The RDF meta model Basic ideas of the RDF Resource instance descriptions in the RDF format Application-specific RDF schemas Limitations of XML compared w/ RDF The Dublin Core Standard Metadata, ontology and information registry

Definations RDF –a uniform format for resource instance as well as schema description Dublin Core –a conceptual schema about web resources Ontology –data about real world objects: classes, inheritance, attribute, relationship Information Registry –data about data: various aspects of on-line documents Information Resource Schema –schema for the information registry All three (the ontology, the registry and the schema) will be specified in the RDF format

Basic Ideas It’s all about machine-understandability and automation –resource discovery in building and maintaining search engines –cataloging for certain Web sites or a digital library –sophisticated communication between intelligent software agents Standardization at two levels –instance level: all resource instances can be described in a uniformed format –schema level: class, attribute and relationship definitions also in a uniformed format. Conventional meta models only enforce the standardized format in the schema level, e.g. O.O., ER or other semantic models.

Resource instance descriptions The RDF meta model contains the following three basic concepts –A resource can be anything describable using RDF. E.g., an entire web page, a whole collection of web pages, a web site. Even an object in the physical world such as a book. –A property is a specific aspect of a resource. It can be a characteristic that belongs to a resource, or a relationship that links one resource with another. –A statement is a piece of description about a particular resource in the RDF format. All RDF keywords for the instance-level description start w/ “rdf:”

Resource instance descriptions (cont’d) A statement about a resource instance has: –the resource’s identifier –one of the resource’s property (defined in an RDF schema) –the value for that property (can be either a literal, or a resource) Cigarette smoking linked to colorectal cancer

Resource instance descriptions (cont’d) A property can be a collection of elements RDF provides three types of collection:, and Collection specification at the instance rather than the schema level <rdf:RDF xmlns:wc=” Cigarette smoking … ……

Resource Schema Specification A schema in RDF is comparable to that in the O.O. model (a set of class definitions) or the ER model (a set of entity specifications) User can define classes as well as an inheritance hierarchy. Attributes of classes are separately defined as ‘properties’ – a major difference from conventional modeling methods. All RDF keywords for the schema-level description start w/ “rdfs:”

Resource Schema Specification: Classes A class definition in RDF is more like a class declaration in O.O. languages. Keywords:,. An example: The set of all medical related documents. <rdfs:subClassOf rdf:resource=” 01/rdf-schema#Resource”/> The set of patients’ records.

Resource Schema Specification: Properties Unlike conventional modeling methods where attributes are subordinates of classes, the ‘Property’ concept in RDF is at the same level with the ‘Class’ concept. Properties are linked w/ classes via the ‘domain’ construct. Benefit: more flexibility and extensibility. An example: … Comparable to: class PatientRecords : MedicalDocument { PatientIDClass patientID; }

Limitations of XML compared w/ RDF XML provides a standardized syntax for interoperability purpose, not on the semantics. –an XML parser can be reused anywhere –the XML parser cannot understand the semantics behind the syntax –for two parties to communicate, they have to agree upon certain semantic aspects of the document App 1 App 2 XML Parser (encoder) XML Parser (decoder) …

Limitations of XML (cont’d) No fixed XML syntax to describe one fact: DTD 1 XML Instance Cigarette … DTD 2 XML Instance Cigarette …

The Dublin Core Standard The Dublin Core Metadata Intiative A standardized conceptual schema for describing web resources Dublin Core Elements (attributes of a web resource) –15 pre-defined elements: Title, Creator, Subject, Description, Publisher, Contributor, Date, Type, Format, Identifier, Source, Language, Relation, Coverage, Rights. Dublin Core also defines a set of keywords associated with each element, called ‘Dublin Core Qualifiers’, to make the element instances more specialized –E.g. a ‘Date’ element may be further refined as ‘Created Date’, ‘Valid Date’, ‘Available Date’, ‘Issued Date’ or ‘Modified Date’. The encoding scheme can be ‘DCMI period’ or ‘W3C-DTF’. The Dublin Core standard can be specified as an RDF Schema

Metadata, Ontology and Information Registry Ontology: –concept (class) graph about real world objects Metadata: –conceptually, people are committed to this de facto definition - ‘structured data about data’ –practically, ‘metadata’ is used to denote both ‘data about data’ and ‘data about real world objects (the ontology)’ Information Registry: –to free ourselves from the ambiguity of ‘metadata’, we can use ‘information registry’ to denote ‘structured data about data’. Both the ontology and the information registry can be represented in the RDF format

Ontology and Information Registry (cont’d)... Information Resources described by Information Registry Ontology abstraction Real World Objects described by abstraction Dublin Core Schema Extended Schema Resource Schema All specified in the RDF format