CIS336 Website design, implementation and management also Semester 2 of CIS219, CIS221 and IT226

1
CIS336Website design, implementation and
management(also Semester 2 of CIS219, CIS221 and
IT226)

• Lecture 2
• XML Documents
• (Based on Møller and Schwartzbach, 2006, Chapter
  2)

David Meredith d.meredith_at_gold.ac.uk www.titanmus
ic.com/teaching/cis336-2006-7.html
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What is XML?

• XML Extensible Markup Language
• XML is a framework for defining markup languages
• It is a subset of SGML
• No fixed collection of tags like HTML
• XML lets us define our own tags, designed for the
  kind of information we want to represent
• Each XML language is targeted at a particular
  application domain (e.g., chemical formulae,
  legal documents, religious texts, music notation)
• All XML languages use the same basic markup
  syntax and can benefit from a common set of
  generic tools for processing documents
• XML is intended to be the future of all
  structured information
• Including all information previously stored in
  relational databases
• Prompted development of powerful query language,
  XQuery, which is designed to replace SQL

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XML and HTML

• XML is not an extensible markup language
• It is not a single markup language at all!
• It defines a class of markup languages and a
  common notation that any markup language can use
• XML is not an extension of or replacement for
  HTML
• HTML should ideally be a particular application
  of XML, i.e., an XML language
• HTML doesnt fit directly into the XML framework
• So W3C designed XHTML which is an XML-compliant
  variant of HTML

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What XML doesnt do

• XML specification says nothing about the
  semantics of the markup tags
• Specified by the individual XML languages
• XML says nothing about how an XML document should
  be rendered in a browser
• Can specify an XML stylesheet (using XSL) that
  defines how each tag should be rendered in a
  browser

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XML and interoperability

• XML is designed to be inherently
  internationalized and platform independent
• All XML documents must use the Unicode character
  set
• contains all international characters, past and
  present
• XML also deals with different line-break
  encodings on different platforms by normalising
  all such breaks to the same sequence of
  characters
• Defined by a public, free specification which can
  be viewed and implemented by anyone

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Development of XML(see http//www.w3.org/xml/)

• XML development started in mid 1990s
• Initial draft specification of XML produced in
  November 1996
• Pure subset of SGML
• XML 1.0 became a W3C recommendation in February
  1998
• Latest version of XML 1.0 is the Fourth Edition,
  published in August 2006
• Available here http//www.w3.org/TR/xml/
• XML 1.1 became a W3C recommendation in February
  1998
• Latest version of XML 1.1 is the Second Edition,
  published in August 2006
• Available here http//www.w3.org/TR/xml11/
• XML 1.1 incorporates recent and future changes in
  the Unicode standard and introduces the idea of
  normalization of character encodings
• XML 1.1 is not fully compatible with XML 1.0
• Many applications written in XML 1.0, so many
  keep with this standard in preference to XML 1.1
• Perhaps standard should have been simpler
• But now huge amount of technology and information
  that relies on the standard so core features will
  probably not change

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Limitations of HTML

• HTML tags used here to indicate structure of
  recipes
• But no way of enforcing correct format for data
• Cannot easily sort recipes or select a subset
  with particular features
• Cannot easily perform computations on the recipe
  data
• HTML is not a good language for making a database
• HTML is designed specifically for the hypertext
  domain, not other domains (like recipes)
• In HTML, syntax and semantics (or structure and
  layout) are intertwined, even if we use cascading
  style sheets
• For data storage, we want to store data with
  logical structure only so that it can be
  processed and formatted in all sorts of different
  ways

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Recipes in XML

• Can define our own recipe markup language in XML,
  RecipeML
• Tags in RecipeML directly correspond to concepts
  in the recipe domain
• e.g., recipe, ingredient, preparation step, etc.
• Similar to identifying key domain abstractions in
  OO software engineering
• XML-ification is the process of developing an XML
  representation for a particular domain
• Essential information is in attributes and text
  between tags
• Tags indicate structure only, not layout
• Tags provide meta-information
• For any domain, usually many possible markup
  designs, e.g.,
• could break up date into day, month and year
• Could enclose ingredient list in ltingredientsgt
  tag
• XML is semi-structured
• Can choose level of detail at which to mark up
  text
• Often have to choose between using attributes or
  elements, e.g.,
• name, amount, unit attributes could be tags

