Chapter 2 Structured Web Documents in XML

1
Chapter 2Structured Web Documents in XML

• Grigoris Antoniou
• Frank van Harmelen

2
An HTML Example

• lth2gtNonmonotonic Reasoning Context-
• Dependent Reasoninglt/h2gt
• ltigtby ltbgtV. Mareklt/bgt and
• ltbgtM. Truszczynskilt/bgtlt/igtltbrgt
• Springer 1993ltbrgt
• ISBN 0387976892

3
The Same Example in XML

• ltbookgt
• lttitlegtNonmonotonic Reasoning
  Context- Dependent Reasoninglt/titlegt
• ltauthorgtV. Mareklt/authorgt
• ltauthorgtM. Truszczynskilt/authorgt
• ltpublishergtSpringerlt/publishergt
• ltyeargt1993lt/yeargt
• ltISBNgt0387976892lt/ISBNgt
• lt/bookgt

4
HTML versus XML Similarities

• Both use tags (e.g. lth2gt and lt/yeargt)
• Tags may be nested (tags within tags)
• Human users can read and interpret both HTML and
  XML representations quite easily
• But how about machines?

5
Problems with Automated Interpretation of HTML
Documents

• An intelligent agent trying to retrieve the names
• of the authors of the book
• Authors names could appear immediately after the
  title
• or immediately after the word by
• Are there two authors?
• Or just one, called V. Marek and M.
  Truszczynski?

6
HTML vs XML Structural Information

• HTML documents do not contain structural
  information pieces of the document and their
  relationships.
• XML more easily accessible to machines because
• Every piece of information is described.
• Relations are also defined through the nesting
  structure.
• E.g., the ltauthorgt tags appear within the ltbookgt
  tags, so they describe properties of the
  particular book.

7
HTML vs XML Structural Information (2)

• A machine processing the XML document would be
  able to deduce that
• the author element refers to the enclosing book
  element
• rather than by proximity considerations
• XML allows the definition of constraints on
  values
• E.g. a year must be a number of four digits

8
HTML vs XML Formatting

• The HTML representation provides more than the
  XML representation
• The formatting of the document is also described
• ?he main use of an HTML document is to display
  information it must define formatting
• XML separation of content from display
• same information can be displayed in different
  ways

9
HTML vs XML Another Example

• In HTML
• lth2gtRelationship matter-energylt/h2gt
• ltigt E M c2 lt/igt
• In XML
• ltequationgt
• ltmeaninggtRelationship matter
• energylt/meaninggt
• ltleftsidegt E lt/leftsidegt
• ltrightsidegt M c2 lt/rightsidegt
• lt/equationgt

10
HTML vs XML Different Use of Tags

• In both HTML docs same tags
• In XML completely different
• HTML tags define display color, lists
• XML tags not fixed user definable tags
• XML meta markup language language for defining
  markup languages

11
XML Vocabularies

• Web applications must agree on common
  vocabularies to communicate and collaborate
• Communities and business sectors are defining
  their specialized vocabularies
• mathematics (MathML)
• bioinformatics (BSML)
• human resources (HRML)

12
Lecture Outline

• Introduction
• Detailed Description of XML
• Structuring
• DTDs
• XML Schema
• Namespaces
• Accessing, querying XML documents XPath
• Transformations XSLT

13
The XML Language

• An XML document consists of
• a prolog
• a number of elements
• an optional epilog (not discussed)

14
Prolog of an XML Document

• The prolog consists of
• an XML declaration and
• an optional reference to external structuring
  documents
• lt?xml version"1.0" encoding"UTF-16"?gt
• lt!DOCTYPE book SYSTEM "book.dtd"gt

15
XML Elements

• The things the XML document talks about
• E.g. books, authors, publishers
• An element consists of
• an opening tag
• the content
• a closing tag
• ltlecturergtDavid Billingtonlt/lecturergt

16
XML Elements (2)

• Tag names can be chosen almost freely.
• The first character must be a letter, an
  underscore, or a colon
• No name may begin with the string xml in any
  combination of cases
• E.g. Xml, xML

