XML Fundamentals

XML Fundamentals

Elliotte Rusty Harold

Monday, March 24, 2003

elharo@metalab.unc.edu

http://www.cafeconleche.org/

Outline

• Part I: XML Overview

• Part II: What is XML Good For?

• Part III: A Practical Example

• Part IV: Well-formedness

• Part V: DTDs and Validity

• Part VI: Namespaces

Part I: XML Overview

XML succeeded, and in ways that weren't expected - at least not by many. Originally it was conceived as a document-oriented technology for robust quality publishing of documents over networks. the original workplan had three pillars - XML syntax, XML link, and XML stylesheets. Schemas were not high on the agenda and XML was not seen as an infrastructure for middleware or glueware. It was expected that at some stage it would be necessary to manage data but there was little activity in this area in 1997. When developing Chemical Markup Language (which must be one of the first published XML applications), I found the lack of datatypes very frustrating!

Well, XML is now a basic infrastructure of much modern information. I doubt that anyone now designs a protocol, or operating system without including XML. Although this list sometimes complains that XML isn't as clean as we would like, it works, and it works pretty well.

--Peter Murray-Rust on the xml-dev mailing list, Thursday, 07 Feb 2002

What is XML?

• Extensible Markup Language

• A syntax for documents

• A Meta-Markup Language

• A Structural and Semantic language, not a formatting language

• Not just for Web pages

XML is a Meta Markup Language

• Not like HTML, troff, LaTeX

• Make up the tags you need as you need them

• The tags you create can be documented in a Document Type Definition (DTD)

• A meta syntax for domain-specific markup languages like MusicML, MathML, and CML

XML describes structure and semantics, not formatting

• XML documents form a tree

• Element and attribute names reflect the kind of the element

• Formatting can be added with a style sheet

A Song Description in HTML

<dt>Hot Cop
<dd> by Jacques Morali, Henri Belolo, and Victor Willis
<ul>
<li>Jacques Morali
<li>PolyGram Records
<li>6:20
<li>1978
<li>Village People
</ul>

A Song Description in XML

<?xml version="1.0" encoding="ISO-8859-1" standalone="yes"?>
<SONG>
  <TITLE>Hot Cop</TITLE>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <LENGTH>6:20</LENGTH>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>
</SONG>

The XML Declaration

• At the top of the document, you normally find an XML declaration:

  <?xml version="1.0" encoding="ISO-8859-1" standalone="yes"?>

• version attribute

  • required

  • always has the value 1.0

• standalone attribute

  • yes

  • no

• encoding attribute

  • UTF-8

  • ISO-8859-1

  • etc.

Elements

• Documents are composed of elements

• An element is delimited by a start-tag and a matching end-tag:

  <COMPOSER>Jacques Morali</COMPOSER>

  • Start-tag <COMPOSER>

  • Contents "Jacques Morali"

  • End-tag </COMPOSER>

• Elements can contain other elements:

  <SONG>
    <TITLE>Hot Cop</TITLE>
    <COMPOSER>Jacques Morali</COMPOSER>
    <COMPOSER>Henri Belolo</COMPOSER>
    <COMPOSER>Victor Willis</COMPOSER>
    <PRODUCER>Jacques Morali</PRODUCER>
    <PUBLISHER>PolyGram Records</PUBLISHER>
    <LENGTH>6:20</LENGTH>
    <YEAR>1978</YEAR>
    <ARTIST>Village People</ARTIST>
  </SONG>
  

Cascading Style Sheets

• A simple and straight-forward language for applying styles like bold and Helvetica to particular XML elements.

• Rather than being stored as part of the document itself, all the style information is placed in a separate document called a style sheet.

• Partially supported by Mozilla, Netscape 6, IE 5.0/5.5, and Opera 4.0/5.0

• Full W3C Recommendation

CSS Stylesheet for Songs

SONG     {display: block}TITLE    {display: block; font-family: Helvetica, sans-serif;          font-size: 20pt; font-weight: bold}COMPOSER {display: block;           font-family: Times, Times New Roman, serif;          font-size: 14pt;          font-style: italic}ARTIST   {display: block;           font-family: Times, Times New Roman, serif;          font-size: 14pt; font-weight: bold;          font-style: italic}PUBLISHER {display: block;            font-family: Times, Times New Roman, serif;           font-size: 14pt}LENGTH    {display: block;            font-family: Times, Times New Roman, serif;           font-size: 14pt}YEAR      {display: block;            font-family: Times, Times New Roman, serif;           font-size: 14pt}

Attaching style sheets to documents

<?xml version="1.0"?>
<?xml-stylesheet type="text/css" href="song1.css"?>
<SONG>
  <TITLE>Hot Cop</TITLE>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <LENGTH>6:20</LENGTH>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>
</SONG>

song.xsl

<?xml version="1.0"?>
<xsl:stylesheet version="1.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

  <xsl:template match="SONG">
    <html>
      <body>
       <h1>
        <xsl:value-of select="TITLE"/> 
        by the 
        <xsl:value-of select="ARTIST"/> 
       </h1>
       <ul>
         <xsl:apply-templates select="COMPOSER"/>
         <li>Publisher: <xsl:value-of select="PUBLISHER"/></li>
         <li>Year: <xsl:value-of select="YEAR"/></li>
         <li>Producer: <xsl:value-of select="PRODUCER"/></li>
       </ul>
      </body>
    </html>
  </xsl:template>

  <xsl:template match="COMPOSER">
    <li>Composer: <xsl:value-of select="."/></li>
  </xsl:template>

</xsl:stylesheet>

Applying an XSLT Style Sheet

• Browser support is weak to non-existent.

• Can use third party tools like Xalan, Saxon, and XT

• Let's use Saxon to apply this stylesheet to compositions.xml.

  • Windows executable:

    C:\> saxon hotcop.xml song.xsl>hotcop.html

  • Java executable:

    C:\> java com.icl.saxon.StyleSheet hotcop.xml song.xsl>hotcop.html

Output

<html>
   <body>
      <h1>Hot Cop 
         by the 
         Village People
      </h1>
      <ul>
         <li>Composer: Jacques Morali</li>
         <li>Composer: Henri Belolo</li>
         <li>Composer: Victor Willis</li>
         <li>Publisher: PolyGram Records</li>
         <li>Year: 1978</li>
         <li>Producer: Jacques Morali</li>
      </ul>
   </body>
</html>

Editing and Saving XML Documents

• Plain ASCII or UTF-8 text

• .xml is standard file extension

• Any standard text editor will work

Well-formedness

Rules:

• Open and close all tags

• Empty-element tags end with />

• There is a unique root element

• Elements may not overlap

• Attribute values are quoted

• < and & are only used to start tags and entity references

• Only the five predefined entity references are used

• Plus more...