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XML Language Syntax, Semantics Use as a Database

• Define syntax of an XML language (like RecipeML,
  XHTML) using XML Schema
• i.e., what tags are allowed and where they can
  appear in the XML document
• e.g., preparation tag can only contain step tags
  and step tag can only contain text
• Define semantics of an XML language using XSLT
• Transforms XML into appropriate XHTML file that
  can be displayed in a web browser
• Use XQuery to search recipe collection and
  extract all sorts of information from it
• For more specialized applications can use a
  general-purpose programming language like Java
• e.g., to write a web-based recipe editor, might
  need to use Servlets and JSP

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XML Trees

• Each XML document represents a hierarchical
  structure called an XML tree
• Various ways of describing the structure of an
  XML tree, but here will adopt XPath Data Model
• XML tree can be represented graphically with root
  node at the top (A in top diagram)
• Edges between nodes represent parent-child
  relationships
• A is parent of B B is child of A in top diagram
• Content of a node is sequence of child nodes
• Sequence (B, C, D) is content of A in top diagram
• Leaf node is one with no children
• E, F, C and D are leaf nodes in top diagram
• XML tree is ordered so ordering of children of a
  node is important
• Two trees at right are not equivalent in XML
• Siblings of a node are the other nodes that are
  children of the parent of the node
• C and D are siblings of B in top diagram
• Ancestors of a node include its parent, its
  parents parent, etc. back to root node
• A and B are the ancestors of F in top diagram
• Descendants of a node include its children, its
  childrens children and so on
• Descendants of A are B, E, F, C and D in top
  diagram

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XML Tree Node Types

• In XPath data model, XML tree is a special
  ordered tree in which each node is one of the
  following types
• Text nodes
• Plain text, not an element, raw data
• Always leaf nodes (i.e., cannot have child nodes)
• Cannot have two consecutive sibling text nodes
• Node labelled with text
• Element nodes
• Logical grouping of information represented by
  descendants
• Node labelled with element name
• Attribute nodes
• Parent is always an element node
• Specify global properties of parent element
• Each attribute is a name-value pair where value
  is always a text string
• Names of attributes of a given element must be
  distinct
• Comment nodes
• Always a leaf node
• Always contains a text string
• Processing instruction nodes
• Used to convey specialized meta-information to
  XML processing tools

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Tree view of XML recipe

• Some subtleties
• Parent of each attribute node is an element node,
  but children of an element node do not include
  attributes
• Attributes of an element node form an unordered
  set but children of an element form an ordered
  set (or sequence)
• Document ordering of nodes
• Node x occurs before node y if its start tag
  occurs earlier in the textual representation of
  the document than that of y
• Parent precedes children, siblings ordered
  left-to-right
• Tree-view conventions
• Root node drawn as a circle
• Element nodes drawn as rounded boxes
• Text nodes drawn as parallelograms
• Attribute nodes drawn as rectangles containing
  name value pairs

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Viewing tree structure in a browser

• If you load an XML file in a modern browser and
  the file has no associated style sheet, then its
  tree structure is shown

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Other XML data models

• Foregoing is how XML document described in XPath
  data model
• In DOM (Document Object Model) and JDOM (Java
  Document Object Model), an XML tree can contain
  other types of nodes such as
• Document Type nodes corresponding to Document
  Type Definitions (DTDs)
• Entity reference nodes which are references to
  XML fragments defined in the DTD schema
• CDATA nodes which are a special type of text node

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Issues in designing an XML language

• Text nodes usually contain the actual information
  or data
• Elements and their attributes used to convey
  logical structure and meta-information about the
  data
• Difference between information and
  meta-information not always obvious
• Some languages use elements for everything
• Others use attributes for everything so that all
  elements are empty
• Most languages use a mixture of elements and
  attributes

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Textual representation of XML documents

