XML Part 5: XLL: The Extensible Linking Language

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XML Part 5 XLL The Extensible Linking Language

• World Wide Web Technology

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XLL The Extensible Linking Language

• XLL has now been divided into two parts
• Xlink deals with the constructs that describe
  links between objects
• Xpointer describes how fragments of a document
  are addressed

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XLink

• What is a Link?
• A link is an explicit relationship between two or
  more data objects or portions of data objects
• Xlink has its origins in HTML, HyTime and TEI
  (Text Encoding Initiative)
• For interoperability with HTMLmany constructs are
  similar

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Links in HTML

• A very simple link type expressed in only 2
  elements ltAgt and ltLINKgt
• The link is expressed at one of its ends (with an
  A element)
• Users can only initiate travel from that end to
  the other
• The behaviour on link traversal is principally
  determined by the browser
• The link goes to only one destination

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Link Elements

• Existence of a link is asserted by a linking
  element
• In HTML, simply ltAgt or ltLINKgt are reserved
• In XML, this is not appropriate because element
  types are extensible. So instead, link elements
  are indicated by the presence of an attribute
  xmllink

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Link Elements

• Possible values of xmllink are
• simple
• extended
• locator
• group
• document
• An element with xmllink set to one of these is
  treated as an element of the indicated link type

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Example

• ltA xmllinksimple hrefhttp//www.w3.org/gt
  The W3Clt/Agt
• asserts a simple link
• Not it is not the type A that asserts the link,
  but the xmllink attribute

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Locators

• What are locators?
• Are data which identify a resource.
• For documents such locators are normally URIs
• For fragments of documents
• recall the target and in HTML
• Xpointers can be used to specify more specific
  resources such as a fragment of a document

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Connectors

• The resource being located is often termed the
  designated resource.
• The designated resource may be a subresource of a
  resource called the containing resource
• The latter is addressed using a URI.
• The subresource is addressed using an XPointer

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Whole document addressed using URI
Doc ltgtfragment resourceltgt
Subresource addressed using XPointer
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Connectors

• Subresource
• URIXpointer
• URIXpointer
• The connector means that the extraction is to
  be done client side
• The connector means that nothing is said about
  whether the extraction is to be server or client
  side. If a URI is not provided, the containing
  resource is considered to be the document
  containing the linking element

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Queries

• A URI by definition, includes an optional query
  component.
• Where this component is used to indicate via an
  Xpointer, a subresource to be extracted on the
  server side, the query should take the form
• URI?XML-XPTRXPointer

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Types of Links

• Inline link
• indicated by a linking element that serves as one
  of its own resources eg ltAgt HTML
• Out-of-line link
• indicated by a linking element that does not
  serve as one of its own resources (may be in none
  of the resources it connects)
• Multidirectional link
• Traversed at more than one of its resources
• Links to multiple resources

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Types of Linking Element

• Simple link
• usually inline and always one-directional
• Extended link
• more general

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Example
Resource which is an addressable unit nasg XML
Locator

• lta NAMElink14 HREFhttp//www.somesite/pub/doc
  4.htmlchap4gt
• points to an ltagt element in the document
  doc4.html lta NAMEchap4gt The targetlt/agt

Linking element
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Other Attributes a link element can specify

• href - locator
• inline - whether the content is a resource of the
  link
• role - role of the remote resource
• title - title of the renote resource
• content-role - role of the local resource
• content-title - title of local resource

Meaning eg. Bib, thesaurii
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Link Behaviour

• show
• new - resource displayed in new window
• replace - replace displayed in same window
• embed - resource embedded
• actuate
• auto - link is automatically traversed
• user - link is traversed by user action
• behavoir

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Simple Link Declaration

• lt!ELEMENT MySimpleLink ANYgt
• lt!ATTLIST MySimpleLink xmllink CDATA FIXED
  Simple href CDATA REQUIRED
  inline (truefalse) true role CDATA IMPLIED
  title CDATA IMPLIED show (embedreplacenew)
  IMPLIED actuate (autouser) IMPLIED
  behavior CDATA IMPLIED content-roleCDATA IMPL
  IED content-title CDATA IMPLIED

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Simple Link

• ltASimpleLink hrefhttp//www.somesite.com/diction
  ary/ shownew content-roleterm
  roledefinitiongtXMLlt/ASimpleLinkgt
• Simple links can be out-of-line

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Extended Links

• ltAnExtended inlinefalsegt ltALocator
  hrefsmith.xml roleEssay/gt ltAlocator
  hrefjones.xml roleRebuttal/gtlt/AnExtendedgt
• ltAnExtended inlinetruegt ltAnimalgtKangaroolt/Anim
  algt ltAlocator hrefkangaroo.gif rolePhoto
  shownew/gt ltAlocator hrefkangaroo.xml
  roleDescription shownew/gtlt/AnExtendedgt

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Extended Link Groups

• In the first extended link example,
  multidirectional links were asserted out of line.
  
• In such cases, it is useful for applications to
  know the documents that together constitute an
  interlinked group
• An extended link group may be used to store a
  list of links to other documents that make up
  such a group.

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Extended Link Groups
Each is a link

• lt!ELEMENT MyGroup (MyDocument)gtlt!ATTLIST
  MyGroup xmllink CDATA FIXED group steps
  CDATA IMPLIEDgtlt!ELEMENT MyDocument
  EMPTYgtlt!ATTLIST MyDocumentxmllink CDATA
  FIXED documenthref CDATA REQUIREDgt

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Attribute Remapping

• Sometimes attributes like role might clash with
  attributes otherwise defined for the element used
  as a linking element.
• For example, considerltperson rolePresentergtHi
  llary Clintonlt/persongt
• If we wanted to make this element a simple link
  to another document, we have a problem in that
  role is already used.

