Sistemi basati su conoscenza XML

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Sistemi basati su conoscenzaXML

• Prof. M.T. PAZIENZA
• a.a. 2004-2005

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Introduction to XML

• HTML (1990) was designed to display data
  (documents), and to focus on how data looks
• XML (1996) was designed to describe data
  (documents), and to focus on what data is
• HTML is about displaying information,
• XML is about describing information
• both derive from SGML (1988)
• XML is a standard for describing content in
  addition to presentation aspects.

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HTML

• HTML is a markup language it augments regular
  text with marks that hold special meaning for
  Web browser handling the document.
• Commands in the language are called tags (start
  end),
• ltTAGgt, lt/TAGgt
• and have a fixed meaning.
• HTML is adequate to represent the structure of
  documents only for display purposes.

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XML (EXtensible Markup Language)

• XML tags are not predefined in XML. The author
  must define his own tags and his own document
  structure.
• XML uses a DTD (Document Type Definition) to
  describe any type data (document).
• XML with a DTD is designed to be
  self-descriptive.
• XML is free and extensible
• XML is as a cross-platform, software and hardware
  independent tool for transmitting information.

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XML does not DO anything

• XML was not designed to DO anything.
• It is just pure information wrapped in XML
  tags. Someone must write a piece of software to
  send it, receive it or display it.
• XML is not a language it is a syntax supporting
  creation of personalized markup languages.

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XML does not DO anything

• Ex
• lt?xml version"1.0"? EncodingISO_8859-1gt
• ltnotegt
• lttogtTovelt/togt ltfromgtJanilt/fromgt
  ltheadinggtReminderlt/headinggt
• ltbodygtDon't forget me this weekend!lt/bodygt
  lt/notegt
• The note has a header, and a message body. It
  also has sender and receiver information. But
  still, this XML document does not DO anything.
  Someone must write a piece of software to send
  it, receive it or display it.

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XML to Exchange Data

• XML was designed to store, carry and exchange
  data.With XML, data can be exchanged between
  incompatible systems.
• In the real world, computer systems and databases
  contain data in incompatible formats.
• Converting the data to XML can greatly reduce the
  complexity of data exchange and create data that
  can be read by many different types of
  applications.

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XML used to Share Data

• With XML, plain text files can be used to share
  data.
• Since XML data is stored in plain text format,
  XML provides a software- and hardware-independent
  way of sharing data.
• This makes it much easier to create data that
  different applications can work with. It also
  makes it easier to expand or upgrade a system to
  new operating systems, servers, applications, and
  new browsers. 

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XML used to Store Data

• With XML, plain text files can be used to store
  data.
• XML can also be used to store data in files or in
  databases. Applications can be written to store
  and retrieve information from the store, and
  generic applications can be used to display the
  data.

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XML Syntax

• The syntax rules of XML are very simple and very
  strict. The rules are very easy to learn, and
  very easy to use.
• Creating software that can read and manipulate
  XML is very easy to do.

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XML Syntax

• Element (also called tag) is the primary building
  block of an XML document. Xml elements are case
  sensitive and must be properly nested. Elements
  are related as parents and children.
• With XML, the tag ltLettergt is different from the
  tag ltlettergt. Opening and closing tags must
  therefore be written with the same case.
• Attributes provide additional information about
  the element. Their values (enclosed in quotes)
  are inside the start tag of an element. An
  attribute is a name-value pair separated by an
  equal sign ()
• Entities are shortcuts for portions of common
  text (entity reference starts with and ends
  with )

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XML Syntax

• Comments arbitrary text- may be inserted
  anywhere in an XML document (comment starts with
  lt!- and ends with -gt)
• Comment      'lt!--' ((Char - '-') ('-' (Char
  - '-'))) '--gt
• An example of a comment
• lt!-- declarations for ltheadgt ltbodygt --gt
• Note that the grammar does not allow a comment
  ending in ---gt.
• The following example is not well-formed
• lt!-- B, B, or B---gt

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XML Syntax

• Document type declaration (DTD) is the set of
  rules that allows to specify own set of elements,
  attributes and entities. A DTD specifies which
  elements can be used and constraints on elements
• A DTD defines the legal elements of an XML
  document that is the legal building blocks of an
  XML document. It defines the document structure
  with a list of legal elements.
• XML Schema is an XML based alternative to DTD.

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Why use a DTD?

• XML provides an application independent way of
  sharing data.
• With a DTD, independent groups of people can
  agree to use a common DTD for interchanging data.
  
• Any application can use a standard DTD to verify
  that data received from the outside world is
  valid.

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XML Syntax

• All XML elements (a part XML declaration )must
  have a closing tag. The XML declaration
• lt?xml version"1.0"? EncodingISO_8859-1gt
• is not a part of the XML document itself. It is
  not an XML element, and it should not have a
  closing tag.
• The XML declaration defines the XML version and
  the character encoding used in the document.