Validity

To be valid an XML document must be

1. Well-formed

2. Must have a Document Type Definition (DTD)

3. Must comply with the constraints specified in the DTD

A DTD for Songs

<!ELEMENT SONG (TITLE, COMPOSER+, PRODUCER*, PUBLISHER*, 
                 LENGTH?, YEAR?, ARTIST+)>

<!ELEMENT TITLE (#PCDATA)>

<!ELEMENT COMPOSER (#PCDATA)>
<!ELEMENT PRODUCER (#PCDATA)>
<!ELEMENT PUBLISHER (#PCDATA)>
<!ELEMENT LENGTH (#PCDATA)>
<!-- This should be a four digit year like "1999",
     not a two-digit year like "99" -->
<!ELEMENT YEAR (#PCDATA)>

<!ELEMENT ARTIST (#PCDATA)>

A Valid Song Document

<?xml version="1.0"?>
<!DOCTYPE SONG SYSTEM "song.dtd">
<SONG>
  <TITLE>Hot Cop</TITLE>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <LENGTH>6:20</LENGTH>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>
</SONG>

Checking Validity

To check validity you pass the document through a validating parser which should report any errors it finds. For example,

% java dom.Counter -v invalidhotcop.xml
[Error] invalidhotcop.xml:10:8: The content of element type "SONG" must match 
"(TITLE,COMPOSER+,PRODUCER*,PUBLISHER*,LENGTH?,YEAR?,ARTIST+)".
invalidhotcop.xml: 862;70;0 ms (7 elems, 0 attrs, 19 spaces, 59 chars)

A valid document:

% java dom.Counter -v validhotcop.xml
validhotcop.xml: 671;70;0 ms (10 elems, 0 attrs, 28 spaces, 98 chars)

A More Complex Example

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<?xml-stylesheet type="text/css" href="song.css"?>
<!DOCTYPE SONG SYSTEM "expanded_song.dtd">
<SONG xmlns="http://metalab.unc.edu/xml/namespace/song"
      xmlns:xlink="http://www.w3.org/1999/xlink">
  <TITLE>Hot Cop</TITLE>
  <PHOTO 
    xlink:type="simple" xlink:show="onLoad" xlink:href="hotcop.jpg"
    ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200"/>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <!-- The publisher is actually Polygram but I needed 
       an example of a general entity reference. -->
  <PUBLISHER xlink:type="simple" xlink:href="http://www.amrecords.com/">
    A &amp; M Records
  </PUBLISHER>
  <LENGTH>6:20</LENGTH>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>
</SONG>
<!-- You can tell what album I was 
     listening to when I wrote this example -->

Attributes

<PHOTO xlink:type="simple" xlink:show="onLoad" xlink:href="hotcop.jpg" ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200" />

• name="value" same as in HTML

• Generally used for meta-information

• Attribute values are quoted with either single or double quotes:

  • Good:

  • <A HREF="http://www.cafeconleche.org/">

  • <DIV ALIGN='CENTER'>

  • <A HREF="http://www.cafeconleche.org/">

  • <EMBED SRC="minnesotaswale.aif" hidden="true">

  • Bad:

  • <A HREF=http://www.cafeconleche.org/>

  • <DIV ALIGN=CENTER>

  • <EMBED SRC=minnesotaswale.aif hidden=true>

  • <EMBED SRC="minnesotaswale.aif" hidden>

Empty-element Tags

<PHOTO xlink:type="simple" xlink:show="onLoad" xlink:href="hotcop.jpg" ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200" />

• Ends with /> instead of >

• <PHOTO/> is semantically the same as <PHOTO></PHOTO>

• Just syntactic sugar

Comments

<!-- You can tell what album I was listening to when I wrote this example -->

• Essentially the same as in HTML

Namespaces

• Let you mix and match different XML vocabularies

• URIs identify elements and attributes that belong to different XML applications

• Prefixes can change if the URI stay the same

<SONG xmlns="http://www.cafeconleche.org/namespace/song"
      xmlns:xlink="http://www.w3.org/1999/xlink">
  <TITLE>Hot Cop</TITLE>
  <PHOTO 
    xlink:type="simple" xlink:show="onLoad" xlink:href="hotcop.jpg"
    ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200"/>
  <COMPOSER>Jacques Morali</COMPOSER>
  <PUBLISHER xlink:type="simple" xlink:href="http://www.amrecords.com/">
    A &amp; M Records
  </PUBLISHER>
  <ARTIST>Village People</ARTIST>
</SONG>

Entity References

A & M Records

• < and & are only used to start tags and entities

  • Good:

    <H1>O'Reilly &amp; Associates</H1>

  • Bad:

    <H1>O'Reilly & Associates</H1>

  • Good:

    <CODE>for (int i = 0; i &lt;= args.length; i++ ) { </CODE>

  • Bad:

    <CODE>for (int i = 0; i <= args.length; i++ ) { </CODE>

• Only the five predefined entity references are used

  • Good:

    • &

    • <

    • >

    • "

    • '

  • Bad:

    • ©

    • ®

    • &tm;

    • α

    • é

    •  

    • etc.

• Entity references must end with a semicolon.

  • < is good

  • &lt is bad

A More Complex DTD

<!ELEMENT SONG (TITLE, PHOTO?, COMPOSER+, PRODUCER*, 
 PUBLISHER*, LENGTH?, YEAR?, ARTIST+)>
<!ATTLIST SONG xmlns       CDATA #REQUIRED
               xmlns:xlink CDATA #REQUIRED>
<!ELEMENT TITLE (#PCDATA)>

<!ELEMENT PHOTO EMPTY>
<!ATTLIST PHOTO xlink:type CDATA #FIXED "simple"
                xlink:href CDATA #REQUIRED
                xlink:show CDATA #IMPLIED
                ALT        CDATA #REQUIRED
                WIDTH      CDATA #REQUIRED
                HEIGHT     CDATA #REQUIRED
>

<!ELEMENT COMPOSER  (#PCDATA)>
<!ELEMENT PRODUCER  (#PCDATA)>
<!ELEMENT PUBLISHER (#PCDATA)>
<!ATTLIST PUBLISHER xlink:type CDATA #IMPLIED
                    xlink:href CDATA #IMPLIED
>

<!ELEMENT LENGTH (#PCDATA)>
<!-- This should be a four digit year like "1999",
     not a two-digit year like "99" -->
<!ELEMENT YEAR (#PCDATA)>

<!ELEMENT ARTIST (#PCDATA)>

Part II: What is XML Good For?

What is XML used for?

• Domain-Specific (Vertical) Markup Languages

• Self-Describing Data

• Interchange of Data Among Applications

• Structured and Integrated Data

Domain-Specific Markup Languages

• Markup language for a vertical market

• Non-proprietary format

• Don't pay for what you don't use

Self-Describing Data

• Much data is lost due to format problems

• XML is very simple

• XML is self-describing

• XML is well documented

An XML Fragment

<PERSON ID="p1100" SEX="M">
  <NAME>
    <GIVEN>Judson</GIVEN>
    <SURNAME>McDaniel</SURNAME>
  </NAME>
  <BIRTH>
    <DATE>21 Feb 1834</DATE>
  </BIRTH>
  <DEATH>
    <DATE>9 Dec 1905</DATE>
  </DEATH>
</PERSON>

Interchange of Data Among Applications

• E-commerce

• Syndication

• EAI and EDI

Can assemble data from multiple sources

• A document can be assembled from multiple physical storage entities

• These may be files, database queries, or anything that can be referred to by a URI

• Can even include non-XML content

XML Applications

• A specific markup language that uses the XML meta-syntax is called an XML application

• Different XML applications have their own more constricted syntaxes and vocabularies within the broader XML syntax

• Further syntax can be layered on top of this; e.g. data typing through schemas

Example XML Applications

• Web Pages

• Mathematical Equations

• Music Notation

• Vector Graphics

• and more...

