XML Technology in E-Commerce

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XML Technology in E-Commerce
Lecture 5 XPath, XLink
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Course Announcements
Examination XTEC 2000/2001
The list for registration is available at INF
room 5047 (Joke Lammerink). Lecture on June 12
will be in CC3. Additional information can be
found in Announcements at URL http//trese.cs.utw
ente.nl/courses/xtec
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Lecture Outline

• XPath
• Purpose
• Data Model
• Location Path Syntax
• Location Path Evaluation and Examples
• XLink
• HTML Links
• Requirements for Linking
• Extended Link Model
• XML representation
• Simple Links

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Overview
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XPathPurpose

• Specifies string-based, non-XML language for
  expressions
• Addresses parts in XML documents
• Provides functions for manipulation of strings,
  numbers and booleans
• Specification address - http//www.w3.org/TR/xpath
  (Recommendation since 16 November 1999)

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XPathApplications

• Specification of patterns of nodes in XML
  documents (used in XSLT templates)
• Specification of points (addresses) in XML
  documents used as anchors for links (XPointer and
  XLink)
• Querying and extracting parts of XML documents
• XPath expressions can be values of XML
  attributes
• XPath expressions can be a part of URIs

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Data Model

• XML document is modeled as a tree of nodes
• Node types
• root nodes
• element nodes
• text nodes
• attribute nodes
• namespace nodes
• processing instruction nodes
• comment nodes
• Nodes are ordered
• Examples - Deitel 11.2, fig. 11.2, page 300

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XPathExample

• Simple DTD
• lt!ELEMENT book (title, chapter) gt
• lt!ELEMENT chapter (title, section) gt
• lt!ELEMENT section (section para) gt
• Example XPath expression that selects element
  nodes
• childchapter/descendantpara
• Selects all paragraphs contained in the chapters
  of the book (provided that the book is the
  context node)

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Expressions

• Expressions are the primary syntactic construct
  in XPath
• Expression evaluation yields object of one of the
  following types
• node set
• boolean
• number
• string
• Location path - expression that selects a set of
  nodes. The result is a node set

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Location path

• Example
• childchapter/descendantpara
• Evaluation is always made against a particular
  context node.
• The example selects all paragraph elements that
  have ancestor among the chapter children of the
  context node

Context node
.
chapter
chapter
para
para
para
para
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Location Path Syntax
childchapter / descendantpara
Location path
Location step
Location step
child para position()1
Location step
Axis
Node test
Predicates

• Axis specifies the tree relationship between the
  context node and the nodes that will be selected
  by the step
• Node test specifies the node type to be
  selected
• Predicate refines the selection

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Axes

• Axes
• self
• parent
• child
• ancestor
• ancestor-or-self
• descendant
• descendant-or-self

• following-sibling
• preceding-sibling
• following
• preceding
• attribute
• namespace

• Axis can be forward or reverse axis. See Deitel
  11.3.1, fig. 11.6, page 305.

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Axes
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Axes
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Node Tests

• Axes have principal node type
• Attribute type for attribute axis
• Namespace type for namespace axis
• Element type for others
• Node tests
• - true for any node of the principal node type
• node() - true for any node of any type
• text() - true for any text node
• comment()
• processing-instruction()
• node name, e.g. chapter

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Predicates

• Predicates refine the selected node set in a
  location step
• Predicates are evaluated sequentially from left
  to right
• Predicates always result in true or false
• Examples
• paraposition()3, para3
• paralast()
• paraid(id_string)
• paraattributetype"warning"

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Location Path Evaluation

• The starting point is the context node. It is
  specified through additional, application
  specific mechanism
• For a given location step and given context node
  a node set is generated by applying axis
  information, node test and predicates
• Each node in the resulting node set becomes a
  context node for the next step. The result is an
  union of the nodes sets generated for each
  context node
• Expression value is the node set generated by the
  last location step

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Location PathAdditional Details

• Absolute and relative location path
• relative childchapter/descendantpara
• absolute / childchapter/descendantpara
• Abbreviated Syntax
• If no axis is specified, child is the default
• attributename _at_name
• selfnode() . (selects the context node)
• parent ..
• /descendant-or-selfnode()/ //
• div/descendant-or-selfnode()/childpara
  div//para

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Location PathAbbreviation Syntax Examples

• /para - selects all para grandchildren of the
  context node
• //olist/item - selects all item elements that
  have olist parent element
• ../_at_lang - selects the lang attribute of the
  parent of the context node
• para_at_type"warning"5 - selects the fifth para
  child element of the context node that has type
  attribute with value warning
• para5 _at_type"warning" - selects the fifth
  para child of context node if it has attribute
  type with value warning

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XPathSummary

• XPath defines syntax for expressions used in
  other XML specifications (XSLT, XPointer)
• XPath defines a tree-like logical model of XML
  documents
• Location paths are expressions that select a set
  of nodes
• Software support MSXML Parser, Apache XML
  Project,

Read Deitel 11 Assignment Deitel Ex 11.3, page
316
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Lecture Outline

• XPath
• Purpose
• Data Model
• Location Path Syntax
• Location Path Evaluation and Examples
• XLink
• HTML Links
• Requirements for Linking
• Extended Link Model
• XML representation
• Simple Links