17
Content of XML Elements

• Content may be text, or other elements, or
  nothing
• ltlecturergt
• ltnamegtDavid Billingtonlt/namegt
• ltphonegt 61 - 7 - 3875 507 lt/phonegt
• lt/lecturergt
• If there is no content, then the element is
  called empty it is abbreviated as follows
• ltlecturer/gt for ltlecturergtlt/lecturergt

18
XML Attributes

• An empty element is not necessarily meaningless
• It may have some properties in terms of
  attributes
• An attribute is a name-value pair inside the
  opening tag of an element
• ltlecturer name"David Billington" phone"61 - 7
  - 3875 507"/gt

19
XML Attributes An Example

• ltorder orderNo"23456" customer"John Smith"
• date"October 15, 2002"gt
• ltitem itemNo"a528" quantity"1"/gt
• ltitem itemNo"c817" quantity"3"/gt
• lt/ordergt

20
The Same Example without Attributes

• ltordergt
• ltorderNogt23456lt/orderNogt
• ltcustomergtJohn Smithlt/customergt
• ltdategtOctober 15, 2002lt/dategt
• ltitemgt
• ltitemNogta528lt/itemNogt
• ltquantitygt1lt/quantitygt
• lt/itemgt
• ltitemgt
• ltitemNogtc817lt/itemNogt
• ltquantitygt3lt/quantitygt
• lt/itemgt
• lt/ordergt

21
XML Elements vs Attributes

• Attributes can be replaced by elements
• When to use elements and when attributes is a
  matter of taste
• But attributes cannot be nested

22
Well-Formed XML Documents

• Syntactically correct documents
• Some syntactic rules
• Only one outermost element (called root element)
• Each element contains an opening and a
  corresponding closing tag
• Tags may not overlap
• ltauthorgtltnamegtLee Honglt/authorgtlt/namegt
• Attributes within an element have unique names
• Element and tag names must be permissible

23
The Tree Model of XML Documents An Example

• ltemailgt
• ltheadgt
• ltfrom name"Michael Maher"
• address"michaelmaher_at_cs.gu.edu.au"/gt
• ltto name"Grigoris Antoniou"
• address"grigoris_at_cs.unibremen.de"/gt
• ltsubjectgtWhere is your draft?lt/subjectgt
• lt/headgt
• ltbodygt
• Grigoris, where is the draft of the paper you
  promised me
• last week?
• lt/bodygt
• lt/emailgt

24
The Tree Model of XML Documents An Example (2)
25
The Tree Model of XML Docs

• The tree representation of an XML document is an
  ordered labeled tree
• There is exactly one root
• There are no cycles
• Each non-root node has exactly one parent
• Each node has a label.
• The order of elements is important
• but the order of attributes is not important

26
Lecture Outline

• Introduction
• Detailed Description of XML
• Structuring
• DTDs
• XML Schema
• Namespaces
• Accessing, querying XML documents XPath
• Transformations XSLT

27
Structuring XML Documents

• Define all the element and attribute names that
  may be used
• Define the structure
• what values an attribute may take
• which elements may or must occur within other
  elements, etc.
• If such structuring information exists, the
  document can be validated

28
Structuring XML Dcuments (2)

• An XML document is valid if
• it is well-formed
• respects the structuring information it uses
• There are two ways of defining the structure of
  XML documents
• DTDs (the older and more restricted way)
• XML Schema (offers extended possibilities)

29
XML Schema

• Significantly richer language for defining the
  structure of XML documents
• Tts syntax is based on XML itself
• not necessary to write separate tools
• Reuse and refinement of schemas
• Expand or delete already existent schemas
• Sophisticated set of data types, compared to DTDs
  (which only supports strings)

30
XML Schema (2)

• An XML schema is an element with an opening tag
  like
• ltschema "http//www.w3.org/2000/10/XMLSchema"
• version"1.0"gt
• Structure of schema elements
• Element and attribute types using data types

31
Element Types

• ltelement name"email"/gt
• ltelement name"head" minOccurs"1"
  maxOccurs"1"/gt
• ltelement name"to" minOccurs"1"/gt
• Cardinality constraints
• minOccurs"x" (default value 1)
• maxOccurs"x" (default value 1)
• Generalizations of ,?, offered by DTDs

