Representing Data with XML February 26, 2004 Neal Arthorne

Data Representation Design goals for data representation:  Portable (platform independent)  Easy for machines to process  Human legible  Flexible and usable over the Internet and other networks  Concisely defined with formal rules

Extensible Markup Language World Wide Web Consortium (W3C) defines the Extensible Markup Language (XML)  W3C also defined HTML, CSS, HTTP, SVG and other markup languages  XML Working group formed in 1996  XML 1.0 (Third Edition) 4 February 2004 (original Recommendation in 1998)

XML Example Queen Victoria Jubilee issue - 5 cents Two portraits of Queen Victoria 1837 and The portrait on the left... Canada x 5¢ Lyndwode Charles Pereira, Peleg Franklin Brownell ,000 American Bank Note Company, Ottawa file://test/stamps/ /Jubilee _cents.jpg Prolog Element Attribute Entity reference

XML XML documents should be well-formed (syntax, closing tags etc) XML documents are “valid” if they are validated with a Document Type Definition (DTD) DTDs provide a grammar for the XML by defining elements, attributes and entities

XML Advantages XML provides:  Logical structure for data in a textual representation  Formal rules for validating documents  Flexibility to define your own markup language  Portability across networks and platforms  Becoming a widely accepted data interchange format  Processed with off-the-shelf tools

XML Disadvantages XML drawbacks:  Not a binary format so it requires a lot of overhead for a little bit of data  Very little support for binary or mixed media data formats (hex or base64 encoding)  Only for data and holds no semantics or reasoning DTDs do not provide:  Data types for each element or attribute  Complex structural rules for documents

XML Schema XML Schema defines a new schema language to replace DTD Standardized by W3C in 2001 Provides data typing and logical structure Written in XML (easy to process) Higher complexity than DTD

XML Schema Example Element nameData type Attribute nameData type An XML document is an ‘instance document’ of an XML Schema

Simple Types Simple Types are of three varieties:  Atomic: Built-in or derived, e.g.  List: multiple items of the same type  Union: Union or two or more Simple Types

Built-in Types XML Schema defines numerous built-in types:  integer, decimal, token, byte, boolean, date, time, short, long, float, anyURI, language Derived types use facets to restrict existing types:  min/maxInclusive, min/maxExclusive, pattern, enumeration, min/maxLength, length, totalDigits, fractionDigits

Complex Types Complex Types define logical structures with attributes and nested elements They use a sequence, choice or all containing elements that use Simple Types or other Complex Types May reference types defined elsewhere in the schema or imported using import statement

In the Schema of Things XML Schema supersedes DTD Defines a typed data format with no semantics or relations between data Next step: higher level of abstraction and the ability to define objects and relations

Resource Description Framework W3C standard for describing resources on the World Wide Web (1999, revised 2004) Generalized to identify objects that may not be retrievable on the Web Objects identified by Uniform Resource Identifiers (URIs) RDF represented by a directed graph and in XML syntax

RDF Example In English: has the full name Federico Diaz and has an employer called Fisher and Sons. Federico Diaz

RDF Parts Each RDF statement is a triple containing a subject (identifier by URI), a predicate (e.g. creator, title, full name) and an object An object can be either a literal value (e.g. Federico Diaz) or another RDF resource All three parts can be identified with an URI and fragment identifier #

RDF Semantics RDF attaches no specific meaning to RDF statements – just like the name of a database field is meaningless to an SQL engine RDF does provide a way to attach data types to literal values, but RDF does not define data types Generally RDF software uses the XML Schema data types  10 Arbitrary XML can also be used as a literal  10

RDF Schema RDF Schema is a ‘vocabulary description language’ that relates resources to each other using RDF RDFS uses ‘classes’ of objects like in Object-Oriented (OO) systems Class properties relate to other classes using OO concepts such as generalization

RDF Schema Use Differs from OO in that Properties are defined in terms of the resources to which they apply (their domain) – they are not restricted to the scope of a single class  domain: Classes to which a Property applies  range: The Class of a Property (i.e. type) Allows new Properties to be created that apply to the same domain without redefining the domain

RDFS Classes Classes introduced by RDFS:  Resource - top level class  Literal – all literal values like text strings  Class – the class of all classes  Datatype – top level RDF datatype Properties introduced by RDFS:  subClassOf  subPropertyOf  domain – domain of a Property  range – range of a Property  label, comment, seeAlso – human readable labels inheritance

RDF/RDFS Lets authors create vocabularies of Classes and Properties and show how the terms should be used to describe resources, e.g.  Property ‘author’ applies to class ‘Book’  Class ‘Employee’ is a subclass of ‘Person’ Does not define descriptive properties such as ‘dateOfIssue’ or ‘title’ but references them using URIs Like in XML/XML Schema, an RDF instance document can be validated against its RDF Schema

Machines Understanding the Web RDF/RDFS along with XML/XML Schema provide a means to describe resources on the web with basic generalization For a higher conceptual level, applications require semantic information Ontologies serve as a starting point for understanding

Ontologies on the Web “Ontologies define the terms used to represent an area of knowledge.” – OWL Use Cases & Requirements, 2004 Example use cases:  A web portal that needs to classify information  Multimedia archive that requires a taxonomy of media or content-specific properties  Corporate portal website that integrates vocabularies from different departments

Web Ontology Language (OWL) Supersedes DAML+OIL  DARPA Agent Markup Language (DAML) was based on RDF/RDFS and includes much of what is now OWL Adds terms used to better describe relations between classes of RDF resources With OWL ontologies can be integrated, extended and shared

OWL Features OWL improvements on RDF/RDFS:  Cardinality min/maxCardinality for Properties with respect to a Class  Equality, disjointness equivalentClass, equivalentProperty, sameAs, differentFrom, disjointWith  Transitive, Symmetric, Functional Properties labelling a Property allows for reasoning  A has B and B has C implies A has C (Transitive)  A has B implies B has A (Symmetric)

OWL Features con’t  Boolean expressions of Class relations unionOf, complementOf, intersectionOf  Property restrictions Limits how properties can be used by an instance of a class  Versioning priorVersion, versionInfo, incompatibleWith, backwardCompatibleWith

Conclusion XMLXML Schema RDFRDF Schema OWL Unicode/ISO byte streams Data formatting and data types Machine data representation Resource description and vocabulary Knowledge processing and reasoning ??? Conceptual level reasoning – ‘smart’ applications Knowledge Data

References World Wide Web Consortium XML XML Schema Part 0: Primer RDF Primer RDF Concepts RDF/XML Syntax RDF Schema OWL Use Cases & Requirements OWL Overview