XML (Extensible Markup Language)

17.1 Background

• A format for data exchange and manipulation
• Easy to learn.
• Self-describing: markup describes the structure and type of data.
• Simple format from management perspective: parsing, transformations, portability, human readable
• It is a standard (was a major benefit for SQL!): a flood of support

17.2 Syntax

• Tags on data elements, identifying their meaning
  <COURSES> 
    <course> 
       <number>670</number> 
       <quarter>Spring</quarter> 
       <days>TR</days> 
    </course> 
    <course> 
       <number>680</number> 
       <quarter>Autumn</quarter> 
       <days>MWF</days> 
    </course> 
  </COURSES> 
   

• Attributes may be added to the tags
  <COURSES> 
    <course title="data bases" year="2001"> 
       <number >670</number> 
       <quarter>Spring</quarter> 
       <days>TR</days> 
    </course> 
    <course title="data structures" year="2000"> 
       <number>680</number> 
       <quarter>Autumn</quarter> 
       <days>MWF</days> 
    </course> 
  </COURSES> 
  

• Cross references may be used to express associations.
  <COURSES> 
    <course ref="sp"> 
       <number>670</number> 
       <days>TR</days> 
    </course> 
    <course ref="au"> 
       <number>680</number> 
       <days>MWF</days> 
    </course> 
    <quarter id="sp">Spring</quarter> 
    <quarter id="au">Autumn</quarter> 
  </COURSES> 
  

• An underlying tree data model
  

• Allows interleaving of elements with cdata (document-centric instead of data-centric).
  <COURSES> 
   Two courses 
    <course ref="sp"> 
       Here is a cse <number>670</number> course 
       offered on <days>TR</days>. 
    </course> 
   and 
    <course ref="au"> 
       We also have a cse <number>680</number> course 
       offered on <days>MWF</days>. 
    </course> 
   and two quarters 
    <quarter id="sp">Spring</quarter> 
   and 
    <quarter id="au">Autumn</quarter>. 
  </COURSES> 
  

17.3 Relations and XML

• Simple

  table col1 col2 col3 a1 b1 c1 a2 b2 c2

  <table>    <row> <col1>a1</col1> <col2>b1</col2> <col3>c1</col3> </row>    <row> <col1>a2</col1> <col2>b2</col2> <col3>c2</col3> </row>  </table>

• Applicable to small subclass of XML structures.
• Used mainly by intermediate servers between databases and users.

17.4 Well Formed Files

• Start and end tags required: <tag>...</tag>

• Empty tags may use short cuts: <tag/>

• Element attributes must be in quotes: <tag attribute="...">...</tag> (or ’...’)

• Proper nesting of tags

• Root element

• Entity codes (http://www.unicode.org/charts/):
  • &#x0634; (Arabic sheen ش)
  • &#x03B1; (Greek alpha $\alpha$)
  • &#x05D0; (Hebrew aleph א)
  • &#x2286; (subset of or equal to ⊆)

• Entity names:
  • &lt; for < (&#x003C;)
  • &gt; for > (&#x003E;)
  • &quot; for " (&#x0022;)
  • &amp; for & (&#x0026;)
  • &apos; for ’ (&#x0027;)

• comments: <!--...-->

• Processing instructions: <?....?>

• CDATA: <![CDATA[...]]>

17.5 Rendering Semantics

• Cascade Style Sheets (CSS)

  <table> <row num="1"> <cell>a1</cell> <cell>b1</cell> <cell>c1</cell> </row> <row num="2"> <cell>a2</cell> <cell>b2</cell> <cell>c2</cell> </row> </table>

  a1 b1 c1 a2 b2 c2

  table { border: 1pt black solid; display: table; } cell { color : red ; font-size : 120%; text-align: center; } row [ num = "1" ] { font-weight: bold; } @media speech { table { pitch: medium; pitch-range: 60; stress: 90; richness: 90 } row [ num = "1" ] { voice-family: harry, male } } @media print {...}

• XSL-FO (Formatting Objects)
  <xsl:template match="chapter/title"> 
    <fo:block text-align="center" space-after="8pt" 
              space-before="16pt" space-after.precedence="3"> 
      <xsl:apply-templates/> 
    </fo:block> 
  </xsl:template> 
   
  <xsl:template match="paragraph"> 
    <fo:block text-indent="2pc" space-after="7pt" 
              space-before.minimum="6pt" space-before.optimum="8pt" 
              space-before.maximum="10pt"> 
      <xsl:apply-templates/> 
    </fo:block> 
  </xsl:template> 
  

17.6 Recommended Standards

http://www.w3.org:

