COMP249: XML

Steve Cassidy and Rolf Schwitter

Introduction to XML

Motivation - why we need another language
Syntax of XML
Some applications of XML
Parsing XML with Javascript
JSON vs XML

Motivations for XML

Deficiencies in HTML
- No meaning in the markup
- Structure is too liberal
Data interchange
- We're forever inventing new languages
- Tackle internationalisation etc. issues once
- Make writing parsers history

History

Late 1980's: Standard Generalised Markup Langugage (SGML)
1998: W3C XML Working Group publishes XML 1.0
1999: Netscape publishes RSS version 0.90
2000: W3C Recommendation: XHTML 1.0
2003: Adam Curry develops iPodder script, podcasting is born
2006: XML 1.1 published

XML Basics

XML is a standard for defining markup applications.
XML defines the syntax of markup.
An XML document is a tree of elements with a single root element.
An XML document is well formed if it follows the rules of XML syntax.
An XML document is valid if its structure matches a given Document Type Definition (DTD).
XHTML and RSS are applications of XML: a markup language which uses the syntax and rules of XML.

An XML Example

<?xml version='1.0'?>
<!DOCTYPE slides SYSTEM "slides.dtd">
<slides>
  <foil>
    <title>XML Basics</title>
    <itemizedlist>
      <listitem>
        XML is a standard for defining 
        markup languages.
      </listitem>
      <listitem>
        XML defines the <emphasis>syntax</emphasis> 
        of markup.
      </listitem>
    </itemizedlist>
  </foil>
</slides>

Explanation

The form of the XML declaration is: <?xml version='1.0'?>.
Nothing can precede the XML declaration, not even whitespace characters.
In this example, slides is the document root name and the DTD is stored in an external file slides.dtd.
The root element is slides, whose opening tag must appear on the first line of the XML.
XML names are used to name elements and attributes.
XML names (e.g. foil, title or itemizedlist) must begin with a letter or underscore and can include digits, hyphens, and periods.
XML names are case sensitive.
All other elements must be nested inside the root element.

An XML Example - Not Well Formed

   <?xml version='1.0'?>
<!DOCTYPE slides SYSTEM "slides.dtd">
<slides>
  <foil>
    <title>XML Basics
    <itemizedlist>
      <listitem>
        Salad < Fish & Chips  
      </LISTITEM>
      <listitem>
        XML defines <bold> the 
        <emphasis>syntax</bold></emphasis> 
        of markup.
      </listitem>
    </itemizedlist>
  </foil>
</slides>

Errors in the above are:

XML declaration (<?xml...) isn't the first thing in the file
No end tag for title
Wrong case for end tag of listitem
bold tag closed inside emphasis tag - overlaps aren't allowed.

XML Document Structure

XML Syntax rules define the low level structure of XML
A Schema defines the higher level structure:
- What tag names
- What each tag can contain
XML Schema languages:
- Document Type Definition (DTD)
- W3C XML Schema
- RelaxNG
- Schematron

We're only going to consider DTDs in this unit and then only at a very superficial level of detail. The important message is that there are ways to define the structure and content of XML documents.

Document Type Definition (DTD)

DTD defines:

What elements can be in an XML document.
What attributes an element can have and restrictions on their values.
What the content model for an element is: what elements can occur within it and in what pattern.
Where text can occur:
- #PCDATA -- parsed character data, entity references will be replaced
- #CDATA -- character data, can contain special chars

Why Use a DTD

To enforce a standard that many people can rely on (for example, XHTML).
To provide structure for document authors to work within (for example, Docbook).
To enable validation of documents.

Example DTD

<!ELEMENT slides (foil+|section+)>
<!ATTLIST slides
 "label	CDATA		#IMPLIED
  status	CDATA		#IMPLIED
  id	ID		#IMPLIED
 revisionflag	(changed | added | deleted | off) #IMPLIED">
<!ELEMENT section (title,foil+)>
<!ELEMENT foil (title,(itemizedlist|para))>
<!ELEMENT itemizedlist (listitem+)>
<!ELEMENT listitem #PCDATA>
<!ELEMENT para #PCDATA>

<!ENTITY comp249 "COMP249 (Web Technology)">
<!ENTITY copy    "&#x00A9;">

Declaring Elements

The form of an element declaration looks as follows:
```
<!ELEMENT element_name (names of child elements)>
```
For example, consider the following declaration:
```
<!ELEMENT slides (foil+|section+)>
```
Modifiers can be added to child element specifications:
- + - one or more occurrences
- * - zero or more occurrences
- ? - zero or one occurrence.
If an element just contains text then it's content model is just #PCDATA.

Declaring Attributes

The form of an attribute declaration looks as follows:

<!ATTLIST element_name attribute_name attribute_type [default_value]>

For example, consider the following declaration:
```
<!ATTLIST slides label CDATA #IMPLIED>
```
Possible default values for attributes are:
- a value - the value used, if none is specified in an element
- #FIXED - the value, which every element will have and which cannot be changed
- #REQUIRED - no default value is given; every instance of the element must specify a value
- #IMPLIED - no default value is given; the value may or may not be specified in an element.

Declaring Entities

Entities can be defined so that they can be referenced anywhere in the content.
The form of an entity declaration is:
```
<!ENTITY entity_name "entity_value">
```
You can define an entity to represent a string:
```
<!ENTITY comp249 "COMP249 (Web Technology)">
```
In an XML document the string can be referenced by:
```
I'm studying &comp249;
```

What DTDs Can't Do

Can't say anything about the textual content of an element.
XML Schema proposals attempt to do this (XML Schema, RELAX-NG).
Equivalent to providing a (programming language) type system for XML.

