(8) XML Use Cases

• XML is a metalanguage supporting application-specific vocabularies
• RSS and Atom [Content Syndication] are XML vocabularies for newsfeeds
  • Doc or Die [http://docordie.blogspot.com/]: RSS feed [http://docordie.blogspot.com/rss.xml] vs. Atom feed [http://docordie.blogspot.com/atom.xml]
  • browsers now incorporate and/or integrate newsfeed readers
• OpenDocument (ODF) is a language for office application documents
  • designed for open and interoperable exchange
  • standardized by ISO (which now also standardizes Microsoft's Open XML)
• Scalable Vector Graphics (SVG) for portable vector graphics
  • designed for embedding in Web pages
  • good example for compound documents: HTML containing SVG [http://www.carto.net/papers/svg/animated_weather_symbols/]
• XML also can be used for representing complex structured documents

(9) Markup?

• Structures are encoded using special characters
  • a fundamental difference in comparison to binary formats
  • markup languages can be read and modified using text-based tools
  • programs must treat markup characters in a special way
• Documents are content interspersed with markup (i.e., structures)
  • XML-aware software interprets the markup
  • XML-unaware software just sees a text file
  • modifications must be made XML-aware (e.g., inserting AT&T as AT&T)
• You have to pay the The Price for Markup [The Price for Markup (1)]
• You need a parser for interpreting the markup

(10) Basic Concepts

• XML Documents have an XML declaration (optional)
• There is exactly one document element (a.k.a. root element)
• Elements may be nested (there is no conceptual limit)
  • elements may be repeated (they can be identified by position)
• Elements are marked up using tags
  • most elements have content, surrounded by start tags and end tags
  • empty elements are allowed and may use a special notation
• Elements may have attributes (zero to any number)
  • attributes can only occur once on an element (i.e., they cannot be repeated)

<?xml version="1.0" encoding="UTF-8"?>
<element>
 <subelement attribute="value">Content</subelement>
 <subelement a2="value2">More Content</subelement>
 <empty-element a3="v3"></empty-element>
 <empty-element a4="v4" a5="v5"/>
</element>

(11) Tree Syntax

• Markup is important, but only a notation
• XML documents are trees with different node types
  • nodes so far: document, element, attribute, text
  

(12) Elements

• Elements can use a wide variety of names [http://www.w3.org/TR/xml/#NT-Name]
  • Allowed: html, id9832798472, _, :, こんにちは
  • Disallowed: leading numbers, spaces, control characters
• Element names usually convey some information about the content
  • this is not reliable and highly language-dependent
  • it is very useful when working with a known vocabulary
  • it is potentially harmful when working with an unknown vocabulary
• Elements are the foundation for XML's versatility
  • they can be nested (<address><city>Berkeley</city><zip>94720</zip>…)
  • they can be repeated (<givenname>Erik</givenname><givenname>Thomas</givenname>)
  • their sequence can convey additional information (given names have a sequence)

(13) Attributes

• Additional information pertaining to elements
• Traditionally, anything that is not considered content
  • SGML is a document markup language
  • XML uses SGML's concepts
  • XML has its roots in the document world
• Elements: Content (i.e., Data); Attributes: Metadata
• Documents often distinguish by what is textual content

 <section id="xml" author="bob">
  <title>Extensible Markup Language (XML)</title>
  <p>XML is based on SGML (Section <ref name="sgml"/>) ...</p>
  <p type="example">XML can be used ...</p>
  <section id="xml-syntax" author="dret">
   <title>XML Syntax</title>
   <p>Section <ref name="sgml-syntax"/> describes ...</p>
  </section>
 </section>

(14) Attribute Syntax

• Naming rules are the same as for Elements [Elements (1)]
• Attributes always appear within an element's start tag
• Attributes are name/value-pairs [http://www.w3.org/TR/xml/#NT-Attribute]
  • the value is enclosed in single or double quotes
• Attribute with a single-quote value: elem attr="Single: '"/
• Attribute with a double-quote value: elem attr='Double :"'/
• How can attribute values contain both?

(15) The Price for Markup

• Markup characters have a special meaning
  • < opens a tag
  • for attribute values, quotes delimit the value
• The literal use of a markup character requires escaping
  • XML's entities can refer to pieces of content
  • entity syntax is &name; for referring to the entity name
  • XML has 5 predefined entities [http://www.w3.org/TR/xml/#sec-predefined-ent]: <, >, &, ', "
• Attribute using both kinds of quotes: <elem attr="Single ' and Double &quot;"/>

<li>Attribute using both kinds of quotes: <code>&lt;elem attr="Single ' and Double &amp;quot;"/></code></li>