• XML document is a Unicode text with markup tags
  and other meta-information representing elements,
  attributes and other nodes
• Text nodes are written as the text they represent
  (character data)
• Element nodes delimited by start and end tags
• ltrelated ref"42"gtGarden Quiche is also
  yummy.lt/relatedgt
• Text in between start and end tags is the content
• This constitutes descendants of the element node
• Attributes written inside element start tag and
  attribute values always written within double or
  single quotes
• ref"42" or ref'42'
• Empty element is one without content (i.e.,
  nothing between start and end tags)
• ltpineapplegtlt/pineapplegt or ltpineapple/gt
• XML document must be well-formed
• Nodes organised into a strictly nested tree
  structure
• Every start tag must have an end tag (or use
  abbreviated form for empty element)
• Elements must nest properly
• Properly nested ltbananagtltorangegtlt/orangegtlt/banana
  gt
• Improperly nested ltbananagtltorangegtlt/bananagtlt/oran
  gegt
• Cf. HTML which allows certain tags (particularly
  many end tags) to be omitted and also allows
  improper nesting
• XML is case-sensitive
• ltTaggtlt/taggt is not well-formed because end tag
  not the same name as start tag

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Textual representation of XML documents

• XML document should begin with an XML
  declaration
• lt?xml version"1.0" encoding"UTF-8" ?gt
• Version attribute indicates version of XML being
  used
• Should be 1.0 or 1.1
• Encoding attribute indicates encoding used in
  file
• All XML parsers required to understand Unicode
  encodings UTF-8 and UTF-16
• Some parsers support other popular encodings like
  ISO-8859-1 but must then be able to convert from
  these encodings to Unicode code points
• Best to use UTF-8 or UTF-16 if possible
• XML declaration followed by root element

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Character data and attribute values

• In character data (text nodes) and attribute
  values, special characters have to be escaped
  using Unicode character references
• N denotes Unicode character with code point N
  represented in decimal
• xN denotes Unicode character with code point N
  represented in hexadecimal
• Some characters are predefined entities in XML
  (see table above)
• Examples
• lt can be referenced as lt, x3C or 60
• can be referenced as amp, x26 or 38
• lt and must be escaped in both character data
  and attribute values
• In attribute values that are enclosed by " or '
  this character must also be escaped
• Also use character references to encode Unicode
  characters that are not accessible from the
  keyboard
• e.g., sake in hiragana script is
  which is encoded as x3055x3051
• Complete list of Unicode character code points is
  available on the Unicode website at
  http//www.unicode.org/charts/

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CDATA sections

• If you have some text that contains lots of
  characters that have to be escaped, then you can
  enclose the text within a CDATA section
• CDATA section corresponds to a CDATA node in the
  DOM and JDOM data models
• For example, in most situations, lt!CDATAaltb
  bgtcgtis equivalent to altb amp bgtc
• Strange syntax for CDATA sections originates in
  SGML

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Comments, processing instructions and DTD
information

• Comment nodes are encoded in the source in the
  same way as in HTML lt!--This is a comment--gt
• A processing instruction is a target-value pair
  delimited by lt?...?gt lt?xml-stylesheet
  type"text/xsl" href"mystyle.xsl"?gtin which
  xml-stylesheet is the target and the string
  type"text/xsl" href"mystyle.xsl" is the
  single value
• Document type nodes (recognized in DOM and JDOM)
  are encoded as follows lt!DOCTYPE gt

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Example XML document

• Contains an XML declaration, followed by a
  document type definition and then a single root
  element named features
• The features element contains a processing
  instruction, some character data and a comment

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White space in XML

• Often convenient to use white space (spaces,
  tabs and new lines) to format source and make it
  more readable
• Usually this white space is not supposed to be
  included in the delivered version of the document
• However, sometimes we want the white space to be
  preserved
• e.g., in poetry or computer programme source code
• By default, the way that white space is handled
  in an XML document is decided by the application
  that is used to process the document
• If we definitely want white space within the
  content of an element to be preserved, then we
  assign the value "preserve" to the attribute
  xmlspace in that element
• Applies to all elements within content of element
  where xmlspace attribute value specified, unless
  overridden by another instance of the xmlspace
  attribute
• Typically, white space handling is defined in the
  DTD for the specific language

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Is XML too verbose?

