XML Linking Language (XLink)

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XML Linking Language (XLink)

• W3C Working Draft - 3/3/1998

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Overview of presentation

• Introduction to links
• Locator syntax
• Link Recognition
• Linking Elements
• Extended Link Groups
• Link Behavior
• Attribute Remapping
• Conformance

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What is a Link ?

• Link is an explicit relation between two or more
  data objects or portions of data objects
• Links are asserted by elements in an XML documents

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Basic Characteristics of Links

• The link is expressed at one of its ends
• Users can initiate travel from that end to the
  other
• The links effect on windows, frames etc. are
  mainly determined by the browsers
• The link goes to only one destination

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Influential Standards

• HTML
• HyTime
• TEI P3 (Text Encoding Initiative Guidelines)
• and others.

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Terminology

• Linking Element - An Element that asserts the
  existence and describes the characteristics of a
  link
• Locator - Data, provided as part of a link, which
  identifies a resource
• Resource - Anything reachable by the use of a
  locator in some linking element
• Traversal - The action of using a link i.e.
  accessing a resource

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Locator Syntax

• Locator for a resource is provided by means of a
  URI
• Xpointers can be used in conjunction with the
  URI, as fragment identifiers or queries, to
  specify a more precise sub-resource
• (A sub-resource is a portion of a resource,
  pointed to as the precise destination of a link)

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Locator Syntax (contd.)

• Locator URI
• Connector (XPointer Name)
• URI Connector (XPointer Name
• Connector
• URI URIchar

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Link Recognition

• The existence of a link is asserted by a linking
  element
• Example
• ltA xmllinksimple
• href http//www.w3.orggtThe W3Clt/Agt
• Attribute list declaration
• lt!ATTLIST A xmllink CDATA
• fixed simplegt

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Linking Elements

• Xlink defines two types of linking elements
• Simple Link
• Extended Link

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Linking Elements (contd.)

• Information associated with a link
• One or more locators to identify the the remote
  resources participating in the link
• Semantics of the link
• Semantics of the remote resource
• Semantics of the local resource, if the link is
  inline

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Linking Elements (contd.)

• Locators
• A link must supply a locator for each remote
  resource
• A locator takes the form of an attribute called
  href
• Example
• lt!ENTITY locator.att
• href CDATA REQUIRED
• gt

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Linking Elements (contd.)

• Link Semantics
• Whether the link is inline or out-of-line
• Inline Links - A link where the content of the
  linking element serves as a resource
• Out-of-line Links - A link whose content does
  not serve as one of the links resources
• The role of a link

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Linking Elements (contd.)

• Link Semantics Example
• lt!ENTITY link-semantics.att
• inline (truefalse) true
• role CDATA IMPLIED
• gt

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Linking Elements (contd.)

• Remote Resource Semantics
• lt!ENTITY remote-resource-semantics.att
• role CDATA IMPLIED
• title CDATA IMPLIED
• show (embedreplacenew) IMPLIED
• actuate (autouser) IMPLIED
• behavior CDATA IMPLIED
• gt

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Linking Elements (contd.)

• Local Resource Semantics
• lt!ENTITY local-resource-semantics.att
• content- role CDATA IMPLIED
• content- title CDATA IMPLIED
• gt

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Linking Elements (contd.)

• Simple Link are usually inline and always
  one-directional.
• Example
• ltmylink xmllinksimple title citation
• hrefhttp//www.a.com/foo.xml shownew
• content-roleReferencegt as in Smith (1997)
• lt/mylinkgt

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Linking Elements (contd.)

• Example Declaration
• lt!ELEMENT simple ANYgt
• lt!ATTLIST simple
• xmllink CDATA FIXED simple
• locator.att
• remote-resource-semantics.att
• local-resource-semantics.att
• simple-link-semantics.att
• gt

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Linking Elements (contd.)

• Extended Links
• Link Together any number of resources, resulting
  in multiple targets
• Link to and from resources that cannot contain
  the links themselves.
• Applying and filtering sets of relevant links on
  demand
• Enabling other advanced hypermedia capabilities

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Linking Elements (contd.)

• Sample Declaration
• lt!ELEMENT simple ANYgt
• lt!ATTLIST simple
• xmllink CDATA FIXED extended
• link-semantics.att
• local-resource-semantics.att
• gt

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Extended Link Groups

• An extended link group element may be used to
  store a list of links to other documents that
  together constitute an interlinked group
• Each such document is identified by means of an
  extended link document element, a special kind
  of locator element

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Extended Link Groups (contd.)

• Sample declarations for extended link group and
  extended link document elements
• lt!ELEMENT group (document)gt
• lt!ATTLIST group
• xmllink CDATA FIXED group
• steps CDATA IMPLIED
• gt
• lt!ELEMENT document EMPTY
• lt!ATTLIST document
• xmllink CDATA FIXED document
• locator.att
• gt

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Link Behavior

• Authors can signal certain intentions as to the
  timing and effects of traversal.
• These intentions expressed along the show and
  actuate axis

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Link Behavior (contd.)

• Show Axis
• The show attribute is used to express a policy as
  to the context in which a resource that is
  traversed is to be displayed or processed.
• It can take on one of three values
• embed
• replace
• new

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Link Behavior (contd.)

• Actuate Axis
• The actuate attribute is used to express a policy
  as to when traversal of a link should occur
• It may take on one of two values
• auto
• user

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Attribute Remapping

• User chosen attribute names can be mapped to the
  default names using the attribute xmlattribute

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Conformance

• An element conforms to Xlink if
• The element has an xmllink attribute whose value
  is one of the attribute values prescribed by this
  specification and
• the element and all of its attributes and content
  adhere to the syntactic requirements imposed by
  the chosen xmllink attribute value, as
  prescribed in this specification.