(17) XML Example

<feed xmlns="http://www.w3.org/2005/Atom" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:geo="http://www.w3.org/2003/01/geo/wgs84_pos#" xmlns:georss="http://www.georss.org/georss" xmlns:woe="http://where.yahooapis.com/v1/schema.rng" xmlns:flickr="urn:flickr:" xmlns:media="http://search.yahoo.com/mrss/">
 <title>The MobApp2010 Pool, with geodata</title>
 <link rel="self" href="http://api.flickr.com/services/feeds/geo/?g=1313291@N20&amp;lang=en-us&amp;format=atom"/>
 <link rel="alternate" type="text/html" href="http://www.flickr.com/photos/"/>
 <id>tag:flickr.com,2005:/photos/public/group/1313291@N20/geo/</id>
 <icon>http://l.yimg.com/g/images/buddyicon.jpg#1313291@N20</icon>
 <subtitle/>
 <updated>2010-01-31T06:10:25Z</updated>
 <generator uri="http://www.flickr.com/">Flickr</generator>
 <entry>
  <title>DSC00809</title>
  <link rel="alternate" type="text/html" href="http://www.flickr.com/photos/47030217@N06/4318229442/"/>
  <id>tag:flickr.com,2005:/photo/4318229442</id>
  <published>2010-01-31T06:10:25Z</published>
  <updated>2010-01-31T06:10:25Z</updated>
  <dc:date.Taken>2010-01-30T21:14:39-08:00</dc:date.Taken>
  <content type="html">&lt;p&gt;&lt;a href=&quot;http://www.flickr.com/people/47030217@N06/&quot;&gt;stoodle246&lt;/a&gt; posted a photo:&lt;/p&gt;
&lt;p&gt;&lt;a href=&quot;http://www.flickr.com/photos/47030217@N06/4318229442/&quot; title=&quot;DSC00809&quot;&gt;&lt;img src=&quot;http://farm5.static.flickr.com/4028/4318229442_5ac597fdf5_m.jpg&quot; width=&quot;240&quot; height=&quot;180&quot; alt=&quot;DSC00809&quot; /&gt;&lt;/a&gt;&lt;/p&gt;</content>
  <author>
   <name>stoodle246</name>
   <uri>http://www.flickr.com/people/47030217@N06/</uri>
  </author>
  <link rel="enclosure" type="image/jpeg" href="http://farm5.static.flickr.com/4028/4318229442_e8ac9d23aa_o.jpg"/>
  <georss:point>37.873633 -122.256975</georss:point>
  <geo:lat>37.873633</geo:lat>
  <geo:long>-122.256975</geo:long>
  <woe:woeid>55858022</woe:woeid>
 </entry>

(18) JSON Example

{
 "title":"The MobApp2010 Pool, with geodata",
 "link":"http://www.flickr.com/photos/",
 "description":"",
 "modified":"2010-01-31T06:10:25Z",
 "generator":"http://www.flickr.com/",
 "items":[
  {
   "title":"DSC00809",
   "link":"http://www.flickr.com/photos/47030217@N06/4318229442/",
   "media":{
    "m":"http://farm5.static.flickr.com/4028/4318229442_5ac597fdf5_m.jpg"
   },
   "date_taken":"2010-01-30T21:14:39-08:00",
   "description":"<p><a href=\"http://www.flickr.com/people/47030217@N06/\">stoodle246<\/a> posted a photo:<\/p> <p><a href=\"http://www.flickr.com/photos/47030217@N06/4318229442/\" title=\"DSC00809\"><img src=\"http://farm5.static.flickr.com/4028/4318229442_5ac597fdf5_m.jpg\" width=\"240\" height=\"180\" alt=\"DSC00809\" /><\/a><\/p> ",
   "published":"2010-01-31T06:10:25Z",
   "author":"nobody@flickr.com (stoodle246)",
   "author_id":"47030217@N06",
   "tags":"",
   "latitude":"37.873633",
   "longitude":"-122.256975"
  },

(19) Object Notation

• Uses a subset of JavaScript syntax (static data structures)
• Data model is derived from JavaScript's data model
  • number, string, and boolean for simple values
  • objects represents sets of values (using curly braces)
  • arrays represent sequences of values (using square brackets)
  • null as a special value
• Based on JavaScript but usable in many languages
  • can be readily mapped to the data models of many programming languages
  • XML is not native in any major programming language (research: E4X [http://en.wikipedia.org/wiki/ECMAScript_for_XML]/XJ [http://www.alphaworks.ibm.com/tech/xj])

(20) JSON via URI Parameter

• Many services provide structured data in various formats
  • logically speaking these are just different representations of the same content
  • depending on trends and technologies, new formats may be added
• URIs allow query strings for transmitting data to a resource
  • HTML forms [../web-fall09/forms] use query strings for submitting form values
• Query strings can also be used manually in a URI
  • hackable URIs are convenient for developers
• http://api.flickr…1313291@N20&lang=en-us&format=rss_200 [http://api.flickr.com/services/feeds/geo/?g=1313291@N20&lang=en-us&format=rss_200]

(21) JSON via Content Negotiation

• XML or JSON are just different representations
  • they represent the same underlying resource (as identified by the URI)
• HTTP Content Negotiation [Web Foundations (URI & HTTP); HTTP Content Negotiation (1)] allows to specify preferences
  • clients specify preferences and server respond with the best match
• HTTP Accept specifies the accepted media type [../web-fall09/mediatypes]
  • resources may be available in HTML, XML, or JSON
  • depending on the HTTP request, the server responds with the best representation
  • reduces processing time on the client and can be cached
• Really smart Ajax frameworks could even hide the content negotiation
  • request JSON or XML, with XML being the lower priority
  • if XML is sent by the server, it has to be parsed into a JavaScript object
  • the end result is always a JavaScript object for the framework user