• Some argue that XML markup is more verbose than
  necessary
• Same information can often be represented much
  more parsimoniously in a relational database
• Leads to (misguided) advice to use attributes in
  preference to elements and short names for both
  attributes and elements
• This usually leads to inflexible and
  incomprehensible language designs!
• Better to disregard such considerations in the
  design phase and then compress files later using
  either a general purpose compression program or
  one that is optimized for XML, such as XMill
  http//sourceforge.net/projects/xmill
• To represent structured information, all you
  really need are text and element nodes
• One simpler alternative is to use something like
  Lisp S-Expressions which date back to 1958
• For example,(collection (recipe
  (title "Rhubarb Cobbler") (date "Wed, 14
  Jun 95") ))represents the same
  asltcollectiongt ltrecipegt lttitlegtRhubarb
  Cobblerlt/titlegt ltdategtWed, 14 Jun
  95lt/dategt lt/recipegtlt/collectiongt

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Applications of XML

• Hundreds of XML applications have been developed
  for many different domains http//xml.coverpages.
  org/xmlApplications.html
• XML languages can be roughlyl classified into
• Data-oriented languages for describing data that
  would traditionally have been stored in
  databases.
• Usually have a flat, wide structure, with the
  root element containing many similar children,
  each with a simple structure
• Document-oriented languages (e.g., XHTML) are for
  annotating the structure of natural language text
• Elements often have mixed content (elements and
  character data)
• Unlike documents in a data-oriented language,
  documents in a document-oriented XML language can
  usually be understood even if the markup tags are
  removed
• Protocols and programming languages including,
  e.g., XML Schema and XSLT
• Usually have most complex syntax
• Hybrids often combine features of data- and
  document-oriented languages
• Typically allow freeform text as content of
  certain elements
• e.g., ltcommentgt element in RecipeML

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Examples of XML languages XHTML

• XHTML 1.0 is W3Cs XML-ification of HTML 4.01
• Apart from the XML declaration and the XHTML
  namespace declaration, XHTML is very similar to
  HTML 4.01
• However, XHTML document must be a well-formed XML
  document, therefore
• Omitting end tags is forbidden in XHTML
• Can abbreviate end tags by using lt/gt notation,
  e.g., ltbr/gt
• XHTML element and attribute names must be lower
  case
• Attribute values cannot be omitted and must be
  surrounded by double or single quotes
• e.g., attribute checked in HTML must be written
  checked"checked"

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XHTML Variants

• XHTML 1.0 Strict
• Clean markup in which all layout is specified
  using CSS
• XHTML 1.0 Transitional
• Additionally permits explicit layout markup like
  bgcolor attribute and font tag
• XHTML 1.0 Frameset
• Allows use of frames
• XHTML 1.1
• Modularization of XHTML 1.0 in which language
  partitioned by functionality into modules, e.g.,
• Structure includes html, head and body tags
• Text includes basic text markup
• Hypertext includes the anchor tag (ltagt)
• Lists ul, ol, dl,
• Forms form, input, select,
• Each module defined using a separate DTD
• Allows specific subsets of the XHTML language to
  be included in new languages

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Other XML languages

• CML (Chemical Markup Language)
• Data-oriented language for representing molecules
  and chemical reactions
• One of the first XML applications
• Supported by wide range of tools such as browsers
  and editors
• WML (Wireless Markup Language)
• Document-oriented XML language that replaces HTML
  on mobile devices that typically have small
  displays, limited user-input facilities and low
  bandwidth
• ebXML (Electronic Business XML Initiative)
• Worldwide initiative to use XML for exchanging
  electronic business data
• Has provided comprehensive standards for business
  processes, data components, collaboration
  protocol agreements, messaging etc.
• Complex language that belongs to protocols and
  programming languages category of XML languages
• ThML (Theological Markup Language)
• Superset of XHTML for markup of theological texts
• Supports references, annotations, glossaries
• MusicXML
• For encoding Western musical staff notation
• Many other XML applications and initiatives
  listed here http//xml.coverpages.org/xmlApplica
  tions.html