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XML Syntax

• All XML documents must have a root tag
• The first tag in an XML document is the root tag.
• All XML documents must contain a single tag pair
  to define the root element (ex.ltnotegt ).
• All other elements must be nested within the root
  element.
• All elements can have sub-elements (children).
  Sub-elements must be correctly nested within
  their parent element
• ltrootgt ltchildgt ltsubchildgt.....lt/subchildgt
  lt/childgt lt/rootgt
• In previous example there are 4 child elements of
  the root (to, from, heading, body)

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XML Syntax

• Attribute values must always be quoted
• With XML, it is illegal to omit quotation marks
  around attribute values. 
• XML elements can have attributes in name/value
  pairs just like in HTML.
• In XML the attribute value must always be quoted.

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XML Syntax

• lt?xml version"1.0"?gt ltnote date12/11/99gt
  lttogtTovelt/togt ltfromgtJanilt/fromgt
  ltheadinggtReminderlt/headinggt ltbodygtDon't forget me
  this weekend!lt/bodygt lt/notegt
• Incorretto
• lt?xml version"1.0"?gt ltnote date"12/11/99"gt
  lttogtTovelt/togt ltfromgtJanilt/fromgt
  ltheadinggtReminderlt/headinggt ltbodygtDon't forget me
  this weekend!lt/bodygt lt/notegt
• corretto

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XML Syntax

• White Space is Preserved
• CR / LF is Converted to LF
• A new line is always stored as LF

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XML Syntax

• There is nothing special about XML. It is just
  plain text with the addition of some XML tags
  enclosed in angle brackets.
• Software that can handle plain text can also
  handle XML. In a simple text editor, the XML tags
  will be visible and will not be handled
  specially.
• In an XML-aware application however, the XML tags
  can be handled specially. The tags may or may not
  be visible, or have a functional meaning,
  depending on the nature of the application.

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XML Elements

• XML Elements are Extensible
• XML documents can be extended to carry more
  information.
• XML Elements have Relationships
• Elements are related as parents and children

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XML Elements

• Book Title My First XML
• Chapter 1 Introduction to XML
• What is HTML
• What is XML
• Chapter 2 XML Syntax
• Elements must have a closing tag
• Elements must be properly nested

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XML element (book description)

• ltbookgt
• lttitlegtMy First XMLlt/titlegt
• ltprod id"33-657" media"paper"gtlt/prodgt
• ltchaptergtIntroduction to XML
• ltparagtWhat is HTMLlt/paragt
• ltparagtWhat is XMLlt/paragt
• lt/chaptergt
• ltchaptergtXML Syntax
• ltparagtElements must have a closing taglt/paragt
• ltparagtElements must be properly nestedlt/paragt
• lt/chaptergt
• lt/bookgt

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XML element (book description)

• book is the root element.
• title, prod, and chapter are child elements of
  book.
• book is the parent element of siblings (or sister
  elements) because they have the same parent.

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Elements have Content

• Elements can have different content types.
• An XML element is everything from (including) the
  element's start tag to (including) the element's
  end tag.
• An element can have element content, mixed
  content, simple content, or empty content. An
  element can also have attributes.

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Elements have Content

• In the book description
• book has element content, because it contains
  other element
• chapter has mixed content because it contains
  both text and other elements
• para has simple content (or text content) because
  it contains only text
• prod has empty content because it carries no
  information.

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Element Naming

• Names can contain letters, numbers, and other
  characters
• Names must not start with a number or punctuation
  character
• Names must not start with the letters xml (or XML
  or Xml ..)
• Names cannot contain spaces

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Element Naming

• XML documents often have a corresponding
  database, in which fields exist corresponding to
  elements in the XML document. A good practice is
  to use the naming rules of the database for the
  elements in the XML documents

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Ex. XML News document

• lt?xml version"1.0"?gt
• ltnitfgt ltheadgt
• lttitlegtColombia Earthquakelt/titlegt lt/headgt
• ltbodygt ltbody.headgt ltheadlinegt lthl1gt143 Dead in
  Colombia Earthquakelt/hl1gt lt/headlinegt
• ltbylinegt ltbytaggtBy Jared Kotler, Associated Press
  Writerlt/bytaggt lt/bylinegt
• ltdatelinegt ltlocationgtBogota,Colombialt/locationgt
  ltstory.dategtMonday January 25 1999 728
  ETlt/story.dategt lt/datelinegt lt/body.headgt lt/bodygt
  lt/nitfgt

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DTD

• A DTD is enclosed in
• lt!DOCTYPE name DTD declaration gt
• where name is the name of the outermost enclosing
  tag, and DTD declaration is the text of the
  rules of the DTD
• The DTD starts with the outermost element, called
  the root of the element

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Internal DTD

• lt?xml version"1.0"?gt lt!DOCTYPE note
• lt!ELEMENT note (to,from,heading,body)gt
• lt!ELEMENT to (PCDATA)gt
• lt!ELEMENT from (PCDATA)gt
• lt!ELEMENT heading (PCDATA)gt
• lt!ELEMENT body (PCDATA)gt gt
• ltnotegt lttogtTovelt/togt ltfromgtJanilt/fromgt
  ltheadinggtReminderlt/headinggt ltbodygtDon't forget me
  this weekend!lt/bodygt lt/notegt
• The DTD is interpreted like this!ELEMENT note
  defines the element "note" as having four
  elements "to,from,heading,body".!ELEMENT to
  defines the "to" element  to be of the type
  "CDATA".!ELEMENT from defines the "from" element
  to be of the type "CDATA"and so on.....