Mathematical Markup Language

<?xml version="1.0"?>
<html>
<head>
<title>Fiat Lux</title>
<meta name="GENERATOR" content="amaya V1.3b" />
</head>
<body>

<p>
And God said,
</p>

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <mrow>
    <msub>
      <mi>&delta;</mi>
      <mi>&alpha;</mi>
    </msub>
    <msup>
      <mi>F</mi>
      <mi>&alpha;&beta;</mi>
    </msup>
    <mo>=</mo>
    <mfrac>
      <mrow>
        <mn>4</mn>
        <mi>&pi;</mi>
      </mrow>
      <mi>c</mi>
    </mfrac>
    <msup>
      <mi>J</mi>
      <mrow>
        <mi>&beta;</mi>
      </mrow>
    </msup>
  </mrow>
</math>

<p>
and there was light.
</p>
</body>
</html>

Channel Definition Format

<?xml version="1.0"?>
<CHANNEL HREF="http://www.cafeconleche.org/index.html">
  <TITLE>Cafe con Leche</TITLE>
  <ITEM HREF="http://www.cafeconleche.org/books.html">
    <TITLE>Books about XML</TITLE>
  </ITEM>
  <ITEM HREF="http://www.cafeconleche.org/tradeshows.html">
    <TITLE>Trade shows and conferences about XML</TITLE>
  </ITEM>
  <ITEM HREF="http://www.cafeconleche.org/lists.htm">
    <TITLE>Mailing Lists dedicated to XML</TITLE>
  </ITEM>
</CHANNEL>

Classic Literature

• The Complete Plays of Shakespeare

• The Bible

• The Koran

• The Book of Mormon

• Joseph Conrad's Heart of Darkness

• The entire Project Gutenberg corpus

Vector Graphics

• Vector Markup Language (VML)

• Internet Explorer 5.0, 5.5

• Microsoft Office 2000

• Scalable Vector Graphics (SVG)

  • Adobe SVG Plug-In

  • Apache Batik

XML for XML

• XSL: The Extensible Stylesheet Language

• The W3C XML Schema Language

• XLink: The Extensible Linking Language

XSL: The Extensible Stylesheet Language

• XSL Transformations

• XSL Formatting Objects

Schemas

• Data typing in XML is weak

• DTDs use a strange non-XML syntax

• Limited compatibility with namespaces

• Limited extensibility

• Schemas fix all these problems

• There are multiple schema languages including:

  • Rick Jelliffe's Schematron

  • Murato Makoto's and James Clark's RELAX NG

  • The W3C XML Schema Language

W3C XML Schema Language Example

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
  <xsd:element name="SONG" type="SongType"/>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE" type="xsd:string" 
                   minOccurs="1" maxOccurs="1"/>
      <xsd:element name="COMPOSER"  type="xsd:string" 
                   minOccurs="1" maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
                   minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
                   minOccurs="0" maxOccurs="1"/>
    
      <xsd:element name="LENGTH" type="xsd:duration" 
                   minOccurs="0" maxOccurs="1"/>
      <xsd:element name="YEAR"   type="xsd:gYear" 
                   minOccurs="1" maxOccurs="1"/>
  
      <xsd:element name="ARTIST" type="xsd:string" 
                   minOccurs="1" maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

</xsd:schema>

XLinks

• Any element can be a link

• Links can be bi-directional

• Links can be separated from the documents they connect

<footnote xlink:type="simple" xlink:href="footnote7.xml">7</footnote>

File Formats, in-house applications, and other behind the scenes uses

• Microsoft Office 2000

• Netscape What's Related

Part III: A Practical Example

A larger example: Music Catalog

• Examine the data

• Design a vocabulary for the data

• Write a style sheet

Sample Catalog

Organizing the Data

• XML documents are trees.

• XML elements contain other elements as well as text

• Within these limits there's more than one way to organize the data

  • Hierarchically

  • Relationally

  • Objects

What is the Root Element

• The catalog?

• A custom Document element?

The Root Element

• Choose catalog for the root element

• Everything else will be a descendant of catalog

• This is not the only possible choice

<?xml version="1.0" encoding="ISO-8859-1"?>
<catalog>
  Everything else will go here...
</catalog>

What are the Immediate Children of the Root?

• Composers?

• Songs/Compositions?

• Categories?

• All of the Above?

Child Elements

<?xml version="1.0" encoding="ISO-8859-1"?>
<catalog>

  <category>
    Small chamber ensembles - 2-4 Players by New York Women Composers
  </category>

</catalog>

White space in XML is not especially significant

<?xml version="1.0" encoding="ISO-8859-1"?>
<catalog><category>Small chamber ensembles 
- 2-4 Players by New York Women Composers</category></catalog>

Composers

Each composer has a name

<?xml version="1.0" encoding="ISO-8859-1"?>
<catalog>

  <category>
    Small chamber ensembles - 2-4 Players by New York Women Composers
  </category>

  <composer>
    <name>Julie Mandel</name>
  </composer>

  <composer>
    <name>Margaret De Wys</name>
  </composer>  
    
  <composer>
    <name>Beth Anderson</name>
  </composer>
    
  <composer>
    <name>Linda Bouchard</name>
  </composer>

</catalog>

Grand Children

• It's better for sorting to divide names into first, middle, and last

• Some (e.g. middle name) elements may be empty

<?xml version="1.0" encoding="ISO-8859-1"?>
<catalog>

  <category>
    Small chamber ensembles - 2-4 Players by New York Women Composers
  </category>

  <composer>
    <name>
      <first_name>Julie</first_name> 
      <middle_name></middle_name> 
      <last_name>Mandel</last_name>
    </name>
  </composer>

  <composer>
    <name>
      <first_name>Margaret</first_name> 
      <middle_name>De</middle_name> 
      <last_name>Wys</last_name>
    </name>
  </composer>  
    
  <composer>
    <name>
      <first_name>Beth</first_name> 
      <middle_name></middle_name> 
      <last_name>Anderson</last_name>
    </name>
  </composer>
    
  <composer>
    <name>
      <first_name>Linda</first_name> 
      <middle_name></middle_name> 
      <last_name>Bouchard</last_name>
    </name>
  </composer>

</catalog>

Attributes

• Some people have the same names

• Use an ID number to disambiguate

• Store the ID number in an id attribute

• name=value

• An element may not have two attributes with the same name

• Attribute values must be quoted

<?xml version="1.0" encoding="ISO-8859-1"?>
<catalog>

  <category>
    Small chamber ensembles - 2-4 Players by New York Women Composers
  </category>

  <composer id="c1">
    <name>
      <first_name>Julie</first_name> 
      <middle_name></middle_name> 
      <last_name>Mandel</last_name>
    </name>
  </composer>

  <composer id="c2">
    <name>
      <first_name>Margaret</first_name> 
      <middle_name>De</middle_name> 
      <last_name>Wys</last_name>
    </name>
  </composer>  
    
  <composer id="c3">
    <name>
      <first_name>Beth</first_name> 
      <middle_name></middle_name> 
      <last_name>Anderson</last_name>
    </name>
  </composer>
    
  <composer id="c4">
    <name>
      <first_name>Linda</first_name> 
      <middle_name></middle_name> 
      <last_name>Bouchard</last_name>
    </name>
  </composer>

</catalog>

Attributes vs. Elements

• Attribute are for meta-data; elements are for data.

• Does the reader want to see the information? If yes, use element content; if no, use attributes

• Attributes are good for ID numbers, URLs, references, and other information not directly relevant to the reader

When not to use attributes

• Attributes can't hold structure well.

• Elements allow you to include meta-meta-data (information about the information about the information).

