(6) Describing Resources

• RDF describes everything in triples
  • making a statement about a resource (identified by a URI [XML Basics; Uniform Resource Identifier (URI) (1)])
  • describing a certain property of the resource (identified by a URI [XML Basics; Uniform Resource Identifier (URI) (1)])
  • specifying a value for that property (a URI [XML Basics; Uniform Resource Identifier (URI) (1)] or a literal)

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:contact="http://www.w3.org/2000/10/swap/pim/contact#">
  <contact:Person rdf:about="http://www.w3.org/People/EM/contact#me">
    <contact:fullName>Eric Miller</contact:fullName>
    <contact:mailbox rdf:resource="mailto:em@w3.org"/>
    <contact:personalTitle>Dr.</contact:personalTitle> 
  </contact:Person>
</rdf:RDF>

(7) RDF Graphs

(8) RDF is Simple and Complex

• RDF's abstract model [http://www.w3.org/TR/rdf-concepts/] is the idea of descriptive triples
  • the actual RDF model is rooted in description logic
  • RDF itself can only describe individuals (something identified by URI)
• RDF/XML [http://www.w3.org/TR/rdf-syntax-grammar/] is an XML syntax for encoding triples
  • the syntax allows a variety of ways to represent the same RDF statements
  • processing RDF/XML with XML tools is likely to fail
  • use RDF parsers to parse all variations of RDF/XML into an abstract RDF graph
• RDF Schema [http://www.w3.org/TR/rdf-schema/] supports the creation of RDF vocabularies
  • describe the classes of things that can be used in statements
  • describe the properties which can be used for each of these classes
  • describe the allowed values for the supported properties

(9) RDF Schema Graph

(10) RDF in Attributes (RDFa)

• Microformats can use any kind of markup design
  • this makes it hard to detect microformats when processing a Web page
  • combining microformats can become complicated and ill-designed
• RDFa defines a syntax for embedding RDF into HTML
  • the vocabulary must be described by some RDF schema language

<p>This document is licensed under a <a xmlns:cc="http://creativecommons.org/licenses/" rel="cc:license" href="http://creativecommons.org/licenses/by/nc-nd/3.0/">Creative Commons License</a>.</p>

• RDFa uses and extends HTML for embedding RDF
  • it uses HTML's rel, rev, href, and src attributes
  • it defines a number of new attributes for HTML elements [http://www.w3.org/TR/rdfa-syntax/#s_metaAttributes]
  • it defines a processing model [http://www.w3.org/TR/rdfa-syntax/#s_model] for deriving RDF triples from these attributes

(12) JavaScript and XML

• The XMLHttpRequest API has been built for requesting XML via HTTP
  • this is useful because XML is the most popular data format
  • all requested data has to be processed by using XML access methods in JavaScript
• JavaScript does not have XML as its internal data model
  • the XML received via XMLHttpRequest has to be parsed into a DOM tree
  • DOM access in JavaScript is inconvenient for complex operations
  • alternatively, the XML can be mapped to JavaScript objects (also requires parsing)
• JavaScript Object Notation (JSON) encodes data as JavaScript objects
  • more efficient for the consumer if the consumer is written in JavaScript
  • this turns the generally usable XML service into a JavaScript-oriented service
  • for large-scale applications, it might make sense to provide XML and JSON
  • using HTTP Content Negotiation [http://dret.net/lectures/web-fall09/foundations#http-conneg], this can be negotiated on the HTTP protocol level

(13) JSON Example

<?xml version="1.0"?>
<menu id="file" value="File">
 <popup>
  <menuitem value="New" onclick="CreateNewDoc()"/>
  <menuitem value="Open" onclick="OpenDoc()"/>
  <menuitem value="Close" onclick="CloseDoc()"/>
 </popup>
</menu>

{ "menu" : {
 "id" : "file",
 "value" : "File",
 "popup" : {
  "menuitem" : [
   { "value" : "New", "onclick" : "CreateNewDoc()" },
   { "value" : "Open", "onclick" : "OpenDoc()" },
   { "value" : "Close", "onclick" : "CloseDoc()" }
  ]
 }
}}

(14) 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 [http://dret.net/lectures/web-fall09/foundations#http-conneg] allows to specify preferences
  • clients specify preferences and server respond with the best match
• HTTP Accept specifies the accepted Media Types [http://dret.net/lectures/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

(15) JSON Media Type

• RFC 4627 [http://dret.net/rfc-index/reference/RFC4627] registers JSON as a media type
• The definition of JSON is derived from ECMAScript's object literals
• JSON is a very limited notation intended for simple structures
  • it allows the four primitive types strings, numbers, booleans, and null
  • it allows the two structured types objects (unordered) and arrays (ordered)
• The value of the media type in this case is the clean integration into the Web
  • information providers may choose to expose their data in JSON and XML
  • HTTP Content Negotiation [http://dret.net/lectures/web-fall09/foundations#http-conneg] can be used to specify which representation is preferred

(17) XML is just a Tool

• XML does not achieve magic interoperability
• Interoperability is mostly a problem of sharing data models
• XML provides a useful set of technologies to support interoperability
• XML-centric XML is more useful than serialized-something XML