32
Attribute Types

• ltattribute name"id" type"ID use"required"/gt
• lt attribute name"speaks" type"Language"
• use"default" value"en"/gt
• Existence use"x", where x may be optional or
  required
• Default value use"x" value"...", where x may
  be default or fixed

33
Data Types

• There is a variety of built-in data types
• Numerical data types integer, Short etc.
• String types string, ID, IDREF, CDATA etc.
• Date and time data types time, Month etc.
• There are also user-defined data types
• simple data types, which cannot use elements or
  attributes
• complex data types, which can use these

34
Data Types (2)

• Complex data types are defined from already
  existing data types by defining some attributes
  (if any) and using
• sequence, a sequence of existing data type
  elements (order is important)
• all, a collection of elements that must appear
  (order is not important)
• choice, a collection of elements, of which one
  will be chosen

35
A Data Type Example

• ltcomplexType name"lecturerType"gt
• ltsequencegt
• ltelement name"firstname" type"string"
• minOccurs"0 maxOccurs"unbounded"/gt
• ltelement name"lastname" type"string"/gt
• lt/sequencegt
• ltattribute name"title" type"string"
  use"optional"/gt
• lt/complexTypegt

36
XML Schema The Email Example

• ltelement name"email" type"emailType"/gt
• ltcomplexType name"emailType"gt
• ltsequencegt
• ltelement name"head" type"headType"/gt
• ltelement name"body" type"bodyType"/gt
• lt/sequencegt
• lt/complexTypegt

37
XML Schema The Email Example (2)

• ltcomplexType name"headType"gt
• ltsequencegt
• ltelement name"from" type"nameAddress"/gt
• ltelement name"to" type"nameAddress"
• minOccurs"1" maxOccurs"unbounded"/gt
• ltelement name"cc" type"nameAddress"
• minOccurs"0" maxOccurs"unbounded"/gt
• ltelement name"subject" type"string"/gt
• lt/sequencegt
• lt/complexTypegt

38
XML Schema The Email Example (3)

• ltcomplexType name"nameAddress"gt
• ltattribute name"name" type"string"
  use"optional"/gt
• ltattribute name"address" type"string"
  use"required"/gt
• lt/complexTypegt
• Similar for bodyType

39
Lecture Outline

• Introduction
• Detailed Description of XML
• Structuring
• DTDs
• XML Schema
• Namespaces
• Accessing, querying XML documents XPath
• Transformations XSLT

40
Namespaces

• An XML document may use more than one DTD or
  schema
• Since each structuring document was developed
  independently, name clashes may appear
• The solution is to use a different prefix for
  each DTD or schema
• prefixname

41
An Example

• ltvuinstructors xmlnsvu"http//www.vu.com/empDT
  D"
• xmlnsgu"http//www.gu.au/empDTD"
• xmlnsuky"http//www.uky.edu/empDTD"gt
• ltukyfaculty ukytitle"assistant professor"
• ukyname"John Smith"
• ukydepartment"Computer Science"/gt
• ltguacademicStaff gutitle"lecturer"
• guname"Mate Jones"
• guschool"Information Technology"/gt
• lt/vuinstructorsgt

42
Namespace Declarations

• Namespaces are declared within an element and can
  be used in that element and any of its children
  (elements and attributes)
• A namespace declaration has the form
• xmlnsprefix"location"
• location is the address of the DTD or schema
• If a prefix is not specified xmlns"location"
  then the location is used by default

43
Lecture Outline

• Introduction
• Detailed Description of XML
• Structuring
• DTDs
• XML Schema
• Namespaces
• Accessing, querying XML documents XPath
• Transformations XSLT

44
Addressing and Querying XML Documents

• In relational databases, parts of a database can
  be selected and retrieved using SQL
• Same necessary for XML documents
• Query languages XQuery, XQL, XML-QL
• The central concept of XML query languages is a
  path expression
• Specifies how a node or a set of nodes, in the
  tree representation of the XML document can be
  reached

45
XPath

• XPath is core for XML query languages
• Language for addressing parts of an XML document.
  