• Not everyone always agrees on what is and isn't meta-data.

• Elements are more extensible in the face of future changes.

Compositions

Let's look at an example of what we want:

Rendered HTML:

Brass Swale (1988) 5", tbn, 2 Bfl tpts, bar. hn

Tonal. Commissioned/Premiered by the Redlands' New Music Ensemble. (A swale is a meadow or a marsh where a lot of wild plants grow together. The composer discovered the word when a horse named Swale won the Kentucky Derby several years ago. Since her work is primarily collage of newly composed musical swatches, she has used the name extensively.) ACA - American Composers Alliance

Or in HTML:

<dt><cite>Brass Swale</cite> (1988) 5", tbn, 2 Bfl tpts, bar. hn</dt>
<dd><p>
Tonal. Commissioned/Premiered by the Redlands' New Music 
Ensemble. (A swale is a meadow or a marsh where a lot of 
wild plants grow together. The composer discovered the word 
when a horse named Swale won the Kentucky Derby several 
years ago. Since her work is primarily collage of newly 
composed musical swatches, she has used the name 
extensively.)  ACA - American Composers 
Alliance</p>
</dd>

Each composition has a

• Title

• Date

• Description

• List of instruments

• Length

• Publisher

• Some pieces may be missing from some compositions

Composition Example in XML

  <composition>
    <title>Brass Swale</title>
    <date>1988</date> 
    <length>5"</length>
    <instruments>tbn, 2 Bfl tpts, bar, hn</instruments>
    <description>
      Tonal. Commissioned/Premiered by the Redlands' New Music
      Ensemble. (A swale is a meadow or a marsh where a lot of
      wild plants grow together. The composer discovered the word
      when a horse named Swale won the Kentucky Derby several
      years ago. Since her work is primarily collage of newly
      composed musical swatches, she has used the name
      extensively.)
    </description>
    <publisher>ACA - American Composers Alliance</publisher>
  </composition>

Further Divisions

  <composition>
    <title>Trio for Flute, Viola and Harp</title>
    <date><year>1994</year></date> 
    <length>13'38"</length>
    <instruments>fl, hp, vla</instruments>
    <description>
      <p>Premiered at Queens College in April, 1996 by Sue Ann Kahn, 
      Christine Ims, and Susan Jolles. In 3 movements :</p>
      <ul>
        <li>mvt. 1: 5:01</li>
        <li>mvt. 2: 4:11</li>
        <li>mvt. 3: 4:26</li>
      </ul>  
    </description>
    <publisher>Theodore Presser</publisher>
  </composition>

Attaching the Composer to the Composition

  <composition composer="c3">
    <title>Trio: Dream in D</title>
    <date><year>1980</year></date> 
    <length>10'</length>
    <instruments>fl, pn, vc, or vn, pn, vc</instruments>
    <description>
      Rhapsodic. Passionate. Available on CD 
      <cite><a href=
       "http://www.amazon.com/exec/obidos/ASIN/B000007NMH/qid=913265342/sr%3D1-2/">
       Two by Three
      </a></cite> from North/South Consonance (1998).
    </description> 
    <publisher></publisher>
  </composition>

Some Keywords For the Search Engines

<?xml version="1.0" encoding="ISO-8859-1"?>
<catalog>

  <category>
    Small chamber ensembles - 2-4 Players by New York Women Composers
  </category>

  <cataloging_info>
    <abstract>Compositions by the members of New York Women Composers</abstract>
    <keyword>music publishing</keyword>
    <keyword>scores</keyword>
    <keyword>women composers</keyword>
    <keyword>New York</keyword>
  </cataloging_info>

  <composer id="c1">
    <name>
      <first_name>Julie</first_name> 
      <middle_name></middle_name> 
      <last_name>Mandel</last_name>
    </name>
  </composer>
  
  ...
  
</catalog>

Standard Signature

• Copyright notice

• Name of maintainer

• Email address of maintainer

• Last modified date

<?xml version="1.0" encoding="ISO-8859-1"?>
<catalog>
...
  <last_updated>July 28, 1999</last_updated>
  <copyright>1999 New York Women Composers</copyright>
  <maintainer email="elharo@metalab.unc.edu" 
              url="http://www.macfaq.com/personal.html">
    <name>
      <first_name>Elliotte</first_name> 
      <middle_name>Rusty</middle_name> 
      <last_name>Harold</last_name>
    </name>
  </maintainer>

</catalog>

CSS Style Sheet for the Catalog

category { display: block; 
          font-family: Helvetica, Arial, sans;
          font-size: 32pt; 
          font-weight: bold; 
          text-align: center
         }
       
catalog { font-family: "New York", "Times New Roman", serif; 
          font-size: 14pt; 
          background-color: white; 
          color: black; 
          display: block
        }
      
composer { display: block; 
           font-family: Helvetica, Arial, sans;
           font-size: 24pt; 
           font-weight: bold; 
           text-align: left
         }  
       
composition title { display: block; 
       font-family: Helvetica, Arial, sans;
       font-size: 18pt; 
       font-weight: bold; 
       text-align: left}
       
composition * {display:list-item}
       
description {display: block}
              
// cataloging_info is only for search engines
cataloging_info { display: none;
       color: #FFFFFF}
       
last_updated, copyright, maintainer {display: block;
       font-size: small}
       
copyright:before {content: "Copyright " }

last_updated:before {content: "Last Modified " }

last_updated {margin-top: 2ex }

Possible Extensions

• Should be able to match composers with compositions

• Should be able to sort composers and compositions by name

• Should be able to include data from attributes; e.g. the maintainer's email address

• Horizontal rules would be nice

• Better header (e.g. title and meta tags) would be nice

Possible Solutions

• CSS Level 3?

• XSL

• XSL + JavaScript

CSS or XSL?

• CSS has broader support.

• CSS is simpler.

• XSL is much more powerful.

• XSL can be used without browser support by transforming to HTML on the server side.

Part IV: Well-formedness

• Every XML document must be well-formed.

• Parsers are not allowed to accept malformed XML documents.

Well-formedness Rules

• Open and close all tags

• Empty-element tags end with />

• There is a unique root element

• Elements may not overlap

• Attribute values are quoted

• < and & are only used to start tags and entities

• Only the five predefined entity references are used

• Plus more...

Open and close all tags

• Good:

  • <p>The quick brown fox jumped over the lazy dog</p>

  • <li>A very <B>important</B> point</li>

  • Copyright 1999 Elliotte Rusty Harold<br></br>

• Bad:

  • The quick brown fox jumped over the lazy dog<p>

  • <li>A very <B>important point

  • Copyright 1999 Elliotte Rusty Harold<br>

Empty-element tags end with />

• <BR/>, <HR/>, and <IMG/> instead of <BR>, <HR>, and <IMG>

• Web browsers deal inconsistently with these

• Can use <BR></BR> <HR></HR> <IMG></IMG> instead

• <BR CLASS="EMPTY"/> seems to work best.

