Sistemi basati su conoscenza XML

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Sistemi basati su conoscenzaXML

• Prof. M.T. PAZIENZA
• a.a. 2002-2003

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Introduction to XML

• XML (1996) was designed to describe data, and to
  focus on what data is
• HTML (1990) was designed to display data, and to
  focus on how data looks
• HTML is about displaying information,
• XML is about describing information
• both derive from SGML (1988)

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What is XML?

• XML stands for EXtensible Markup Language
• XML was designed to describe data.
• XML tags are not predefined in XML. You must
  define your own tags.
• XML uses a DTD (Document Type Definition) to
  describe the data.
• XML with a DTD is designed to be
  self-descriptive.

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XML

• XML is free and extensible
• XML tags are not predefined. You must "invent"
  your own tags.
• 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.
• XML was designed to store, carry and
  exchange/send data.
• It is just pure information wrapped in XML tags.
  Someone must write a piece of software to send
  it, receive it or display it.

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XML does not DO anything

• Ex.
• 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

• With XML, data can be exchanged between
  incompatible systems.
• In the real world, computer systems and databases
  contain data in incompatible formats. One of the
  most time-consuming challenges for developers has
  been to exchange data between such systems over
  the Internet.
• Converting the data to XML can greatly reduce
  this complexity 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.
• Because of this, 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.
• Attributes provide additional information about
  the element. Their values (enclosed in quotes)
  are inside the start tag of an element

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XML Syntax

• Entities are shortcuts for portions of common
  text (entity reference starts with and ends
  with )
• Comments may be inserted anywhere in an XML
  document(comment starts with lt!- and ends with
  -gt)
• Document type declaration (DTD) is the set of
  rules that allows to specify own set of elements,
  attributes and entities

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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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An example XML document

• lt?xml version"1.0"?gt
• ltnotegt
• lttogtTovelt/togt
• ltfromgtJanilt/fromgt ltheadinggtReminderlt/headinggt
  ltbodygtDon't forget me this weekend!lt/bodygt
• lt/note

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XML Syntax

• All XML elements (a part XML declaration )must
  have a closing tag
• The declaration is not a part of the XML document
  itself. It is not an XML element, and it should
  not have a closing tag

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XML Syntax

• XML tags are case sensitive
• With XML, the tag ltLettergt is different from the
  tag ltlettergt.
• Opening and closing tags must therefore be
  written with the same case

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XML Syntax

• All XML elements must be properly nested
• Improper nesting of tags makes no sense to XML

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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.
• 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

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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/paragtù
• lt/chaptergt
• lt/bookgt

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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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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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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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• 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