Ziele des Kapitels

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Ziele des Kapitels über XML

• Studierende verstehen die Bedeutung von XML.
• Studierende erhalten einen Überblick über die XML
  Sprachfamilie.
• Studierende lernen, einfache XML Dokumente und
  deren Layout zu spezifizieren.

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XML Kapitel Überblick

• XML in 7 points
• Überblick über XML Entwurfsziele und XML als
  Familie von Technologien
• XML Motivation und erste Beispiele
• Grenzt XML von HTML und SGML ab und zeigt
  einfache Anwendungsbeispiele
• XML Schema and UML
• Ausführliches Beispiel der Abbildung
• XML Tutorial
• Enthält Spezialbeispiele zur XML Sprachfamilie

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XML

• Extensible Markup Language, abbreviated XML,
  describes a class of data objects called XML
  documents and partially describes the behavior of
  computer programs which process them. XML is an
  application profile or restricted form of SGML,
  the Standard Generalized Markup Language ISO
  8879. By construction, XML documents are
  conforming SGML documents.
• see http//www.w3.org

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XML

• Definition A software module called an XML
  processor is used to read XML documents and
  provide access to their content and structure.
• Definition It is assumed that an XML processor
  is doing its work on behalf of another module,
  called the application.

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XML

• XML documents are made up of storage units called
  entities, which contain either parsed or unparsed
  data.
• Parsed data is made up of characters, some of
  which form character data, and some of which form
  markup.
• Markup encodes a description of the document's
  storage layout and logical structure. XML
  provides a mechanism to impose constraints on the
  storage layout and logical structure.

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Design Goals for XML (1)

• The design goals for XML are
• 1. XML shall be straightforwardly usable over the
  Internet.
• 2. XML shall support a wide variety of
  applications.
• 3. XML shall be compatible with SGML.
• 4. It shall be easy to write programs which
  process XML documents.
• 5. The number of optional features in XML is to
  be kept to the absolute minimum, ideally zero.
•  

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Design Goals for XML (2)

• 6. XML documents should be human-legible and
  reasonably clear.
• 7. The XML design should be prepared quickly.
• 8. The design of XML shall be formal and concise.
• 9. XML documents shall be easy to create.
• 10. Terseness in XML markup is of minimal
  importance.

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XML in 7 pointssee also http//www.w3.org/1999/XM
L-in-10-points

• XML, XLink, Namespace, DTD, Schema, CSS,
  XHTML,... If you are new to XML, it may be hard
  to know where to begin.
• XLink, XPointer generalized link concepts
• XSL more powerful than CSS, serves formatting
  purposes for XML documents

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1. XML is a method for putting structured data in
a text file

• Structured data" spreadsheets, address books,
  configuration parameters, financial transactions,
  technical drawings, etc.
• Text format allows one to look at the data
  without the program that produced it. XML is a
  set of rules, guidelines, conventions, for
  designing text formats for such data, in a way
  that produces files that are easy to generate and
  read (by a computer) and that are unambiguous and
  platform-independent.

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2. XML looks a bit like HTML but isn't HTML

• Like HTML, XML makes use of tags (words bracketed
  by 'lt' and 'gt') and attributes (of the form
  name"value")
• While HTML specifies what each tag attribute
  means (and often how the text between them will
  look in a browser), XML uses the tags only to
  delimit pieces of data, and leaves the
  interpretation of the data completely to the
  application that reads it.
• E.g. If you see "ltpgt" in an XML file, don't
  assume it is a paragraph. Depending on the
  context, it may be a price, a parameter, a
  person, etc.

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3. XML is text, but isn't meant to be read

• XML files are text files, but theyare not meant
  to be read by humans. They are text files,
  because that allows experts (such as programmers)
  to more easily debug applications.
• The rules for XML files are much stricter than
  for HTML. A forgotten tag, or an attribute
  without quotes makes the file unusable, while in
  HTML such practice is often explicitly allowed,
  or at least tolerated.

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4. XML is a family of technologies

• We will look at the following technologies
• XML
• DTD (Document Type Definition)
• XML Schema, XSchema
• XPath, XPointer
• XInclude,
• XSLT, CSS
• XLink

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4. XML is a family of technologies

• XML 1.0 specification that defines what "tags"
  and "attributes" are, but around XML 1.0, there
  is a growing set of optional modules that provide
  sets of tags attributes, or guidelines for
  specific tasks.

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4. XML is a family of technologies

• When starting with XML, it's important to
  realize that XML is not a markup language itself
  (like HTML), but it provides rules (like SGML)
  for defining a markup language. The names of the
  tags are up to the authors.
• Example ltmyFirstTaggt
• ltHello/gt
• ltWorld/gt
• lt/myFirstTaggt
• So XML is about the characters, tags can consist
  of and defines a set of rules for
  well-formedness. (every opening tag must have a
  closing tag, )

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DTD's solve the problem of defining the structure
of a document. Example
ltmyFirstTag myFirstAttribute"Hello World"gt
ltHello/gt ltWorld/gt lt/myFirstTaggt
A correct DTD tells, how the tags should be
arranged, to form a valid document, or what
attributes a tag can include. In our case a DTD
tells the following