There is a unique root element

• One element completely contains all other elements of the document

• This is HTML in HTML files

• The XML declaration and xml-stylesheet processing instruction are not elements

Elements may not overlap

• If an element contains a start tag for an element, it must also contain the corresponding end tag

• Empty elements may appear anywhere

• Every non root element has a parent element

Attribute values are quoted

• Good:

• <A HREF="http://www.cafeconleche.org/">

• <DIV ALIGN="CENTER">

• <A HREF="http://www.cafeconleche.org/">

• <EMBED SRC="minnesotaswale.aif" hidden="hidden">

• Bad:

• <A HREF=http://www.cafeconleche.org/>

• <DIV ALIGN=CENTER>

• <EMBED SRC=minnesotaswale.aif hidden=hidden>

• <EMBED SRC="minnesotaswale.aif" hidden>

< and & are only used to start tags and entities

• Good:

  
  <H1>O'Reilly &amp; Associates</H1>
  

• Bad:

  
  <H1>O'Reilly & Associates</H1>
  

• Good:

  <CODE>for (int i = 0; i &lt;= args.length; i++ ) { </CODE>
  

• Bad:

  <CODE>for (int i = 0; i <= args.length; i++ ) { </CODE>
  

Only the five predefined entity references are used

• Good:

• &amp;

• &lt;

• &gt;

• &quot;

• &apos;

• Bad:

• &copy;

• &reg;

• &tm;

• &alpha;

• &eacute;

• &nbsp;

• etc.

• Entity references must end with a semicolon.

  • < is good

  • &lt is bad

Character References

• Decimal:

  ¡	¡
  ¢	¢
  £	£
  ¤	¤
  ¥	¥
  ¦	¦

• Hexadecimal

  ¡	¡
  ¢	¢
  £	£
  ¤	¤
  ¥	¥
  ¦	¦

Part V: DTDs

Well-formedness vs. validity

• There are two levels of conformance to XML

• Well-formed documents are correct with or without a DTD. They adhere to the basic syntax rules of XML

• Valid documents also adhere to the constraints specified in a DTD

• All valid documents are well-formed; not all well-formed document are valid.

DTDs and Validity

• A Document Type Definition (DTD) describes the elements and attributes that may appear in a document

• Validation compares a particular document against a DTD

• Well-formedness is a prerequisite for validity

What is a DTD?

• A DTD lists the elements, attributes, and entities contained in a document

• A DTD defines the relationships between different elements and attributes

• A DTD for songs:

  <!ELEMENT SONG (TITLE, COMPOSER+, PRODUCER*, PUBLISHER*, 
                   LENGTH?, YEAR?, ARTIST+)>
  
  <!ELEMENT TITLE (#PCDATA)>
  
  <!ELEMENT COMPOSER (#PCDATA)>
  <!ELEMENT PRODUCER (#PCDATA)>
  <!ELEMENT PUBLISHER (#PCDATA)>
  <!ELEMENT LENGTH (#PCDATA)>
  <!-- This should be a four digit year like "1999",
       not a two-digit year like "99" -->
  <!ELEMENT YEAR (#PCDATA)>
  
  <!ELEMENT ARTIST (#PCDATA)>

• Normally stored in a separate file

Validity

To be valid an XML document must be

1. Well-formed

2. Must have a document type declaration

3. Must comply with the constraints specified in the DTD

A Valid Song Document

<?xml version="1.0"?>
<!DOCTYPE SONG SYSTEM "song.dtd">
<SONG>
  <TITLE>Hot Cop</TITLE>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <LENGTH>6:20</LENGTH>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>
</SONG>

Checking Validity

To check validity you pass the document through a validating parser which should report any errors it finds. For example,

% java sax.Counter -v invalidhotcop.xml
Error at (file file:/D:/speaking/SD99EAST/dtds/invalidhotcop.xml, line 10, char
8): Element "<SONG>" is not valid because it does not follow the rule, "(TITLE,C
OMPOSER+,PRODUCER*,PUBLISHER*,LENGTH?,YEAR?,ARTIST+)".
invalidhotcop.xml: 281 ms

A valid document:

% java sax.Counter -v validhotcop.xml
validhotcop.xml: 170 ms

Internal DTD Subsets

<?xml version="1.0"?>
<!DOCTYPE SONG [
  <!ELEMENT SONG (TITLE, COMPOSER+, PRODUCER*, PUBLISHER*, 
                  LENGTH?, YEAR?, ARTIST+)>

  <!ELEMENT TITLE (#PCDATA)>

  <!ELEMENT COMPOSER (#PCDATA)>
  <!ELEMENT PRODUCER (#PCDATA)>
  <!ELEMENT PUBLISHER (#PCDATA)>
  <!ELEMENT LENGTH (#PCDATA)>
  <!-- This should be a four digit year like "1999",
     not a two-digit year like "99" -->
  <!ELEMENT YEAR (#PCDATA)>

  <!ELEMENT ARTIST (#PCDATA)>
]>
<SONG>
  <TITLE>Hot Cop</TITLE>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <LENGTH>6:20</LENGTH>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>
</SONG>

The importance of validation

• Ensures that data is correct before feeding it into a program

• Ensures that a format is followed

• Establishes what must be supported

• Not all documents need to be valid; sometimes well-formed is enough

Where are DTDs Important?

• Domain-Specific Markup Languages

• Self-Describing Data

• Interchange of Data Among Applications

• Structured and Integrated Data

Domain-Specific Markup Languages

• A DTD precisely describes the format

• DTDs verify that documents adhere to the format

• Ensures interoperability of unrelated tools

Self-Describing Data

• DTDs explain the format so reverse engineering isn't as necessary

• Comments in DTDs can go even further

<!-- This should be a four digit year like "1999",
     not a two-digit year like "99" -->
<!ELEMENT YEAR (#PCDATA)>

Interchange of Data Among Applications

• E-commerce and syndication

• DTDs make sure that two independent applications speak the same language

• DTDs detect malformed data

• DTDs verify correct data

Structured and Integrated Data

• Can specify relationships between elements using element declarations

• Can assemble data from multiple sources using external entity references declared in the DTD

An Example Document

<?xml version="1.0" encoding="ISO-8859-1"?>
<catalog>

  <category>
    Small chamber ensembles - 2-4 Players by New York Women Composers
  </category>

  <cataloging_info>
    <abstract>Compositions by the members of New York Women Composers</abstract>
    <keyword>music publishing</keyword>
    <keyword>scores</keyword>
    <keyword>women composers</keyword>
    <keyword>New York</keyword>
  </cataloging_info>

  <last_updated>July 28, 1999</last_updated>
  <copyright>1999 New York Women Composers</copyright>
  <maintainer email="elharo@metalab.unc.edu" 
              url="http://www.macfaq.com/personal.html">
    <name>
      <first_name>Elliotte</first_name> 
      <middle_name>Rusty</middle_name> 
      <last_name>Harold</last_name>
    </name>
  </maintainer>

  <composer id="c1">
    <name>
      <first_name>Julie</first_name> 
      <middle_name></middle_name> 
      <last_name>Mandel</last_name>
    </name>
  </composer>

  <composer id="c2">
    <name>
      <first_name>Margaret</first_name> 
      <middle_name>De</middle_name> 
      <last_name>Wys</last_name>
    </name>
  </composer>  
    
  <composer id="c3">
    <name>
      <first_name>Beth</first_name> 
      <middle_name></middle_name> 
      <last_name>Anderson</last_name>
    </name>
  </composer>
    
  <composer id="c4">
    <name>
      <first_name>Linda</first_name> 
      <middle_name></middle_name> 
      <last_name>Bouchard</last_name>
    </name>
  </composer>
    