• XHTML: html, head, title, body, br, div, em, p, pre, span, strong a, dl, dt, dd, ol, ul, li, img, ...
  <html> 
  <head> 
     <title>A document</title> 
  </head> 
  <body> 
    <p>...Some standard and <br/> 
       <em>emphasized</em>text...</p> 
    <p>Course options: 
       <ul> 
          <li>CSE 670</li> 
          <li>CSE 680</li> 
       </ul> 
    </p> 
  </body> 
  </html> 
  

• MathML: math, mi, mn, mo, mtext, mspace, mrow, mfrac, msqrt, mroot, mfenced, msub, msup, msubsup, munder, mover, munderover, mmultiscripts, mtable, mtr, mlabeledtr, mtd, ....
  <math> 
  <mrow> 
    <mo stretchy="true"> ( </mo> 
    <mfrac> 
      <mi> a </mi> <mi> b </mi> 
    </mfrac> 
    <mo stretchy="true"> ) </mo> 
  </mrow> 
  </math> 
  

• SVG: rect, circle, ellipse, line, polyline, polygon, ....
  <svg> 
    <rect x="10" y="10" width="30" height="30" fill="blue"/> 
    <circle cx="600" cy="200" r="100" 
          fill="red" stroke="blue" stroke-width="10"  /> 
    <defs> 
      <rect id="MyRect" x="0" y="0" width="60" height="10"/> 
    </defs> 
    <use xlink:href="#MyRect" 
         transform="translate(20,2.5) rotate(10)" /> 
  </svg> 
  

17.7 Name Spaces

• A mechanism for separating the XML vocabularies
• Tag and attribute names: namespace + local name, separated by a colon
• Non-prefixed tag and attribute names are assumed to carry a default namespace.
• A name space is defined through a xmlns attribute
• A xmlns associates a name with a uniform resource identifier (URI).

17.8 Document Type Definitions (DTD’s)

• DTDs serve as grammars that XML documents must adhere to.

• Valid documents must conform to a DTD (besides being well formed).

• CDATA stands for character data.
• PCDATA stands for parsed character data.

17.9 A Java DTD-Based Validator

<..Dtd.java..>
 import java.io.File;
 import javax.xml.parsers.*;
 import org.xml.sax.*;
 import org.xml.sax.ext.*;
 
 class Dtd {
   static public void main(String[] args) throws Exception {
     SAXParserFactory factory = SAXParserFactory.newInstance();
     factory.setValidating( args.length>1 );
     SAXParser saxParser = factory.newSAXParser();
     XMLReader xmlReader = saxParser.getXMLReader();
     xmlReader.parse ( new File( args[args.length-1] ).toURL().toString() );
 } }
 -_-_-

javac Dtd.java

This command compiles the program

java Dtd xml-filename

This command checks whether the given file is well formed.

java Dtd validate xml-filename

This command checks whether the given file validates against the specified DTD.

17.10 XML Schema (XML Schema Definition, XSD)

• Proposed alternatives for DTDs.
• Supports data types, including definition of complex data types from simpler types
• Expressed in XML (can be manipulated by standard XML tools).

Elements and Attributes

• Definitions through named types
  <element name="..."  type="..."/> 
  
  <attribute name="..."  type="..."/> 
  

• Through anonymous types
  <element name="..."> 
    ...type definition... 
  </element> 
  
  <attribute name="..."> 
    ...type definition... 
  </element> 
  

• Default values
  <xs:element name="color" type="xs:string" default="red"/> 
  

• Fixed values
  <xs:attribute name="color" type="xs:string" fixed="red"/> 
  

• A simple element contains no elements or attributes

Simple Types

• Primitive: string, integer, decimal, boolean, date, time, ID, IDREF, ...

  ID and IDREF support key and referential integrity constraints.

• Type constructors:
  <simpleType name="..."> 
     ... 
  </simpleType> 
  

• Union constructor: data can be of any of the specified sub-types (heterogeneous type)
  <simpleType name="..."> 
     <union memberType="integer date" /> 
  </simpleType> 
  

• List constructor: multi-valued data
  <simpleType name="..."> 
     <list itemType="integer" /> 
  </simpleType> 
  

• Derived types via restriction on basic types
  <simpleType name="course"> 
     <restriction base="string"> 
        <pattern value="CSE[1-9][0-9]{2}"/> 
     </restriction> 
  </simpleType> 
  

Complex Types

• Type constructors:
  <complexType name="..."> 
    ... 
  </complexType> 
  

• For document model (text around elements):
  <complexType mixed="true"> 
     ... 
  </complexType> 
  

• Type compositors—combining into groups

  all	all the children, in any order <all>     <element name="name"  type="string"/>     <element name="ssn"   type="integer"/>  </all>
  choice	exactly one child
  sequence	all the children, in the given order
  Occurrence Indicators
  minOccurs maxOccurs	<sequence>     <element name="name"  type="string" maxOccurs="7" />     <element name="ssn"   type="integer"/>  </sequence>  Can be ‘unbounded’.