• The name of the main tag is "myFirstTag"
• myFirstTag has an attribute myFirstAttribute
• Inside myFirstTag there must exist two tags,
  "Hello" and "World"

The main problem of the DTD concept is, that it
tells nothing about the content of a tag (data
type, format, pattern, )
ltmyFirstTaggt12.3lt/myFirstTaggt ltmySecondTaggtText
Contentlt/mySecondTaggt
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4. XML is a family of technologies

• XML Schemas 1 and 2 help developers to precisely
  define their own XML-based formats.
• XSchema fills the gaps of the DTD concept. In
  fact you can replace your DTD's entirely by an
  "XML Schema
• In addition to a DTD you can write exact rules
  about the content of attributes and tags. This
  includes
• data types (integer, string),
• patterns (the content has to be a valid email
  address)
• lists of tokens

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4. XML is a family of technologies

• If we talk about XML as set of rules that define
  the syntax of a document, then XSchema's purpose
  is to specify a pattern/semantic for a special
  purpose.
• Example
• myFirstTag has to contain a number between 5 and
  10.7, that has to have 5 decimal places (6.30020)

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4. XML is a family of technologies

• "XPath (XML Path Language) is a language for
  addressing parts of an XML document, designed to
  be used by both XSLT and XPointer.
• In other words XPath provides the functionality
  to jump to a certain part of an XML Document.
  It's like a bookmark to identify a certain
  point/part of an XML Document.

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4. XML is a family of technologies

• "XPointer, which is based on XPath, supports
  addressing into the internal structures of XML
  documents. It allows for traversals of a document
  tree and choice of its internal parts based on
  various properties, such as element types,
  attribute values, character content, and relative
  position.
• Basically works like anchors in html
• ltlink xlinkhref"mydocument.xmlxpointer(//AAA/BB
  B1)"/gt

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4. XML is a family of technologies

• XInclude allows to include documents / parts of
  documents into a XML document (mostly like
  programming languages do through include).

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4. XML is a family of technologies

• CSS, the style sheet language, is applicable to
  XML as it is to HTML.
• XSL (Extensible Stylesheet Language) is the
  advanced language for expresing style sheets.
• A transformation expressed in XSLT describes
  rules for transforming a source tree into a
  result tree. The transformation is achieved by
  associating patterns with templates.
• A pattern is matched against elements in the
  source tree.
• A template is instantiated to create part of the
  result tree.
• The result tree is separate from the source tree.
  
• The structure of the result tree can be
  completely different from the structure of the
  source tree.

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In constructing the result tree, elements from
the source tree can be filtered and reordered,
and arbitrary structure can be added.
A transformation expressed in XSLT is called a
stylesheet. This is because, in the case when
XSLT is transforming into the XSL formatting
vocabulary, the transformation functions as a
stylesheet.
In other words XSL provides you with a way to
transform any XML input to match a certain
purpose.

• Examples
• transform an order, saved in XML, to a delivery
  note
• transform UML logic, saved in XML, to class
  definitions in a given programming language.

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4. XML is a family of technologies

• Xlink (still in development) describes a standard
  way to add hyperlinks to an XML file. XPointer
  XPath are syntaxes for pointing to parts of an
  XML document.

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"XLink (XML Linking Language) allows elements to
be inserted into XML documents in order to create
and describe links between resources. It uses
XML syntax to create structures that can describe
links similar to the simple unidirectional
hyperlinks of today's HTML, as well as more
sophisticated links."
With XLink's you can replace today's lta hrefgt
links, and tie links together like resource
chains. XLink's can contain information about the
context, they are used in and about additional
information available.
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4. XML is a family of technologies (3)

• The DOM is a standard set of function calls for
  manipulating XML (and HTML) files from a
  programming language.
• XML Namespaces is a specification that describes
  how you can associate a URL with every single tag
  and attribute in an XML document. What that URL
  is used for is up to the application that reads
  the URL, though.
• RDF, W3C's standard for metadata, uses it to link
  every piece of metadata to a file defining the
  type of that data.

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5. XML is verbose, but that is not a problem

• Since XML is a text format, and it uses tags to
  delimit the data, XML files are nearly always
  larger than comparable binary formats. That was a
  conscious decision by the XML developers. The
  advantages of a text format are evident (see 3
  above), and the disadvantages can usually be
  compensated at a different level.
• Communication protocols such as modem protocols
  and HTTP/1.1 (the core protocol of the Web) can
  compress data on the fly.

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6. XML is new, but not that new

• Development of XML started in 1996 and it is a
  W3C standard since February 1998.
• The technology isn't very new. Before XML there
  was SGML, developed in the early '80s, an ISO
  standard since 1986, and widely used for large
  documentation projects.
• For HTML, development started in 1990.
• The designers of XML simply took the best parts
  of SGML, guided by the experience with HTML, and
  produced something that is no less powerful than
  SGML, but vastly more regular and simpler to use.

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7. XML is license-free, platform-independent and
well-supported

• By choosing XML as the basis for some project,
  you buy into a large and growing community of
  tools and engineers experienced in the
  technology. Opting for XML is a bit like choosing
  SQL for databases you still have to build your
  own database and your own programs/procedures
  that manipulate it, but there are many tools
  available and many people that can help you.
• XML isn't always the best solution, but it is
  always worth considering.