  <composition composer="c1">
    <title>Trio for Flute, Viola and Harp</title>
    <date><year>(1994)</year></date> 
    <length>13'38"</length>
    <instruments>fl, hp, vla</instruments>
    <description>
      <p>Premiered at Queens College in April, 1996 by Sue Ann Kahn, 
      Christine Ims, and Susan Jolles. In 3 movements :</p>
      <ul>
        <li>mvt. 1: 5:01</li>
        <li>mvt. 2: 4:11</li>
        <li>mvt. 3: 4:26</li>
      </ul>  
    </description>
    <publisher>Theodore Presser</publisher>
  </composition>

  <composition composer="c2">
    <title>Charmonium</title>
    <date><year>(1991)</year></date> 
    <length>9'</length>
    <instruments>2 vln, vla, vc</instruments>
    <description>
      Commissioned as quartet for the Meridian String Quartet. 
      Sonorous, bold. Moderate difficulty. Tape available.
    </description> 
    <publisher></publisher>
  </composition>

  <composition composer="c1">
    <title>Invention for Flute and Piano</title>
    <date><year>(1994)</year></date> 
    <length></length>
    <instruments>fl, pn</instruments>
    <description>3 movements</description> 
    <publisher></publisher>
  </composition>

  <composition composer="c3">
    <title>Little Trio</title>
    <date><year>(1984)</year></date> 
    <length>4'</length>
    <instruments>fl, guit, va</instruments>
    <description></description> 
    <publisher>ACA</publisher>
  </composition>

  <composition composer="c3">
    <title>Dr. Blood's Mermaid Lullaby</title>
    <date><year>(1980)</year></date> 
    <length>3'</length>
    <instruments>fl or ob, or vn, or vc, pn</instruments>
    <description></description> 
    <publisher>ACA</publisher>
  </composition>

  <composition composer="c3">
    <title>Trio: Dream in D</title>
    <date><year>(1980)</year></date> 
    <length>10'</length>
    <instruments>fl, pn, vc, or vn, pn, vc</instruments>
    <description>
      Rhapsodic. Passionate. Available on CD 
      <cite><a href="http://www.amazon.com/exec/obidos/ASIN/B000007NMH/qid%3D913265342/sr%3D1-2/">Two by Three</a></cite> 
      from North/South Consonance (1998).
    </description> 
    <publisher></publisher>
  </composition>

  <composition composer="c4">
    <title>Propos II</title>
    <date><year>(1985)</year></date> 
    <length>11'</length>
    <instruments>2 tpt</instruments>
    <description>Arrangement from Propos</description> 
    <publisher></publisher>
  </composition>

  <composition composer="c4">
    <title>Rictus En Mirroir</title>
    <date><year>(1985)</year></date> 
    <length>14'</length>
    <instruments>fl, ob, hpschd, vc</instruments>
    <description></description> 
    <publisher></publisher>
  </composition>

</catalog>

Element Declarations

• Each tag must be declared in a <!ELEMENT> declaration.

• A <!ELEMENT> declaration gives the name and content model of the element

• The content model uses a simple regular expression-like grammar to precisely specify what is and isn't allowed in an element

Content Specifications

• ANY

• #PCDATA

• Sequences

• Choices

• Mixed Content

• Modifiers

• EMPTY

ANY

<!ELEMENT catalog ANY>

• A catalog can contain any child element and/or raw text (parsed character data)

#PCDATA

• Parsed Character Data; i.e. raw text, no markup. For example,

  <year>1984</year>

<!ELEMENT year (#PCDATA)>

#PCDATA

• Valid:

<year>1999</year>
<year>99</year>
<year>1999 C.E.</year>
<year>
 The year of our Lord one thousand, nine hundred, and ninety-nine
</year>

• Invalid:

<year>
<month>January</month>
<month>February</month>
<month>March</month>
<month>April</month>
<month>May</month>
<month>June</month>
<month>July</month>
<month>August</month>
<month>September</month>
<month>October</month>
<month>November</month>
<month>December</month>
</year>

#PCDATA

There are a number of elements in the example document that only contain PCDATA:

<!ELEMENT category (#PCDATA)>
<!ELEMENT abstract (#PCDATA)>
<!ELEMENT keyword (#PCDATA)>
<!ELEMENT last_updated (#PCDATA)>
<!ELEMENT copyright (#PCDATA)>
<!ELEMENT first_name (#PCDATA)>
<!ELEMENT middle_name (#PCDATA)>
<!ELEMENT last_name (#PCDATA)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT year (#PCDATA)>
<!ELEMENT instruments (#PCDATA)>
<!ELEMENT publisher (#PCDATA)>
<!ELEMENT length (#PCDATA)>

Comments in DTDs

• DTDs seem as obfuscated as C.

• Comments can improve this by giving example elements

• Comments are the same as in HTML; e.g. <!-- Comment -->

<!-- e.g. "1999 New York Women Composers", 
     not "Copyright 1999 New York Women Composers" -->
<!ELEMENT copyright (#PCDATA)>

Child Elements

    <date><year>1994</year></date> 

• To declare that a date element must have a year child:

<!ELEMENT date (year)>

Child Elements

You only have to declare the immediate children

   <maintainer email="elharo@metalab.unc.edu" 
              url="http://www.macfaq.com/personal.html">
    <name>
      <first_name>Elliotte</first_name> 
      <middle_name>Rusty</middle_name> 
      <last_name>Harold</last_name>
    </name>
  </maintainer>

  <composer id="c1">
    <name>
      <first_name>Julie</first_name> 
      <middle_name></middle_name> 
      <last_name>Mandel</last_name>
    </name>
  </composer> 

• To declare that a maintainer element must have a name child:

<!ELEMENT maintainer (name)>
<!ELEMENT composer (name)>

Sequences

    <name>
      <first_name>Elliotte</first_name> 
      <middle_name>Rusty</middle_name> 
      <last_name>Harold</last_name>
    </name>

• Separate multiple required child elements with commas; e.g.

  <!ELEMENT name (first_name, middle_name, last_name)>
  

• A list of child elements separated by commas is called a sequence

More Sequences

• The element being described must have only child elements, no mixed content

• You must know the order of the child elements

• You must know the type of each child element

• You must know the number of child elements

• The number can be relaxed with wild cards

One or More Children +

  <cataloging_info>
    <abstract>Compositions by the members of New York Women Composers</abstract>
    <keyword>music publishing</keyword>
    <keyword>scores</keyword>
    <keyword>women composers</keyword>
    <keyword>New York</keyword>
  </cataloging_info>

• The + suffix indicates that one or more of that element is required at that point

<!ELEMENT cataloging_info (abstract, keyword+)>

Zero or More Children *

• The * suffix indicates that zero, one, or more of that element is required at that point

<!ELEMENT catalog (category, cataloging_info, last_updated, copyright, 
                   maintainer, composer*, composition*)>

Zero or One Children ?