RSS: Really Simple Syndication

RSS is a standard format for web site summaries.
An RSS summary, at a minimum, is a document describing a "channel" consisting of URL-retrievable items. Each item consists of a title, link, and brief description. (RSS Specification)
RSS defines a set of standard metadata elements for describing web resources (in particular, news feeds).
Recently Atom has beenproposed as a successor to RSS.

An RSS Example

  <xml version="1.0" encoding="ISO-8859-1"?> 
  <rss version="0.91">
  <channel>

    <title>Announcements for COMP249</title>
    <link>http://www.comp.mq.edu.au/units/comp249/cgi-bin/news</link>
    <description>
      This is an news service for COMP249 units established by Steve Cassidy.
    </description>
    <language>en-us</language>

    <item>
      <title>ANZAC Day Tutorials</title>
      <link>http://www.comp.mq.edu.au/units/comp249/cgi-bin/news/2004/04/28#anzac-tut.txt</link>
      <description>
        If you missed a tutorial this week because of the ANZAC day holiday.
        Please try to attend another session or submit your work directly to 
        your tutor for assessment.
      </description>
    </item>

    <item>
      <title>Assignment 3</title>
      <link>http://www.comp.mq.edu.au/units/comp249/cgi-bin/news/2004/04/23#ass3.txt</link>
      <description>
        Assignment 3 is now available.
      </description>
    </item>

    <item>
      <title>Assignment 1 Marks</title>
      <link>http://www.comp.mq.edu.au/units/comp249/cgi-bin/news/2004/04/16#ass1marks.txt</link>
      <description>
        Marks for Assignment 1 have been sent to your ics email address (s123456@ics.mq.edu.au).
      </description>
    </item>

  </channel>
</rss>

XML Bookmark Exchange Language

Web browsers store bookmarks in a variety of formats, some provide tools to import bookmarks from other browsers, but often you can't go back again. XBEL acts as an intermediate format between the Internet Explorer, Netscape and other formats.

Bookmarks organised into folders.
Bookmark entries and folders can have various properties encoded as sub-elements: title, description, info, metadata.
Bookmark url is stored as an attribute.

XBEL: An example

<?xml version="1.0"?>
<!DOCTYPE xbel PUBLIC "+//IDN python.org//DTD XML Bookmark Exchange Language 1.0//EN//XML"
"http://www.python.org/topics/xml/dtds/xbel-1.0.dtd">
<xbel>
  <info>
    <metadata owner="Norman Walsh <ndw@nwalsh.com>" />
  </info>

  <folder id="perlbookmarks">
    <title>Perl</title>

    <bookmark href="http://www.perl.com/">
      <title>Perl.com</title>
    </bookmark>

    <bookmark href="http://www.activestate.com/">
      <title>ActiveState Tool Corp. (Perl for Win32)</title>
    </bookmark>

    <bookmark href="http://194.247.167.1/DADA/PERL/">
      <title>dada meets Perl!</title>
    </bookmark>
  </folder>
</xbel>

VoiceXML

VoiceXML is an XML-based markup language for the development of voice applications.
Using only a traditional Web infrastructure, you can create applications that are accessible over the telephone.
VoiceXML is designed for creating audio dialogs that feature synthesized speech, digitized audio, recognition of spoken and DTMF key input, recording of spoken input, telephony, and mixed initiative conversations.

VoiceXML Example

      <?xml version="1.0" encoding="ISO-8859-1"?>
      <vxml version="2.0" lang="en">
        <form>

          <field name="city">
            <prompt> Where do you want to travel to? </prompt>
              <option> Edinburgh </option>
              <option> New York </option>
              <option> London </option>
              <option> Paris </option>
              <option> Stockholm </option>
          </field>

          <field name="travellers" type="number">
            <prompt> How many are travelling to <value expr="city"/>? </prompt>
          </field>

          <block>
            <submit next="http://localhost/handler" namelist="city travellers"/>
          </block>

        </form>
      </vxml>

XML on the Web - AJAX

AJAX - Asynchronous Javascript and XML
Javascript makes a request to the server from within a page
Recieves an XML response, updates the page using the contents
XML is a data exchange language, easily parsed by Javascript
But, is there a better alternative?

JSON

Javascript Object Notation
Use Javascript as a data exchange language
No need to parse it, just evaluate and you have the data

{
     "Name": "John Smith",
     "address": {
         "streetAddress": "21 2nd Street",
         "city": "New York", 
     },
     "phoneNumbers": [ "212 555-1234", "646 555-4567"  ]
 }

XML vs. JSON

JSON is purely for data interchange
XML is more general: a markup langauge
JSON needs no parser - unless you want to be secure
XML allows for validation independant of the application
JSON is easier to write code for in Javascript
XML is better when there might be more than one kind of consumer for the data

Parsing XML in Javascript

var parser=new DOMParser();
var doc=parser.parseFromString(txt,"text/xml");
/* now access elements using the DOM API */
to = doc.getElementsByTagName("to")[0].childNodes[0].nodeValue;

Source: w3schools

Parsing JSON in Javascript

var p = eval("(" + JSON_text + ")");
        /* or using a parser (safer) */
        var p = json_parse(JSON_text);
        /* now access the data elements */
        to = p.to

Summary

XML is a simple markup language with hierarchical structure.
Documents can be shown to be well formed or valid.
DTD allows standard document types to be defined (eg. XHMTL).
JSON is an alternative data format for sending data structures to web browsers.
Parsers process the XML or JSON to give programs access to the data.