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CDATA Sections

• CDATA sections may occur anywhere character data
  may occur they are used to escape blocks of text
  containing characters which would otherwise be
  recognized as markup.
• CDATA sections begin with the string "lt!CDATA"
• and end with the string
• "gt"

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CDATA Sections

• CDSect      CDStart CDataCDEnd
• CDStart      'lt!CDATA
• CData      (Char - (Char 'gt' Char))
• CDEnd      'gt
• Within a CDATA section, only the CDEnd string is
  recognized as markup, so that left angle brackets
  and ampersands may occur in their literal form
  they need not (and cannot) be escaped using
  "lt" and "amp". CDATA sections cannot nest.

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CDATA Sections

• An example of a CDATA section, in which
  "ltgreetinggt" and "lt/greetinggt" are recognized as
  character data, not markup
• lt!CDATAltgreetinggtHello,world!lt/greetinggtgt

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External DTD

• This is the same XML document with an external
  DTD 
• lt?xml version"1.0"?gt
• lt!DOCTYPE note SYSTEM "note.dtd"gt
• ltnotegt
• lttogtTovelt/togt
• ltfromgtJanilt/fromgt
• ltheadinggtReminderlt/headinggt
• ltbodygtDon't forget me this weekend!lt/bodygt
• lt/notegt

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External DTD

• This is a copy of the file "note.dtd" containing
  the Document Type Definition
• lt?xml version"1.0"?gt
• lt!ELEMENT note (to,from,heading,body)gt
• lt!ELEMENT to (PCDATA)gt
• lt!ELEMENT from (PCDATA)gt
• lt!ELEMENT heading (PCDATA)gt
• lt!ELEMENT body (PCDATA)gt

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XML document (with DTD)

• An example of an XML document with a document
  type declaration
• lt?xml version"1.0"?gt
• lt!DOCTYPE greeting SYSTEM "hello.dtd"gt
  ltgreetinggtHello, world!lt/greetinggt
• The system identifier "hello.dtd" gives the
  address (a URI reference) of a DTD for the
  document

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XML document (with DTD)

• The declarations can also be given locally, as in
  this example
• lt?xml version"1.0" encoding"UTF-8" ?gt lt!DOCTYPE
  greeting lt!ELEMENT greeting (PCDATA)gt gt
  ltgreetinggtHello, world!lt/greetinggt

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XML document (with DTD)

• If both the external and internal subsets are
  used, the internal subset is considered to occur
  before the external subset.
• This has the effect that entity and
  attribute-list declarations in the internal
  subset take precedence over those in the external
  subset.

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Language identification

• In document processing, it is often useful to
  identify the natural or formal language in which
  the content is written.
• A special attribute named xmllang may be
  inserted in documents to specify the language
  used in the contents and attribute values of any
  element in an XML document.
• In valid documents, this attribute, like any
  other, must be declared if it is used.

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Language identification

• A simple declaration for xmllang might take the
  form
• xmllang NMTOKEN IMPLIED
• The intent declared with xmllang is considered
  to apply to all attributes and content of the
  element where it is specified, unless overridden
  with an instance of xmllang on another element
  within that content.
• Specific default values may also be given, if
  appropriate. In a collection of French poems for
  English students, with glosses and notes in
  English, the xmllang attribute might be declared
  this way
• lt!ATTLIST poem xmllang NMTOKEN 'fr'gt
• lt!ATTLIST gloss xmllang NMTOKEN 'en'gt
• lt!ATTLIST note xmllang NMTOKEN 'en'gt

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Language identification

• ltp xmllang"en"gtThe quick brown fox jumps over
  the lazy dog.lt/pgt
• ltp xmllang"en-GB"gtWhat colour is it?lt/pgt
• ltp xmllang"en-US"gtWhat color is it?lt/pgt
• ltsp who"Faust" desc'leise' xmllang"de"gt
  ltlgtHabe nun, ach! Philosophie,lt/lgt ltlgtJuristerei,
  und Medizinlt/lgt ltlgtund leider auch Theologielt/lgt
  ltlgtdurchaus studiert mit heißem Bemüh'n.lt/lgt
• lt/spgt

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Well formed XML documents

• A well formed XML document has correct XML
  syntax (i.e. is a document that conforms to the
  XML syntax rules.
• A valid XML document is a well formed XML
  document which also conforms to the rules of a
  DTD (Document Type Definition).

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• http//www.xml.com/pub/a/98/10/guide0.html
• http//xmlfiles.com/xml/default.asp
• http//www.brics.dk/amoeller/XML/index.html
• http//msdn.microsoft.com/library/default.asp?url
  /library/en-us/xmlsdk30/htm/xmtutxmltutorial.asp
• http//www.w3.org/TR/2000/REC-xml-20001006sec-wel
  l-formed