XML Linking Language, orXLink, is anXMLmarkup language andW3C specification that provides methods for creating internal and external links within XML documents, and associating metadata with those links.[1]
XLink 1.1 is a W3C recommendation[2] and the successor of XLink 1.0, which was also a W3C recommendation.[3][4]
XLink defines a set ofattributes that may be added to elements of otherXML namespaces. XLink provides two kinds of hyperlinking for use in XML documents.Simple links connect only two resources, similar toHTML links.Extended links can link an arbitrary number of resources.
Asimple link creates a unidirectionalhyperlink from one element to another via aURI. Example:
<?xml version="1.0"?><documentxmlns="http://example.org/xmlns/2002/document"xmlns:xlink="http://www.w3.org/1999/xlink"><headingid="someHeading">SomeDocument</heading><para>Hereis<anchorxlink:type="simple"xlink:href="#someHeading">alink</anchor>totheheader.</para><para>Itisananchorthatpointstotheelementwiththeid"someHeading"onthecurrentpage.</para></document>
Extended links allow multiple resources, either remote or local, to be connected by multiple arcs. Anarc is information about the origin, destination and behavior of a link between two resources. The origin and destination resources are defined by labels. By using one or more arcs, an extended link can achieve specific sets of connections between multiple resources.
For example, if all resources in an extended link were given the labelA, then an arc within that link declaringfrom="A", to="A" would form connections between all resources.
Extended links do not need to be contained in the same document as the elements they link to. This makes it possible to associate metadata or other supplementary information with resources without editing those resources.
XLink also supports richer information about link types and the roles of each resource in an arc.
Hypertext links inScalable Vector Graphics can currently be defined as simple XLinks.[5] Theworking draft of SVG 1.2 proposes using extended XLinks as well.[6] In the SVG 2 specification, XLink was deprecated in favor of non-namespaced equivalent attributes.[7]
TheResource Directory Description Language, an extension toXHTML Basic that is used to describeXML Namespaces, uses simple XLinks.[8]
TheeXtensible Business Reporting Language has used simple and extended XLinks since the XBRL 2.0 specification was published in 2001.[9] Most large XBRL taxonomies contain extensive linkbases. As of 2009, XBRL is probably the most extensive use of XLink in production systems.
TheMetadata Encoding and Transmission Standard, supported and maintained by theLibrary of Congress for describing file aggregations, uses simple XLinks in pointing to file locations as well as linkbases which describe relationships among external files (though these restrictto andfrom attributes to type IDREF instead of NMTOKEN).[10][11]
Geography Markup Language uses simple XLinks to implement referencing. In particular, GML uses xlink:href to support a graph model for geospatial information. GML's graph model is essentially the same asRDF, on which early versions of GML were based. The GML specification constrains the semantics of XLinks to be essentially the same as rdf:resource (from the RDF/XML syntax) i.e. the referent can logically be placed in-line and the data is still valid.
Mozilla Firefox has supported simple XLinks since version 1.5, but only forSVG andMathML documents. It is unsupported in other XML documents.[12] Only thexlink:href,xlink:show,xlink:target andxlink:title attributes are supported.[13]
Prince supports simple XLinks.[14]
