role>
       <xlink:title>The link title</xlink:title>
    </rdf:Description>
  </rdf:RDF>
  ...
</element>

Ap: Extended Link mapping to RDF

<element xmlns:xlink="http://www.w3.org/1999/namespace/xlink/"
	 xlink:type="extended">gt;
    <xlink:locator href="Source" role="role1"/>gt;	 
    <xlink:locator href="Target" role="role2"/>gt;	 
    <xlink:arc from="role1" to="role2"
	       show="embed" actuate="auto"/>gt;
    <xlink:title="The link title"/>gt;	 
    ...
</element>gt;

translates to
  	 
<element>gt;	 
  <rdf:rdf xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
           xmlns:xlink="http://www.w3.org/1999/namespace/xlink/">gt;
    <rdf:description about="#xptr(../..)">gt;
        <xlink:type rdf:resource="xlink:extended"/>gt;
	<xlink:locator rdf:about="source" rdf:id="role1"/>gt;
	<xlink:locator rdf:about="target" rdf:id="role2"/>gt;
	<xlink:arc>gt;
	    <xlink:from rdf:resource="#role1">gt;
	    <xlink:to rdf:resource="#role2">gt;
	    <xlink:show rdf:resource="xlink:embed"/>gt;
	    <xlink:actuate rdf:resource="xlink:auto"/>gt;
	</xlink:arc>gt;
        <xlink:title>gt;the link title</xlink:title>gt;
    </rdf:description>gt;
  </rdf:rdf>gt;
  ...
</element>gt;