(6) JSON is the new XML

• XML's tree model is not natural for developers
  • developers tend to think in objects or similar concepts
  • mapping bidirectionally creates unhappiness and friction
• JSON takes a minimalist approach at representing object structures
  • the only supported concepts are objects (name/value pairs) [http://tools.ietf.org/html/rfc4627#section-2.2] and arrays [http://tools.ietf.org/html/rfc4627#section-2.3]
  • these concepts can be nested as deeply as required
• JSON's main reason for success is structural alignment
  • most developers can work with JSON directly in their language
  • XML's more sophisticated features often are not required
• JSON is a poster child of the Pareto Principle [http://en.wikipedia.org/wiki/Pareto_principle] (or maybe 95/5)

(7) 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()" }
  ]
 }
}}

(9) Linked Data Principles

1. Use URIs as names for things.
2. Use HTTP URIs so that people can look up those names.
3. When someone looks up a URI, provide useful information, using a standard data model.
4. Include links to other URIs, so that people can discover more things.

(10) URIs as Names

• A URI can identify any abstract concept, not just Web pages
• The Semantic Web is mainly about how to assign URIs to any concept
• DBpedia [http://dbpedia.org/] is a project creating URIs for every topic in Wikipedia
  • Spoon: http://dbpedia.org/resource/Spoon_%28band%29 [http://dbpedia.org/resource/Spoon_%28band%29]
  • Mysticism: http://dbpedia.org/resource/Mysticism [http://dbpedia.org/resource/Mysticism]
• Anyone can create URIs for concepts
  • Panthera tigris: http://lod.taxonconcept.org/ses/QMUrD [http://lod.taxonconcept.org/ses/QMUrD]
• Multiple people or groups can create URIs for the same concept
  • All the URIs for Panthera tigris [http://sameas.org/html?uri=http%3A%2F%2Flod.geospecies.org%2Fses%2FQMUrD]

(11) RDF as Semantic Web Foundation

• The standard data model for Semantic Web standards is the Resource Description Framework (RDF)
• Data in RDF is just lists of statements
  • Spoon is a music group
  • Spoon is named Spoon
  • Spoon has a member Britt Daniel
• A statement consists of three parts: [subject] [predicate] [object]
  • [Spoon] [is a] [music group]
  • [Spoon] [is named] ["Spoon"]
  • [Spoon] [has a member] [Britt Daniel]

(12) Semantic Web Data Model (abstract)

(13) Semantic Web Data Model (URI concepts)

(14) Linking Data Sets

(17) What is the Content of an XML Document?

• An interesting (and fruitless) discussion
  • the content is whatever you consider it to be
  • agreement between peers is necessary for data exchange
  • agreement between specification writers and toolmakers is necessary to provide tools
• DOM and XSLT were two early arrivals
  • both had an idea (and a model) of what the content of an XML document is
  • they did not have the exact same idea
• Set a normative standard for an XML document's content
  • the Infoset defines what is represented in the tree
  • people should be confident to get this information when using XML technologies

(18) Infoset Example

(19) What is Not in the Infoset

• Do not rely on information not available in the Infoset [http://www.w3.org/TR/xml-infoset/#omitted]
  • order of attributes
  • type of quotes around attribute values
  • notation of empty elements (<elem></elem> vs. <elem/>)
  • how lines are terminated
  • entities and character references
• XML contains all this information if used as XML document
• many XML technologies are in fact Infoset technologies
  • XSD, XSLT, XQuery, SOAP, …

(21) References in Instances

• DTD [Document Type Definition (DTD)] and XSD [XML Schema (XSD) – Part I] IDs are defined in the schema
  • the important information is the (attribute) type in the schema
  • the name of the attribute can be freely chosen by the schema designer
• xml:id [http://www.w3.org/TR/xml-id/] establishes a well-know attribute name for IDs
  • the IDness of an attribute is established by its name
  • IDs can be found without any schema (there does not even has to be one)
• xml:id uses XML's own namespace to identify identifiers
  • only W3C-blessed specifications are allowed to extend the XML namespace
  • http://www.w3.org/XML/1998/namespace [http://www.w3.org/XML/1998/namespace] contains a mix of various specs

<section id="introduction">

<section xml:id="introduction">

(23) Including Trees into Trees

• XInclude [http://www.w3.org/TR/xinclude/] defines an inclusion facility for XML
  • it is a separate standard and not part of XML itself
  • XML processors might or might not support XInclude
• XInclude is defined as replacing XInclude elements with included content
  • logically speaking, XInclude is thus defined as a tree transformation
  • practically speaking, it can thus be implemented in XSLT [http://dret.net/projects/xipr/]

<x xmlns:xi="http://www.w3.org/2001/XInclude">
	<xi:include href="something.xml"/>
	<xi:include xpointer="xmlns(xi=http://www.w3.org/2001/XInclude)xpointer(x/xi:include[1])" parse="xml"/>
</x>

(24) XInclude Schema

<!ELEMENT xi:include (xi:fallback?)>
<!ATTLIST xi:include
	xmlns:xi		CDATA	   #FIXED	"http://www.w3.org/2001/XInclude"
	href			CDATA	   #IMPLIED
	parse		   (xml|text)  "xml"
	xpointer		CDATA	   #IMPLIED
	encoding		CDATA	   #IMPLIED
	accept		  CDATA	   #IMPLIED
	accept-language CDATA	   #IMPLIED
>

(26) XML and Programming

• XML is a format for structured data
  • trees do not map very well to most programming languages
  • for working with XML, some mapping into the language is required
• There are two basic approaches for programming with XML:
  1. use special functions to work on XML documents as external data objects
  2. map XML documents to native data structures of the programming language
• A third approach is to have an XML programming language
  • XSLT [XML Transformations (XSLT) – Part I] is an example for an XML programming language
  • XSD [XML Schema (XSD) – Part I] and XPath [XML Path Language (XPath)] become an integral part of Java in XJ [http://www.research.ibm.com/xj/]

(27) XML and Programming Languages

• Most programming languages do not support XML natively
  • a certain impedance mismatch between both models in unavoidable
• Function libraries (or their equivalent) can provide XML processing facilities
  • SAX [Simple API for XML (SAX) (1)] as an event-based API for accessing XML documents
  • DOM [Document Object Model (DOM) (1)] as a tree-based API for accessing XML documents
• Mapping between XML and the programming language can take two forms
  • using hand-crafted code (based on XML functions) that performs the mapping
  • generating code using an XML schema and/or target data structures in the language
• Generating mapping code can be done in two ways
  • using a generic XML Data Binding [XML Data Binding (1)] framework for the mapping
  • using hand-crafted code that can be better tailored to the schemas

(28) Typical XML & Programming Problem

• Asynchronous JavaScript and XML (Ajax) [../web-fall10/ajax] is based on HTTP & XML
  • JavaScript code can communicate with the server using XMLHttpRequest [http://www.w3.org/TR/XMLHttpRequest/]
  • in theory, the server sends XML data which is processed by the script
• XML parsing and processing is inconvenient in JavaScript
  • there is a impedance mismatch between JavaScript and XML
  • if the client is slow and the XML is big, parsing can be time-consuming
  • if all clients are JavaScript, then sending XML is not really necessary
• JavaScript Object Notation (JSON) is a JavaScript-centric data model
  • JavaScript code can directly instantiate JSON structures as runtime objects
  • any non-JavaScript client (if there are any) will have to use JSON as well