  <composition composer="c1">
    <title>Trio for Flute, Viola and Harp</title>
    <date><year>1994</year></date> 
    <length>13'38"</length>
    <instruments>fl, hp, vla</instruments>
    <description>
      <p>Premiered at Queens College in April, 1996 by Sue Ann Kahn, 
      Christine Ims, and Susan Jolles. In 3 movements :</p>
      <ul>
        <li>mvt. 1: 5:01</li>
        <li>mvt. 2: 4:11</li>
        <li>mvt. 3: 4:26</li>
      </ul>  
    </description>
    <publisher>Theodore Presser</publisher>
  </composition>

• Suffixing an element name with a question mark (?) in the content model indicates that either 0 or 1 (but not more than one) of that element are expected at that position

<!ELEMENT composition 
   (title, date, length?, instruments, description?, publisher?)>

Choices

• A choice indicates one element or another but not both

• A choice is signified by a vertical bar |

• There can be two or more elements in a choice

<!ELEMENT date (year | ISODate)>
<!ELEMENT year (#PCDATA)>
<!ELEMENT ISODate (#PCDATA)>

Mixed Content

<!ELEMENT description (#PCDATA | ul | a | cite | p)*>
<!ELEMENT cite (#PCDATA | a)*>
<!ELEMENT ul (li*)>
<!ELEMENT li (#PCDATA)>
<!ELEMENT a (#PCDATA)>
<!ELEMENT p (#PCDATA)>

• Mixed content is both #PCDATA and child elements in a choice, followed by an asterisk

• This is the only way to combine PCDATA with child elements in a content model

  • #PCDATA must come first

  • #PCDATA cannot be used in a sequence

Content Models You Can't Declare

• Mixed content with other content models

• Exactly one element of a given type but in any position (The SGML & operator)

• Between M and N of a given element

• Restrictions on the PCDATA; e.g. that the year element must contain a four-digit year

Attribute Declarations

Recall this element:

  <maintainer email="elharo@metalab.unc.edu" 
              url="http://www.macfaq.com/personal.html">
    <name>
      <first_name>Elliotte</first_name> 
      <middle_name>Rusty</middle_name> 
      <last_name>Harold</last_name>
    </name>
  </maintainer>

It is declared like this:

<!ELEMENT maintainer (name)>
<!ATTLIST maintainer email CDATA "webmaster@nywc.org">
<!ATTLIST maintainer url CDATA "http://www.ibiblio.org/nywc">

The general format of an <!ATTLIST> declaration is:

<!ATTLIST Element_name Attribute_name Type Default_value>

Multiple Attribute Declarations

  <maintainer email="elharo@metalab.unc.edu" 
              url="http://www.macfaq.com/personal.html">
    <name>
      <first_name>Elliotte</first_name> 
      <middle_name>Rusty</middle_name> 
      <last_name>Harold</last_name>
    </name>
  </maintainer>

It is declared like this:

<!ELEMENT maintainer (name)>
<!ATTLIST maintainer email CDATA "webmaster@nywc.org">
<!ATTLIST maintainer url CDATA "http://www.ibiblio.org/nywc">

But it can also be declared in a single <!ATTLIST> declaration like this:

<!ATTLIST maintainer email CDATA "webmaster@nywc.org" url CDATA "http://www.ibiblio.org/nywc/">

This is more obvious with better indentation:

<!ATTLIST maintainer email CDATA "webmaster@nywc.org" 
                     url   CDATA "http://www.ibiblio.org/nywc/">
                     

Attribute Default Values

• A literal string value

• One of these three keywords

• #REQUIRED

• #IMPLIED

• #FIXED

#REQUIRED

• No default value is provided in the DTD

• Document authors must provide an attribute value for each element

<!ELEMENT maintainer (name)>
<!ATTLIST maintainer email CDATA #REQUIRED 
                     url   CDATA #REQUIRED>

#IMPLIED

<!ELEMENT a (#PCDATA)>
<!ATTLIST a href CDATA #IMPLIED>

• No default value in the DTD

• Author may (but does not have to) provide a value with each element

#FIXED

• Value is the same for all elements

• Default value must be provided in DTD

• Document author may not change default value

<!ELEMENT maintainer (name)>
<!ATTLIST maintainer email CDATA #FIXED "webmaster@nywc.org" 
                     url   CDATA #REQUIRED>

Attribute Types

• CDATA

• ID

• IDREF

• IDREFS

• ENTITY

• ENTITIES

• NOTATION

• NMTOKEN

• NMTOKENS

• Enumerated

CDATA

• Most general attribute type

• Value can be any string of text not containing a raw less-than sign (<) or quotation marks (")

<!ATTLIST maintainer email CDATA #REQUIRED 
                     url   CDATA #IMPLIED>

ID

• Value must be an XML name

  • May include letters, digits, underscores, hyphens, and periods

  • May not include whitespace

  • May or may not have the name "id" or "ID"

  • May contain colons only if used for namespaces

• Value must be unique within ID type attributes in the document

• Generally the default value is #REQUIRED

<!ELEMENT composer (name)>
<!ATTLIST composer id ID #REQUIRED>

IDREF

• Value matches the ID of an element in the same document

• Used for links and the like

• Multiple elements may share the same IDREF values

<!ELEMENT composition (title, date, length?, 
   instruments, description?, publisher?)>
<!ATTLIST composition composer IDREF #REQUIRED>

IDREFS

• A list of ID values in the same document

• Separated by white space

<!ELEMENT composition (title, date, length?, 
   instruments, description?, publisher?)>
<!ATTLIST composition composer IDREFS #REQUIRED>

Finished DTD

<!ELEMENT category (#PCDATA)>
<!ELEMENT abstract (#PCDATA)>
<!ELEMENT keyword (#PCDATA)>
<!ELEMENT last_updated (#PCDATA)>
<!ELEMENT copyright (#PCDATA)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT instruments (#PCDATA)>
<!ELEMENT publisher (#PCDATA)>
<!ELEMENT length (#PCDATA)>

<!ELEMENT date (year | ISODate)>
<!ELEMENT year (#PCDATA)>
<!ELEMENT ISODate (#PCDATA)>

<!ELEMENT catalog (category, cataloging_info, last_updated, 
   copyright, maintainer, (composer | composition)*)>

<!ELEMENT cataloging_info (abstract, keyword+)>

<!ELEMENT description (#PCDATA | ul | a | cite | p)*>
<!ELEMENT cite (#PCDATA | a)*>
<!ELEMENT ul (li*)>
<!ELEMENT li (#PCDATA)>
<!ELEMENT a (#PCDATA)>
<!ELEMENT p (#PCDATA)>

<!ELEMENT maintainer (name)>
<!ELEMENT name (first_name, middle_name, last_name)>
<!ELEMENT first_name (#PCDATA)>
<!ELEMENT middle_name (#PCDATA)>
<!ELEMENT last_name (#PCDATA)>
<!ATTLIST maintainer email CDATA #REQUIRED 
                     url   CDATA #IMPLIED>
                     
<!ELEMENT composer (name)>
<!ATTLIST composer id ID #REQUIRED>

<!ELEMENT composition (title, date, length?, 
                          instruments, description?, publisher?)>
<!ATTLIST composition composer IDREFS #REQUIRED>

<!ATTLIST a href CDATA #REQUIRED>

Part VI: Namespaces

Raison d'etre

1. To distinguish between elements and attributes from different vocabularies with the same names.

2. To group all related elements and attributes together so that a parser can easily recognize them.

The Need for Namespaces

• The XLink specification defines an attribute with the name href. The XHTML specification also uses href attributes on some elements. And the XInclude specification uses href attributes.

• An XSLT style sheet that will transform XHTML documents containing both Scalable Vector Graphics (SVG) pictures and MathML equations into XSL-Formatting object documents.

  • The a, title, script, style and font elements in XHTML and SVG

  • The table element in XHTML and XSL-FO

  • The text element in XSLT and SVG

  • The set element in MathML and SVG

• An XSLT stylesheet that transforms a style sheet in an older version of the XSLT specification to a style sheet in a newer version of the XSLT specification.