• It operates on the tree data model of XML
• It has a non-XML syntax

46
Types of Path Expressions

• Absolute (starting at the root of the tree)
• Syntactically they begin with the symbol /
• It refers to the root of the document (situated
  one level above the root element of the document)
• Relative to a context node

47
An XML Example

• ltlibrary location"Bremen"gt
• ltauthor name"Henry Wise"gt
• ltbook title"Artificial Intelligence"/gt
• ltbook title"Modern Web Services"/gt
• ltbook title"Theory of Computation"/gt
• lt/authorgt
• ltauthor name"William Smart"gt
• ltbook title"Artificial Intelligence"/gt
• lt/authorgt
• ltauthor name"Cynthia Singleton"gt
• ltbook title"The Semantic Web"/gt
• ltbook title"Browser Technology Revised"/gt
• lt/authorgt
• lt/librarygt

48
Tree Representation
49
Examples of Path Expressions in XPath

• Address all author elements
• /library/author
• Addresses all author elements that are children
  of the library element node, which resides
  immediately below the root
• /t1/.../tn, where each ti1 is a child node of
  ti, is a path through the tree representation

50
Examples of Path Expressions in XPath (2)

• Address all author elements
• //author
• Here // says that we should consider all elements
  in the document and check whether they are of
  type author
• This path expression addresses all author
  elements anywhere in the document

51
Examples of Path Expressions in XPath (3)

• Address the location attribute nodes within
  library element nodes
• /library/_at_location
• The symbol _at_ is used to denote attribute nodes

52
Examples of Path Expressions in XPath (4)

• Address all title attribute nodes within book
  elements anywhere in the document, which have the
  value Artificial Intelligence
• //book/_at_title"Artificial Intelligence"

53
Examples of Path Expressions in XPath (5)

• Address all books with title Artificial
  Intelligence
• /book_at_title"Artificial Intelligence"
• Test within square brackets a filter expression
• It restricts the set of addressed nodes.
• Difference with query 4.
• Query 5 addresses book elements, the title of
  which satisfies a certain condition.
• Query 4 collects title attribute nodes of book
  elements

54
Tree Representation of Query 4
55
Tree Representation of Query 5
56
Examples of Path Expressions in XPath (6)

• Address the first author element node in the XML
  document
• //author1
• Address the last book element within the first
  author element node in the document
• //author1/booklast()
• Address all book element nodes without a title
  attribute
• //booknot _at_title

57
Lecture Outline

• Introduction
• Detailed Description of XML
• Structuring
• DTDs
• XML Schema
• Namespaces
• Accessing, querying XML documents XPath
• Transformations XSLT

58
Displaying XML Documents

• ltauthorgt
• ltnamegtGrigoris Antonioult/namegt
• ltaffiliationgtUniversity of Bremenlt/affiliationgt
• ltemailgtga_at_tzi.delt/emailgt
• lt/authorgt
• may be displayed in different ways
• Grigoris Antoniou Grigoris Antoniou
• University of Bremen University of Bremen
• ga_at_tzi.de ga_at_tzi.de

59
Style Sheets

• Style sheets can be written in various languages
• E.g. CSS2 (cascading style sheets level 2)
• XSL (extensible stylesheet language)
• XSL includes
• a transformation language (XSLT)
• a formatting language
• Both are XML applications

60
XSL Transformations (XSLT)

• XSLT specifies rules with which an input XML
  document is transformed to
• another XML document
• an HTML document
• plain text
• The output document may use the same DTD or
  schema, or a completely different vocabulary
• XSLT can be used independently of the formatting
  language

61
XSLT (2)

• Move data and metadata from one XML
  representation to another
• XSLT is chosen when applications that use
  different DTDs or schemas need to communicate
• XSLT can be used for machine processing of
  content without any regard to displaying the
  information for people to read.
• In the following we use XSLT only to display XML
  documents

62
XSLT Transformation into HTML

• ltxsltemplate match"/author"gt
• lthtmlgt
• ltheadgtlttitlegtAn authorlt/titlegtlt/headgt
• ltbody bgcolor"white"gt
• ltbgtltxslvalue-of select"name"/gtlt/bgtltbrgt
• ltxslvalue-of select"affiliation"/gtltbrgt
• ltigtltxslvalue-of select"email"/gtlt/igt
• lt/bodygt
• lt/htmlgt
• lt/xsltemplategt