• Type extension
  <extension base="ns:myType"> 
    <sequence> 
      <element name="date"    type="date"/> 
      <element name="address" type="string"> 
    </sequence> 
  </extension> 
  

• Type restriction
  <restriction base="integer"> 
     <attribute name="status" type="string"/> 
  </restriction> 
  

  When a complex type is restricted, all the declarations must be repeated (with possible restrictions, e.g., maxOccurs="...").

Example

XML Code from a Database

From Relations to XML Sub-Schemas

XML Schemas for the Database

Primary Key and Unique Declarations

• A ‘key’ stands for a primary key
• The declaration of a key may include more than one field
• A ‘unique’ is defined similarly to a key; it allows NULL values

Foreign Key Declaration

[full code]

17.11 A Java XMLSchema-Based Validator

<..Xsd.java..>
 import javax.xml.validation.SchemaFactory;
 import javax.xml.XMLConstants;
 import javax.xml.transform.stream.StreamSource;
 import javax.xml.validation.Schema;
 import javax.xml.validation.Validator;
 
 class Xsd {
   public static void main(String[] args) {
     try {
        SchemaFactory factory = SchemaFactory.newInstance(
                                XMLConstants.W3C_XML_SCHEMA_NS_URI );
        Schema schema = factory.newSchema(new StreamSource( args[0] ));
        Validator validator = schema.newValidator();
        validator.validate(new StreamSource( args[1] ));
     } catch (Exception e) {
        System.err.println("--- Error --- " + e);
 } } }
 -_-_-

javac Xsd.java

This command compiles the program

java Xsd schema-filename xml-filename

This command checks whether the given file validates against the specified Schema.

17.12 XML Path (XPath)

A simple query language

Navigation Space (The Data Model)

• Addresses parts of an XML document.
• Uses a model of a tree of nodes: elements, attributes, text.

Navigation Directions

Directions of tree traversal:

• self
• child, descendant, descendant-or-self, attribute
• parent, ancestor, ancestor-or-self
• following-sibling, preceding-sibling
• / (root)

Path and Query Directives

The respond of a query is a set of nodes.

• child::course — selects the ‘course’ element children of the context node
• descendant::days – day element descendants
• attribute::attr — attr attribute

• child::* — all element children
• attribute::* — all attributes

• child::text() — text children
• child::node() — all children
• child::comment()

• child::course/descendant::days

• child::course[position()=1], child::course[1] (=last(), =last()-1, >1)

• child::course[@attr="database"] (’database’)

• child::course[ position()=2 ][ @attr="database"]

• self::*[ child::course and ... or ...]

Sample Program

<..Path.java..>
 import org.w3c.dom.NodeList;
 import org.xml.sax.InputSource;
 import javax.xml.xpath.XPath;
 import javax.xml.xpath.XPathConstants;
 import javax.xml.xpath.XPathFactory;
 
 
 class Path {
   public static void main(String[] args) {
     String expression;
     try {
       XPath xpath = XPathFactory.newInstance().newXPath();
       InputSource document = new InputSource("course.xml");
 
       expression = "COURSES/course";
       NodeList nameNodes = (NodeList) xpath.evaluate(expression,
               document, XPathConstants.NODESET);
       System.out.println("Number of courses: " + nameNodes.getLength());
 
       expression = "COURSES/course[1]/days";
       String days = (String) xpath.evaluate(expression, document,
               XPathConstants.STRING);
       System.out.println("Days: " + days);
 
     } catch (Exception e) {
         System.err.println("--- Error --- " + e);
 } } }
 -_-_-

17.13 Query Languages

• XSLT
  <xsl:template match="example"> 
    <div style="border: solid red"> 
       <xsl:apply-templates/> 
    </div> 
  </xsl:template> 
  

• xquery
  FOR     ... 
  WHERE   ... 
  RETURN  ... 
  
  for $pn in distinct-values(document("catalog.xml")//partno) 
  let $i := document("catalog.xml")//item[partno = $pn] 
  where count($i) >= 3 
  order by $pn 
  return 
     <well-supplied-item> 
        {$pn} 
        <avgprice> {avg($i/price)} </avgprice> 
     </well-supplied-item> 
  

17.14 Sample Problems

1. Give a XML document representing the relation ...
2. Give a XML document obeying the DTD ...
3. Give a XML document obeying the XML schema ...
4. Give a DTD appropriate for the XML document ...
5. Give a DTD appropriate for the XML schema ...
6. Give a XML schema appropriate for the XML document ...
7. Give a XML schema appropriate for the DTD ...

Reference: Ch. 27 (5th ed) and 26 (4th ed) in textbook.