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Attribute remapping

• Xlink gets around this problem by letting you
  remap attribute names
• lt!ATTLIST person xmllink CDATA FIXED simple
  xmlattributes CDATA FIXED role LinkRole href
  CDATA REQUIREDgt
• This means that within person elements, the
  attribute LinkRole is used in place of role

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Attribute Remapping

• ltperson rolePresenter LinkRoleHomePagehref
  http//www.somesite.com/jlygt JJYlt/persongt

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XPointer

• What is an address?
• Often refers to a physical loocation
• An address in a general sense is any structured
  expression that helps to locate a particular
  object

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Locators

• Data which identify a resource.
• Normally URIs
• Xpointers can be used in conjunction with URIs to
  specify a more specific resource
• a fragment of a document

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Fragments in HTML

• followed by a string of characters is the
  fragment identifier in HTML
• In the target document, the fragment is indicated
  by an ltAgt element with an attribute NAME matching
  the fragment identifier

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Extended Pointers

• URI provides a means of addressing any document
  on the web
• an Xpointer provides a means for addressing any
  element or string of textual content in a given
  document
• URI identifies the containing resource
• Xpointer identifies the designated resource

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Documents as trees
MEMO
TO
FROM
MESSAGE
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Documents as trees
Root
MEMO
Parent
Child
TO
FROM
MESSAGE
Sibling
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Location Terms

• Absolute terms
• Relative terms
• Span terms
• Attribute Terms
• String Data Terms

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Xpointer Structure
MEMO
Root()
Child(1)
TO
FROM
MESSAGE
root().child(1)
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Xpointer Structure

• Root() is an example of an absolute location term
• child(1) is an example of a relative location
  term
• It selects an element on the basis of its
  relationship with a location resource expressed
  in the preceding part of XPointer

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Absolute Location Terms

• root() - the root of the document (assumed if no
  absolute term)
• origin() - the element asserting the link (the
  linking element)
• id(Name) the element with an ID attribute of Name
• html(Name) - the element of type ltAgt with
  attribute NAME of value Name

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Location Source

• Relative Location Terms like child() work in
  conjunction with a location source to indicate
  the designated resource
• An absolute location term is needed
• The process is iterative - their new location
  provides the location source for the next
  relative instruction

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Relative location terms

• Child(2)
• Keyword
• child
• Arguments
• (2)

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Relative Keywords

• child()
• descendent()
• ancestor()
• preceding() - preceding node
• following() - following node
• psibling() - preceding sibling
• fsibling() - following sibling

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Keywords

• Each relative keyword identifies a set of
  locations known as candidate locations
• child() for example identifies all the children
  of the location source
• More information is needed to which candidate
  locations in the designated resource are being
  addressed

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Instance Numbers

• Keyword(Instance Number)
• positive number
• negative number
• all

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Positive Instance Numbers
Location Source
1
5
2
3
4
6
Children
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Negative Instance Numbers
Location Source
-6
-2
-5
-4
-3
-1
Children
The candidate locations are counted from last to
first. ancestor(-1) is the root of the element
tree.
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Selection by Element

• Keyword(InstanceNumber, ElementName)
• child(3, DIV1).child(4,DIV2).child(29,P)
• descendant(-1,EXAMPLE)

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Selection by Node Type

• element
• pi
• comment
• text
• cdata
• all
• Only string data terms can follow a selection of
  pi, comment, text and cdata
• Ancestor, descendant and child are only
  applicable with element selection

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Selection by Attribute

• Keyword(InstanceNumber, ElementName, Attribute1,
  Value1)
• Keyword(InstanceNumber, ElementName, Attribute1,
  Value1, Attribute 2, Value2,..)
• Attribute can be
• AttributeName

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Selection by Attribute

• Value can be
• IMPLIED - no value specified and no default
• - any value even if defaulted
• Name - case insensitive value Name
• Name - case sensitive value Name

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Span Terms

• Span(XPointer1, XPointer2)
• This keyword locates a sub-resource starting at
  the beginning of the data selected by the first
  argument and continuing through to the end of the
  data selected by its second argument.
• Both elements are interpreted relative to the
  location source for the spanning location term
  itself
• id(a16).span(child(1),child(3))selects the first
  3 children of the element with ID a16

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Attribute Terms

• attr(Name)
• returns the value of the attribute with name Name
  on the element indicated by the location source
  of this term

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String Data Terms (I)

• String(InstanceNumber, String)
• String identifies the candidate strings within
  the location source and InstanceNumber selects
  which occurrence of the string is being addressed
• Example
• id(x37).string(3,the) - finds the third
  instance of the in the element with ID x37 and
  select the position just preceding it

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String Data Terms

• As with other instance numbers, InstanceNumber
  here can be negative (counting backwards from end
  of location source) or all.
• String can be null
• Example id(x37).string(3,)
• means the position immediately preceding the
  third character

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String Data Terms (II)

• string(InstanceNumber, String, Position)
• Position identifies an offset from the start of
  the candidate string(s) to the beginning of the
  desired final string match. A negative number
  counts left from end.
• id(x37).string(3,the,2) addresses the position
  immediately preceding the h (the second
  character) in the third occurrence of the in
  the element with ID x37

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String Data Terms (III)

• string(InstanceNumber, String, Position, Length)
• Length specifies the number of characters to
  address. If omitted, zero is assumed which means
  a position between characters is addressed.
• id(x37).string(3,the,2,1) - addresses the
  letter h in the third occurrence of the element
  with ID x37.