Introduction to XML

eXtensible Markup Language

<firm id="bsi001">
  <name>Boot &amp; Shoe Inc.</name>
  <address>
    <street>246 Walker Drive</street>
    <town>Lacetown</town>
    <state>OH</state>
    <zipcode>44312</zipcode>
    <phone>732754689</phone>
  </address>
  <specialization>
    Shoemaking, cobler and cordwainer work: shoes, boots, sandals, clogs and moccasins.
  </specialization>
</firm>

Resources: http://www.kosek.cz

History

• 1960s - markup, i.e. inserting mark into a document
  • e.g. Formatting (TeX)
• 1969 - GML = Generalized Markup Language
  • Goldfarb, Mosher and Lorie, legal texts for IBM
• 1986 - SGML: Standard Generalized Markup Language, ISO 8879
  • Too complicated !
• 1992 - HTML (Hypertext Markup Language)
  • Basics from SGML, very simple
• 1996 - W3C new directions for a new markup language specified, major design decisions
  • 1998 - XML 1.0

XML versus Database

Database Table

Surname	Firstname	E-mail	Phone
Smith	John	jsmith@gmail.com	2554119897
Walker	David	dwalk@hotmail.com	8446225653

The same data in XML

<addressbook>
  <person>
    <surname>Smith</surname>
    <firstname>John</firstname>
    <email>jsmith@gmail.com</email>
    <phone>2554119897</phone>
  </person>
  <person>
    <surname>Walker</surname>
    <firstname>David</firstname>
    <email>dwalk@hotmail.com</email>
    <phone>8446225653</phone>
  </person>
</addressbook>

Advantages of XML

• Open format (specification for free from w3c)
• Easily understandable, self-documented files
• Text-oriented: no specialized tools required, abundance of text editors
• Higher information content (compared to formatting markups)
• Easily convertable to other formats
• Easy and efficient parsing / structure checking
• Support for referencing

Quick Syntax Tour

Terms

XML document text file in the XML format Element(s) what a document consists of Tags mark element boundaries (start/end tags) Attributes another information associated with elements

Quick Syntax Tour (2)

• Tags
  • Start tag <element_name>
  • End tag </element_name>
  • Empty element <element_name/>
• Elements can be embeded, but they cannot cross
  → XML Document = tree of elements
• Special symbols <, >, and & must be encoded using entities ("escape sequences") &lt;, &gt;, and &amp;
• Attribute values must be enclosed in quotes or apostrophes (another needed entities: &quot; and &apos;)

Quick Syntax Tour (3)

XML document can (should) contain instructions for xml processor
Declaration header XML:

<?xml version="1.0" encoding="utf-8" ?>

Document type declaration:

<!DOCTYPE MojeKniha SYSTEM "MojeKniha.DTD">

Comments

<!-- Here be dragons -->

(not allowed inside tags, cannot contain --)

Well-Formed XML Document

the document conforming all syntactic requirements

Time to experiment

Document Type Definition (DTD)

• DTD
  • Came from SGML
  • Formal set of rules for describing document structure
  • Declares element names, their embeding, attribute names and values…
  • Example: a document consisting of a sequence of chapters, each chapter contains a title and a sequence of sections, sections contain paragraphs…
• DTD location
  • external DTD: stand-off file
  • internal DTD: inside the XML document
• Validation
  • Process of checking whether a document fulfills the DTD requirements
  • If OK: valid document

DTD Structure

Four types of declarations

• Declaration of elements <!ELEMENT …>
• Declaration of attributes <!ATTLIST …>
• Declaration of entities
• Declaration of notations

Declaration of Elements

• <!ELEMENT name content>
• A name must start with a letter, can contain numbers and some special symbols: .-_:
• Empty element: <!ELEMENT name EMPTY>
• Element without content limitations: <!ELEMENT name ANY>

Declaration of Elements

Elements containing text

• Reserved name PCDATA (Parseable Character DATA)
• Example: <!ELEMENT title (#PCDATA)>

Element content description

• Regular expressions
• Order: sequence connector , and option connector |
• Quantity control: ? + *
• Example of mixed content element:

  <!ELEMENT emph (#PCDATA|sub|super)* >

Declaration of Attributes

• <!ATTLIST element_name attributes_declaration>
• Attribute declaration
  • Attribute name
  • Attribute type
  • Default value (optional)
• Example

  <!ATTLIST author firstname CDATA surname CDATA>

Declaration of Attributes

Types of Attribute Values

• CDATA: simple string
• NMTOKEN: string constraint as an attribute name
• NMTOKENS: sequence of tokens separated by spaces
• ID: identifier
• IDREF: link to an identifier
• IDREFS: sequence of links
  …

Required Attribute

• #REQUIRED at the end of the attribute declaration

Implicit Attribute

• if not present, the application assumes a default value
• #IMPLIED at the end of the attribute declaration

DTD Sample

<!ELEMENT collection (description,recipe*)>

<!ELEMENT description ANY>

<!ELEMENT recipe (title,ingredient*,preparation,comment?,nutrition)>

<!ELEMENT title (#PCDATA)>

<!ELEMENT ingredient (ingredient*,preparation)?>
<!ATTLIST ingredient name CDATA #REQUIRED
                     amount CDATA #IMPLIED
                     unit CDATA #IMPLIED>

<!ELEMENT preparation (step*)>

<!ELEMENT step (#PCDATA)>

<!ELEMENT comment (#PCDATA)>

<!ELEMENT nutrition EMPTY>
<!ATTLIST nutrition protein CDATA #REQUIRED
                    carbohydrates CDATA #REQUIRED
                    fat CDATA #REQUIRED
                    calories CDATA #REQUIRED
                    alcohol CDATA #IMPLIED>

Validation

xmllint (distributed with libxml)