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Namespaces in XML

• Not part of the XML specification
• Defined separately from XML specification
• For XML 1.0
• http//www.w3.org/TR/xml-names/
• For XML 1.1
• http//www.w3.org/TR/xml-names11/
• XML namespaces provide a simple method for
  qualifying element and attribute names used in
  Extensible Markup Language documents by
  associating them with namespaces identified by
  IRI references(http//www.w3.org/TR/xml-names/)

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XML NamespacesMotivating problem

• The (fictitious) XML language, WidgetML, is
  designed for describing widgets
• Can include explanatory text written in XHTML
  within a WidgetML document
• i.e., WidgetML uses XHTML as a sublanguage
• Example above describes a widget called gadget
  with a medium-sized head and a big gizmo
  subwidget
• XHTML message contained within info element
• Both XHTML and non-XHTML part of WidgetML use
  tags big and head
• Means that big and head tags can mean different
  things within a WidgetML document, depending on
  context
• Demonstrates need to be able to avoid name
  clashes when combining languages that may use
  elements with the same names to mean different
  things
• Programming languages uses name spaces and
  qualified names to avoid clashing names
• In XML we also use namespaces and each namespace
  is identified by a unique URI
• E.g., XHTML namespace is associated with the URI,
  http//www.w3.org/1999/xhtml
• WidgetML developed by a company called Widget
  Inc. whose domain is www.widget.inc, so assigns a
  URI under their domain to the WidgetML namespace,
  such as http//www.widget.inc/widgetml/

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Namespaces

• So now, instead of just writing ltheadgtlt/headgt
  inside the info element, we can
  write lthttp//www.w3.org/1999/xhtmlheadgt
  lt/http//www.w3.org/1999/xhtmlheadgtto specify
  that we mean the head tag from XHTML, not the
  head tag from WidgetML
• But prefixing every tag with a long URI would
  lead to an extremely verbose and incomprehensible
  document
• Instead, we assign a short name to the namespace
  we want to use within an element and declare this
  association between the short name and the
  namespace as an attribute in the start tag of the
  containing element ltinfo xmlnsfoo"http//www.w
  3.org/TR/xhtml1gt ltfooheadgtlt/fooheadgt lt/
  infogtWe can then prefix the tags within the
  containing element with the short name to
  indicate that they are from the declared
  namespace
• The attribute, xmlnsfoo"http//www.w3.org/TR/xht
  ml1",
• Declares the namespace named http//www.w3.org/TR/
  xhtml1
• Gives this namespace the prefix, foo

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Namespaces

• Namespace declaration applies to all contents of
  element in whose start tag it occurs
• Can use any name as a prefix except one that
  contains a colon or one starting with the letters
  XML (in any combination of upper or lower case)
• NCName (No Colon Name) is one that does not
  contain a colon
• QName (Qualified name) may be an NCName or an
  NCName prefixed with a namespace prefix and a
  colon
• Unprefixed element names are assigned a default
  namespace
• Default namespace can be overridden by setting
  attribute xmlns to the URI for the namespace to
  be used for unprefixed tags ltwidget
  typegadget xmlnshttp//www.widget.incgt

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Default namespaces dont apply to attributes!

• 1 and 2 are equivalent
• The size attribute does not come from the
  http//www.widget.inc namespace
• In 3, the size attribute does come from the
  http//www.widget.inc namespace

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RecipeML
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Example RecipeML document

• An example RecipeML document is available
  at http//www.brics.dk/ixwt/examples/recipes.xml

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Summary

• XML is a framework for developing markup
  languages in any conceivable domain
• XML is just a notation for hierarchically
  structuring textual data
• Strength of XML is that it is a widely accepted
  standard supported by many generic languages and
  tools
• Means you get lots of free infrastructure if you
  build on it
• Considered XML in the form of trees and in its
  textual representation
• Considered the namespace mechanism for resolving
  name conflicts