Namespaces disambiguate elements

• Namespaces disambiguate elements with the same name from each other by attaching different prefixes to names from different XML applications.

• Each prefix is associated with a URI.

  • Names whose prefixes are associated with the same URI are in the same namespace.

  • Names whose prefixes are associated with different URIs are in different namespaces.

Namespace Syntax

• Elements and attributes that are in namespaces have names that contain exactly one colon. They look like this:

  rdf:description
  xlink:type
  xsl:template
  

• Everything before the colon is called the prefix

• Everything after the colon is called the local part.

• The complete name including the colon is called the qualified name.

Namespace URIs

• Each prefix in a qualified name is associated with a URI.

• For example, all elements in XSLT 1.0 style sheets are associated with the http://www.w3.org/1999/XSL/Transform URI.

• The customary prefix xsl is a shorthand for the longer URI http://www.w3.org/1999/XSL/Transform.

• You can't use the URI in the element name directly.

  {http://www.w3.org/1999/XSL/Transform}template

Binding Prefixes to Namespace URIs

• Prefixes are bound to namespace URIs by attaching an xmlns:prefix attribute to the prefixed element or one of its ancestors.

  <svg:svg xmlns:svg="http://www.w3.org/2000/svg" 
   width="12cm" height="10cm">
    <svg:ellipse rx="110" ry="130" />
    <svg:rect x="4cm" y="1cm" width="3cm" height="6cm" />
  </svg:svg>
  

• Bindings have scope within the element where they're declared.

• An SVG processor can recognize all three of these elements as SVG elements because they all have prefixes bound to the particular URI defined by the SVG specification.

Binding Prefixes to Namespace URIs Example

<xhtml:html xmlns:xhtml="http://www.w3.org/1999/xhtml"
            xmlns:xlink="http://www.w3.org/1999/xlink">
  <xhtml:head><xhtml:title>Three Namespaces</xhtml:title></xhtml:head>
  <xhtml:body>
    <xhtml:h1 align="center">An Ellipse and a Rectangle</xhtml:h1>
    <svg:svg xmlns:svg="http://www.w3.org/2000/svg" 
             width="12cm" height="10cm">
      <svg:ellipse rx="110" ry="130" />
      <svg:rect x="4cm" y="1cm" width="3cm" height="6cm" />
    </svg:svg>
    <xhtml:p xlink:type="simple" 
      xlink:href="ellipses.html">
      More about ellipses
    </xhtml:p>
    <xhtml:p xlink:type="simple" xlink:href="rectangles.html">
      More about rectangles
    </xhtml:p>
    <xhtml:hr/>
    <xhtml:p>Last Modified February 13, 2000</xhtml:p>    
  </xhtml:body>
</xhtml:html>

Unprefixed attributes are never in any namespace

• Unprefixed attributes are never in any namespace.

• Being an attribute of an element in the http://www.w3.org/1999/xhtml namespace is not sufficient to put the attribute in the http://www.w3.org/1999/xhtml namespace.

• The only way an attribute belongs to a namespace is if it has a declared prefix, like xlink:type and xlink:href.

URIs matter; not prefixes

• Many XML applications have recommended prefixes. For example, SVG elements often use the prefix svg and Resource Description Framework (RDF) elements often have the prefix rdf. However, these prefixes are simply conventions, and can be changed based on necessity, convenience or whim.

• Before a prefix can be used, it must be bound to a URI.

• These URIs are standardized, not the prefixes.

• The prefix can change as long as the URI stays the same.

Namespace URIs do not necessarily point to a document, page, or schema

• Purely formal

• Can point somewhere but do not have to

• Parsers compare namespace URIs on a character by character basis. These are three different namespaces:

  • http://www.w3.org/1999/XSL/Transform

  • http://www.w3.org/1999/XSL/Transform/

  • http://www.w3.org/1999/XSL/Transform/index.html

The Default Namespace

• Indicate that an unprefixed element and all its unprefixed descendant elements belong to a particular namespace by attaching an xmlns attribute with no prefix:

  <DATASCHEMA xmlns="http://www.w3.org/2000/P3Pv1">
    <DATA name="vehicle.make" type="text" short="Make" 
          category="preference" size="31"/>
    <DATA name="vehicle.model" type="text" short="Model" 
          category="preference" size="31"/>
    <DATA name="vehicle.year" type="number" short="Year" 
          category="preference" size="4"/>
    <DATA name="vehicle.license.state." type="postal." short="State" 
          category="preference" size="2"/>
    <DATA name="vehicle.license.number" type="text" 
          short="License Plate Number" category="preference" size="12"/>
  </DATASCHEMA>
  

• Both the DATASCHEMA and DATA elements are in the http://www.w3.org/2000/P3Pv1 namespace.

• Default namespaces apply only to elements, not to attributes. Thus in the above example the name, type, short, category, and size attributes are not in any namespace.

• You can change the default namespace within a particular element by adding an xmlns attribute to the element.

<html xmlns="http://www.w3.org/1999/xhtml"
      xmlns:xlink="http://www.w3.org/1999/xlink">
  <head><title>Three Namespaces</title></head>
  <body>
    <h1 align="center">An Ellipse and a Rectangle</h1>
    <svg xmlns="http://www.w3.org/2000/svg" 
         width="12cm" height="10cm">
      <ellipse rx="110" ry="130" />
      <rect x="4cm" y="1cm" width="3cm" height="6cm" />
    </svg>
    <p xlink:type="simple" xlink:href="ellipses.html">
      More about ellipses
    </p>
    <p xlink:type="simple" xlink:href="rectangles.html">
      More about rectangles
    </p>
    <hr/>
    <p>Last Modified February 13, 2000</p>    
  </body>
</html>

Namespaces and DTDs

• DTDs must declare the qualified names

  <!ELEMENT svg:text (#PCDATA)>

• If the prefix changes, the DTD needs to change too.

• Parameter entity references can help when the prefix changes or is removed:

  <!ENTITY % mathml-colon               ''>
  <!ENTITY % mathml-prefix              ''>
  
  <!ENTITY % mathml-exp                 '%mathml-prefix;%mathml-colon;exp' >
  <!ENTITY % mathml-abs                 '%mathml-prefix;%mathml-colon;abs' >
  <!ENTITY % mathml-arg                 '%mathml-prefix;%mathml-colon;arg' >
  <!ENTITY % mathml-real                '%mathml-prefix;%mathml-colon;real' >
  <!ENTITY % mathml-imaginary      '%mathml-prefix;%mathml-colon;imaginary' >
  

• DTDs need to declare the xmlns and xmlns:prefix attributes too:

  <!ATTLIST svg xmlns (CDATA) #FIXED "http://www.w3.org/2000/svg">

  <!ATTLIST svg:svg xmlns:svg (CDATA) #FIXED "http://www.w3.org/2000/svg">

  However, you should not rely on this. Always explicitly specify the namespace declaration in the instance documents.

To Learn More

• This presentation: http://www.cafeconleche.org/slides/sd2003west/xmlfundamentals

• XML in a Nutshell, second edition

  • Elliotte Rusty Harold and W. Scott Means

  • O'Reilly & Associates, 2002

  • ISBN 0-596-00292-0

  • http://www.cafeconleche.org/books/xian2/

  
  

• XML Bible, second edition

  • Elliotte Rusty Harold

  • Hungry Minds, 2001

  • ISBN 0-7645-4760-7

  • http://www.cafeconleche.org/books/bible2/