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Linking

• Linking in the context of WWW
• Link is a relationship between resources
• Resource - any addressable unit of information or
  service on the Web
• Examples - HTML pages, images, documents,
  programs (services), database queries
• URIs are used for addressing a resource
• Links are asserted by linking elements
• Example - hyperlinks in HTML

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HTML Links
Web Resource
Web page

• Inline link - the link is embedded in the one
  end
• Dedicated link element - A element
• Only specific part of the page can be addressed -
  marked by ltA nameanchor/gt element
• Links have fixed behavior
• Links have only two ends

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Requirements for Linking

• Addressing - any element of an XML document must
  be addressable. Also, document authors must be
  able to select single character or portion of
  text
• Relationships between more than two resources
• Any element in an XML document must be able to
  assert a link
• Attaching external links without resource
  modification must be possible (so called third
  party links)
• Separation of link semantics from link type
• Support for the basic link semantics (inclusion,
  expansion)

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XLink

• Specifies language for link description used in
  XML documents
• Satisfies the previously formulated requirements
• Provides support for simple links (similar to
  HTML links) and for more sophisticated links
• Depends on other standard for addressing
  XPointer
• Specification address
• http//www.w3.org/TR/2000/PR-xlink-20001220/
  (Status proposed recommendation)

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XLinkBasic Terms

• XLink link is an explicit relationship between
  resources
• The link is made explicit by a linking element
• Using or following a link is called traversal. It
  always involves two resources
• Information about how to traverse a pair of
  resources is called arc
• Local resource - the linking element by itself or
  a child of the linking element
• Remote resource - addressed by a URI

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XLinkMarkup Design

• XLink defines a namespace with URI
• http//www.w3.org/1999/xlink
• The namespace contains the following attributes
• type, href, role, arcrole, title, show, actuate,
  label, from, and to
• XLink defines two kinds of links
• Extended
• Simple

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Extended Link Model
Arc
Locator to remote resource
Locator to remote resource
LabelL3
LabelL4
Extended link
Local resource
Arc
LabelL5
LabelL1
LabelL2
Locator to remote resource
Locator to remote resource
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Extended LinkXML Representation

• Example link that represents the relationship
  between student, course and students grade
• Representation of the link element
• ltcourseload
• xmlnsxlinkhttp//www.w3.org/1999/xlink
• xlinktypeextendedgt
• ...elements for student, course and grade...
• traversal information as arc elements
• lt/courseloadgt

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XML RepresentationExternal Resources

• Student and course information are external
  resources represented by a locator type element
• ltstudent
• xlinktypelocator
• xlinkhrefhttp//some.site/student007
• xlinklabelstudent/gt
• ltcourse
• xlinktypelocator
• xlinkhrefhttp//some.site/course6
• xlinklabelcourse /gt

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XML RepresentationLocal Resources

• Grade information is local resource represented
  by resource type element
• ltgrade
• xlinktyperesource
• xlinklabelgradegt5.5lt/gradegt

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XML RepresentationArcs

• We can specify two arcs between the student and
  the course and the student and his grade
• ltarcEl xlinktypearc
• xlinkfromstudent
• xlinktocourse
• xlinktitlepartcipates/gt
• ltarcEl xlinktypearc
• xlinkfromstudent
• xlinktograde
• xlinktitlehas grade/gt

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XML RepresentationOther XLink Attributes

• Semantic attributes
• role can be applied to extended, locator and
  resource type elements. It asserts a property
  that describes the semantics of the resource. The
  value is URI
• arcrole used on arc type elements
• title provides human readable information
• ltarcEl xlinktypearc
• xlinkfromstudent
• xlinktograde
• xlinkarcrolehttp//some.site/arcinfo
  .xml
• xlinktitlestudents grade/gt

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Simple Links

• Simple links can be perceived as a special kind
  of extended link, with some limitations imposed
• associates exactly two resources one local and
  one remote
• specifies an arc from local to remote resource
• provides a short syntax for the equivalent
  extended link
• ltstudent xlinktypesimple
• xlinkhrefhttp//some.site.com/student.h
  tmlgt
• John Smith
• lt/studentgt

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Behavioral Attributes

• Describe the intended behavior for simple and arc
  type elements
• show attribute
• Specifies the desired presentation of the ending
  resource
• Values new, replace, embed, other, none
• actuate attribute
• Specifies the desired timing of traversal
• Values onLoad, onRequest, other, none

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XLink and DTDs

• In XLink linking elements are asserted via a set
  of attributes
• The syntax is rather verbose, a large set of
  attributes must be specified
• DTD can be used to provide default and fixed
  values for attributes. Attributes can be omitted
  in instance documents
• See Deitel 14.3

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XLinkSummary

• XLink provides powerful mechanism for links
  definition in XML documents
• Multi-ended, third party links with additional
  meta information through extended link type
• HTML like links through simple link type
• XLink depends on XPointer as a mechanism for
  addressing elements and points in XML documents

Read Deitel 14.1-14.3, skip 14.4-14.6 Assignment
Deitel Ex 14.3, page 394. For some hints see the
course web site.