63
Style Sheet Output

• lthtmlgt
• ltheadgtlttitlegtAn authorlt/titlegtlt/headgt
• ltbody bgcolor"white"gt
• ltbgtGrigoris Antonioult/bgtltbrgt
• University of Bremenltbrgt
• ltigtga_at_tzi.delt/igt
• lt/bodygt
• lt/htmlgt

64
Observations About XSLT

• XSLT documents are XML documents
• XSLT resides on top of XML
• The XSLT document defines a template
• In this case an HTML document, with some
  placeholders for content to be inserted
• xslvalue-of retrieves the value of an element
  and copies it into the output document
• It places some content into the template

65
A Template

• lthtmlgt
• ltheadgtlttitlegtAn authorlt/titlegtlt/headgt
• ltbody bgcolor"white"gt
• ltbgt...lt/bgtltbrgt
• ...ltbrgt
• ltigt...lt/igt
• lt/bodygt
• lt/htmlgt

66
Auxiliary Templates

• We have an XML document with details of several
  authors
• It is a waste of effort to treat each author
  element separately
• In such cases, a special template is defined for
  author elements, which is used by the main
  template

67
Example of an Auxiliary Template

• ltauthorsgt
• ltauthorgt
• ltnamegtGrigoris Antonioult/namegt
• ltaffiliationgtUniversity of Bremenlt/affiliationgt
• ltemailgtga_at_tzi.delt/emailgt
• lt/authorgt
• ltauthorgt
• ltnamegtDavid Billingtonlt/namegt
• ltaffiliationgtGriffith Universitylt/affiliationgt
• ltemailgtdavid_at_gu.edu.netlt/emailgt
• lt/authorgt
• lt/authorsgt

68
Example of an Auxiliary Template (2)

• ltxsltemplate match"/"gt
• lthtmlgt
• ltheadgtlttitlegtAuthorslt/titlegtlt/headgt
• ltbody bgcolor"white"gt
• ltxslapply-templates select"authors"/gt
• lt!-- Apply templates for AUTHORS children
  --gt
• lt/bodygt
• lt/htmlgt
• lt/xsltemplategt

69
Example of an Auxiliary Template (3)

• ltxsltemplate match"authors"gt
• ltxslapply-templates select"author"/gt
• lt/xsltemplategt
• ltxsltemplate match"author"gt
• lth2gtltxslvalue-of select"name"/gtlt/h2gt
• Affiliationltxslvalue-of
• select"affiliation"/gtltbrgt
• Email ltxslvalue-of select"email"/gt
• ltpgt
• lt/xsltemplategt

70
Multiple Authors Output

• lthtmlgt
• ltheadgtlttitlegtAuthorslt/titlegtlt/headgt
• ltbody bgcolor"white"gt
• lth2gtGrigoris Antonioult/h2gt
• Affiliation University of Bremenltbrgt
• Email ga_at_tzi.de
• ltpgt
• lth2gtDavid Billingtonlt/h2gt
• Affiliation Griffith Universityltbrgt
• Email david_at_gu.edu.net
• ltpgt
• lt/bodygt
• lt/htmlgt

71
Explanation of the Example

• xslapply-templates element causes all children
  of the context node to be matched against the
  selected path expression
• E.g., if the current template applies to /, then
  the element xslapply-templates applies to the
  root element
• I.e. the authors element (/ is located above the
  root element)
• If the current context node is the authors
  element, then the element xslapply-templates
  select"author" causes the template for the
  author elements to be applied to all author
  children of the authors element

72
Explanation of the Example (2)

• It is good practice to define a template for each
  element type in the document
• Even if no specific processing is applied to
  certain elements, the xslapply-templates element
  should be used
• E.g. authors
• In this way, we work from the root to the leaves
  of the tree, and all templates are applied

73
Summary

• XML is a metalanguage that allows users to define
  markup
• XML separates content and structure from
  formatting
• XML is the de facto standard for the
  representation and exchange of structured
  information on the Web
• XML is supported by query languages

74
Points for Discussion in Subsequent Chapters

• The nesting of tags does not have standard
  meaning
• The semantics of XML documents is not accessible
  to machines, only to people
• Collaboration and exchange are supported if there
  is underlying shared understanding of the
  vocabulary
• XML is well-suited for close collaboration, where
  domain- or community-based vocabularies are used
• It is not so well-suited for global communication.