XLink

1
XLink

• Van Lepthien
• CSCI 7818
• Fall 2000

2
Overview

• What is XLink?
• W3C Stuff
• XLink Elements
• Linkbases
• Traversals
• Implementations
• Comments
• References

3
What is XLink?

• XLink allows elements to be inserted into XML
  documents in order to create and describe links
  between resources.

4
What is XLink?

• Define link relationships among more than two
  resources.
• Associate metadata with a link
• Define links to associate resources not in the
  document containing the link

5
What is XLink?

• Enables hyperwebs
• (this slide for Kens benefit)

6
W3C Stuff

• XML XLink Requirements Version 1.0 W3C Note
  24-Feb-1999(http//www.w3.org/TR/NOTE-xlink-req/
  )
• Candidate Recommendation 3 July 2000 XML
  Linking Language (XLink) Version 1.0 -
  (http//www.w3.org/TR/xlink/)
• Namespacexmlnsxlinkhttp//www.w3.org/1999/xli
  nk

7
XLink Scope

• Element Conformance
• Application Conformance

8
Element Conformance

• An element contains a type attribute from the
  XLink namespace with a value of arc,
  extended, locator, resource, simple,
  title, or none.
• An element adheres to the specified constraints
  for the type.
• XLink does not define elements.
• It defines attributes and how the containing
  elements are to be used.

9
Application Conformance

• The recommendation describes processing that an
  application must do to conform, and, if optional
  features are implemented, what must be done to
  make the optional features conform.

10
XLink Element Types

• Links
• simple
• extended
• Resources (used in extended links)
• resource (local resource)
• locator (remote resource)
• arc (used in extended links)
• title (may be in a child element of an extended-,
  locator- or arc-type element)
• none

11
Link - simple

• Defines an outbound link from a local resource to
  a remote resource.
• Similar to HTML references
• lta hrefurigt lt/agt or ltimg hrefurigt
• Attributes
• href URI of a resource. Xpointer support is
  explicitly expected.
• role URI of a resource that describes the role
  of the element
• arcrole URI of a resource that describes the
  role of the link
• show describes how the href resource is to be
  presented on transversal
• actuate describes when the transversal to href
  resource is to happen
• title a human readable description of the
  element

12
simple - Link Example
XML Document ltdoc xmlnsxlink/xlinkgt ltmyta
g xlinktypesimple xlinkrolerole_uri
xlinkhrefref_uri xlinkshownew
xlinkactuateonLoadgt lt/mytaggt lt/docgt
Role definition
Linked resource
Instruction to open the link in a new window when
the link is traversed
Instruction to traverse the link when loading the
original document
13
Link - extended

• An extended link associates an arbitrary number
  of resources.
• An extended link may be stored separately from
  all resources that are being associated.
• Enables linking of objects that have no provision
  of internal links.

14
Link - extended

• A link element with XLink typeextended
  contains a set of elements with XLink types of
  resource, locator, arc, and title.

15
resource

• An element with XLink typeresource defines a
  local resource.
• Attributes
• role URI of a resource that describes the role
  of the element
• title a human readable description of the
  element
• label provides a label to which an arc type
  element may refer

16
locator

• An element with XLink typelocator defines a
  remote resource.
• Attributes
• href URI of a resource (required)
• role URI of a resource that describes the role
  of the element
• title a human readable description of the
  element
• label provides a label to which an arc type
  element may refer

17
arc

• An element with XLink typearc defines links
  between (or among) resources. The links can be
• between a location in the document and a remote
  resource (an outbound arc)
• between a remote resource and a location in the
  document (an inbound arc)
• between remote resources (a remote arc)

18
arc

• Attributes
• from specifies the label of starting resources
  for a transversal
• to specifies the label of ending resources for
  a transversal
• show describes how the href resource is to be
  presented on transversal
• actuate describes when the transversal to href
  resource is to happen
• arcrole URI of a resource that describes the
  role of the arc
• title a human readable description of the
  element

19
title

• An element with XLink typetitle permits more
  information than a title attribute alone would. A
  title element must be a child of the element it
  supplies title information for.
• An element with XLink typetitle only has XLink
  specified meaning if it is the child of an
  element with an XLink type of extended, locator,
  or arc.

