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Ziele des Kapitels

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Title: Ziele des Kapitels


1
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.

2
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

3
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

4
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.

5
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.

6
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.
  •  

7
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.

8
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

9
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.

10
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.

11
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.

12
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

13
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.

14
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, )

15
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
16
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

17
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)

18
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.

19
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

20
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).

21
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.

22
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.

23
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.

24
"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.
25
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.

26
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.

27
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.

28
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.
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