20
extended - LinkExample

• XML Document
• ltdoc xmlnsxlink/xlinkgt
• ltorder xlinktypeextended gt
• ltcar xlinktyperesource xlinklabelc1gtPorsch
  e lt/cargt
• ltcar xlinktyperesource xlinklabelc1gtFerrar
  i lt/cargt
• ltaccount xlinktypelocator xlinklabela1
• xlinkhrefhttp//www.chargeme.com/cgi/citiba
  nk-12-3123
• xlinktitleBrian Temple/gt
• ltlocation xlinktyperesource xlinklabeld1gt
  
• ltattngtMatthias Hauswirthlt/attngt
• ltstreetgtlt/streetgt ltzipgt80303lt/zipgt
• lt/ location gt
• ltcharge_to xlinktypearc xlinkfromc1
  xlinktoa1
• xlinktitleBrian pays/gt
• ltdeliver_to xlinktypearc xlinkfromc1
  xlinktod1/gt
• lt/ordergt
• lt/docgt

BankAccount
21
none

• An element with XLink typenone has no XLink
  specified meaning.
• This can be used to enable one to turn XLink on
  and off in individual elements in an element type
  that is XLink aware.

22
noneExample
DTD lt!ELEMENT ref (stuff)gt lt!ATTLIST ref
xlinktype (simplenone) none xlinkhref
CDATA IMPLIED gt
Markup ltref xlinktypesimple
xlinkhrefhttp//x.comgt ltstuffgtstufflt/stuffgt
lt/refgt ltrefgt ltstuffgtstufflt/stuffgt lt/refgt
ref element with link
ref element without link
23
Behavioral Attributes

• The two behavior attributes, show and actuate,
  give an XLink application instructions on how and
  when, respectively, to display the ending
  resources of links.

24
show attribute

• The show attribute on a simple link or an arc
  describes how the application interpreting XLink
  should present the ending resource of the link or
  arc.
• The value of the show attribute is one of new,
  replace, embed, other, or none
• Linkbase arcs must be treated as if shownone,
  regardless of the value specified.

25
show attribute values

• new the application should open the ending
  resource in a new window, frame, or similar
  context.
• replace the application should load the
  ending resource in the same window, frame, etc.
  as the starting resource.
• embed the application should load the the
  ending resource in place of the starting
  resource.
• other the application should use other markup
  in the link to determine how the ending resource
  should be handled.
• none the application is not constrained on
  how to load the ending resource.

26
actuate attribute

• The actuate attribute on a simple link or an arc
  describes when the application interpreting XLink
  should make the traversal.
• The value of the actuate attribute is one of
  onLoad, onRequest, other, or none
• Linkbase arcs must be treated as if
  actuateonLoad, regardless of the value
  specified.

27
actuate attribute values

• onLoad the application should traverse to the
  ending resource upon loading of the starting
  resource.
• onRequest the application should traverse to
  the ending resource after loading of the starting
  resource, but only when some explicit event (an
  mouse click, etc.) initiates the traversal.
• other the application should use other markup
  in the link to determine how the traversal
  occurs.
• none the application is not constrained on
  how it handles the traversal.

28
Linkbases

• A linkbase is used to gather links in one place,
  external to the items linked.
• A linkbase is specified in an external link with
  an arc type element with arcrolehttp//www.w3.or
  g/1999/xlink/properties/linkbase
• A linkbase must be XML.

29
Traversals

• In an simple link a traversal is specified by the
  implicit local resource and the explicit remote
  resource designated in the href attribute.
• In an extended link, a set of traversals are
  specified by the pairs each made up of a
  from-node, consisting of a locator type element
  whose label matches the value of the from
  attribute in an arc, and a to-node, consisting of
  a locator type element whose label matches the
  value of the to attribute in the same arc.

30
TraversalsExample

• XML Document
• ltdoc xmlnsxlink/xlinkgt
• ltorder xlinktypeextended gt
• ltcar xlinktyperesource xlinklabelc1gtPorsch
  e lt/cargt
• ltcar xlinktyperesource xlinklabelc1gtFerrar
  i lt/cargt
• ltaccount xlinktypelocator xlinklabela1
• xlinkhrefhttp//www.chargeme.com/cgi/citiba
  nk-12-3123
• xlinktitleBrian Temple/gt
• ltlocation xlinktyperesource xlinklabeld1gt
  
• ltattngtMatthias Hauswirthlt/attngt
• ltstreetgtlt/streetgt ltzipgt80303lt/zipgt
• lt/ location gt
• ltcharge_to xlinktypearc xlinkfromc1
  xlinktoa1
• xlinktitleBrian pays/gt
• ltdeliver_to xlinktypearc xlinkfromc1
  xlinktod1/gt
• lt/ordergt
• lt/docgt

Traverals for the charge_to arc carPorsche
?account carFerrari ?account
Traverals for the deliver_to arc carPorsche
?location carFerrari ? location
31
Unclear areas

• simple link role
• Two arcs in an extended link cannot have the same
  from and to values
• show attribute values replace and embed
• The choice of the terms replace and embed for
  values of the show attribute are not clear

32
Unclear areas simple link role

• In a simple link, a role can be specified, but it
  not clear whether the role is being defined for
  the local resource or the remote resource.(In
  the XLink Candidate Recommendation,
  http//www.w3.org/TR/2000/CR-xlink-20000703/Overvi
  ew.html, the example of single link functionality
  implemented in an extended link, the role appears
  as an attribute of the locator type element,
  i.e., the remote resource. But an example does
  not a specification make. And there is no reason
  the local resource should not have a role
  described.

33
Unclear areas Two arcs in an extended link
cannot have the same from and to values

• In an extended link, only one arc can specify a
  pair of xlinkfrom and xlinkto values. If there
  are multiple relationships between a pair of
  objects, they cannot be expressed as arcs from
  one object to the other distinguished by
  different arcrole attribute values.
• This may be intended to prevent multiple
  traversals where they might not be wanted, but
  also prevents multiple traversals where they
  might be needed.

34
Unclear areasshow attribute valuesreplace and
embed

• The choice of the terms replace and embed for
  values of the show attribute are not clear.
• The description of replace says the application
  should load the resource in the same window,
  frame, pane, or other relevant presentation
  context in which the starting resource was
  loaded.
• The description for embed says the application
  should load it in place of the starting
  resource.

35
Implementations

• 1998 - University of Southhampton (see CARR in
  the references)
• 1998? - XLink in DSSSL Package. There is a
  reference on xml.com to an implementation of
  XLink, but the server does not respond.
• 1999 XLinkFilter (see ST LAURENT in the
  references)
• None of the major browsers have support.

36
Comments

• I have ignored XPointer.
• I would like to do a comparison of XLink and
  Chimera functionality.
• The lack of implementations is disturbing even
  recognizing that XLink is still a Candidate
  Recommendation.
• The Unclear Areas popped right out from reading
  the recommendation. This is not a good sign.
• XML support for extended hyperlinks is a good
  thing.

37
References

• CARR Leslie Carr, Initial Experiences of an
  XLink Implementation http//journals.ecs.soton.ac
  .uk/xml4j/xlinkexperience.html
• JIRAT Jiri Jirat, XLink Reference
  http//zvon.org/xxl/xlink/Output/xlink_refs.html
  
• MALER Eve Maler, XLink and Xpointer Overview
  http//www.oasis-open.org/cover/xlinkMaler980402.h
  tml
• ST LAURENT Simon St.Laurent, XLinkFilter An
  Open Source Java XLink SAX Parser Filter
  http//www.simonstl.com/projects/xlinkfilter/index
  .html
• vdVLIST Eric van der Vlist, XML Linking
  Technologies http//www.xml.com/pub/2000/10/04/li
  nking/index.html
• XMLCOM XML Resource Guide XLink
  http//www.xml.com/pub/Guide/XLink