RDF and OWL Developing Semantic Web Services by H. Peter Alesso and Craig F. Smith

1. RDF and OWLDeveloping Semantic Web Servicesby H. Peter Alesso and Craig F. Smith  CMPT 455/826 - Week 6, Day Sept-Dec 2009 – w6d2

2. RDF • Subject: • The resource (a person, place, or thing) that the statement describes. A RDF Resource can be anything in the data model (document, user, product, etc). A RDF Resource is uniquely identified by a URI. • Predicate: • The property (name, city, title, color, shape, characteristic) of the subject (person, place, or thing). A RDF Property is uniquely identified by a URI. • Object: • The value (Peter, San Jose, “War and Peace,” blue, circle, strong) for the property (name, city, title, color, shape, characteristic) of the subject (person, place, or thing). This value can be any valid RDF data type. RDF supports all of the XML data types. Sept-Dec 2009 – w6d2

3. RDF structure • The graphical nature of XML is a tree structure while the graphical nature of RDF is a much flatter, triple-based directed graph (see Chapter 12). • While XML is hierarchical, which means that all related elements are nested within elements they relate to, RDF is not nested. • In RDF you can associate two separate XML structures with each other through a URI. RDF/XML is well formed XML, but includes the URI of the relationship. • URIs can use the same simply naming way to refer to resources under different protocols: HTTP, FTP, GOPHER, EMAIL, URLs (Uniform Resource Locator), a widely used type of URI. In addition, RDF adds a layer of complexity on XML that is more difficult to read. Sept-Dec 2009 – w6d2

4. RDF structure example Sept-Dec 2009 – w6d2

5. RDF Namespaces • An RDF goal is to record knowledge in machine-understandable format and provide mechanisms to allow the combination of the data. • By combining combinations of models, additions can be incorporated without impacting an existing RDF Schema. • The namespace declaration for RDF vocabularies points to the URI of the RDF Schema document for the vocabulary. Sept-Dec 2009 – w6d2

6. RDF Schema • An RDF Schema provides some of the same functionality as a relational database schema. • It provides the resources necessary to describe • objects and their properties. Sept-Dec 2009 – w6d2

7. RDF Vocabularies • RDF Schema create domain-specific vocabularies that are then used to model resources. • The only limitation is the need for industry (domain) cooperation in developing an interoperable vocabulary. • Vocabulary definition starts by identifying and creating domain elements and their properties within the given domain. Sept-Dec 2009 – w6d2

8. RDF Concepts • The RDF Schema approach is to: • define a class • (and indicate its relationships to other classes). • then separately, define properties • and then associate them with a class. • The advantage of this approach is that anyone, anywhere, anytime • can create a property and state that it is usable with the class. • When we want to define a new vocabulary or ontology, • we define new classes of things and new properties. Sept-Dec 2009 – w6d2

9. RDF Constraints • Constraints on properties are made with two RDF-defined properties on the properties: • the rdfs:domain property, which restricts the set of resources that may have a given property (ie., its domain). • the rdfs:range property, which restricts the set of values for a given property (i.e., its range). • The domain and range force subjects and objects of a Property to be a certain type. • NOTE: Users are not generally allowed to define other properties on properties in RDF Sept-Dec 2009 – w6d2

10. Ontologies • Ontologies are collections of information • tht help compare conceptual information across two knowledge bases on the Web, • by helping a system to know when any two given terms are being used to mean the same thing. Sept-Dec 2009 – w6d2

11. Limitations of RDF • Although RDF and RDFS are building blocks for defining a Semantic Web, together they still lack sufficient expressive power. • For example, they can’t define: • (1) the properties of properties, • (2) necessary and sufficient conditions for class membership, or • (3) equivalence and disjointness of classes. • In addition, the only constraints expressible are domain and range constraints on properties. Sept-Dec 2009 – w6d2

12. OWL, RDF, and XML Sept-Dec 2009 – w6d2

13. Ontologies • An ontology includes four concepts: • Classes. • Relationships between classes. • Properties of classes. • Constraints on relationships between the classes and properties of classes. Sept-Dec 2009 – w6d2

14. OWL • An OWL document identifies: • Class hierarchy—defines class/subclass relationships. • Synonym—identifies equivalent classes and equivalent properties. • Class association—maps one or more classes to one or more classes, through the use of a property (i.e., domain/range). • Property metadata—contains metadata for properties. • Class definition—specifies the composition of classes. • OWL defines the classes and properties, as well as their relationship to each other in the document, and as a result they are extremely similar to RDF Schema. • However, OWL has several concepts unique to it, such as Boolean combination of class expressions and property restrictions, which add a layer of reasoning to applications. • RDF Schema and OWL are compatible, which is why there are RDFS elements within the OWL element set. Sept-Dec 2009 – w6d2

15. Handbook of Ontologies for Business Interactionedited by Peter Rittgen Overview of Semantic Technologies by Anne M. Cregan Sept-Dec 2009 – w6d2

16. Key strategies of semantic technologies • Tagging physical data with metadata describing the data. • Metadata is unlimited, in the sense that it can describe anything about the data. Additionally, because it links directly to the data it is about, the tag provides a handle for data identification and retrieval. • Metadata tags are organized into ontologies, • which capture the logical and conceptual relationships between the tags, and provide a semantic map overarching the data. • Aligning and mapping ontologies produces a semantic map • over all the data sources, creating semantic interoperability, and providing the possibility for coordinated and seamless searching, querying, and processing over the virtual data structure. • As ontologies are underpinned by formal logics, • they support automated reasoning over the amassed data. Semantic interoperability thus provides a basis for semantic brokers and semantic services. Intelligent agents then may compose these services to perform more complex tasks on behalf of the user. Sept-Dec 2009 – w6d2

17. Model Driven Architecture and Ontology Development by Dragan Gaševic, Dragan Djuric  and Vladan Devedzic Sept-Dec 2009 – w6d2

18. Ontology Development (1) • Determine the domain and scope of the ontology, this should help create a clear vision of • the ontology's coverage, • its intended use, • the types of questions the information in the ontology should provide answers to, • and maintenance guidelines. Sept-Dec 2009 – w6d2

19. Ontology Development (2) • Consider reusing existing ontologies • since ontology development is hard work, • it is always a good idea to check if someone else has already done the work and made the result publicly available, so that it can be refined and extended to suit a particular domain or task. • Language conversion, interoperability, and tool support issues are important here as well. Sept-Dec 2009 – w6d2

20. Ontology Development (3) • Enumerate important terms in the ontology • this is where building the terminology starts. • Define the classes and the class hierarchy • this step, closely intertwined with the next one, can be performed • top-down (identifying the most general concepts and classes first), • bottom-up (identifying the most specific ones first), • middle-out (starting from some important middle-layer concepts and expanding the hierarchy in both directions), • or using a combination of these approaches. Sept-Dec 2009 – w6d2

21. Ontology Development (4) • Define the properties (slots) of classes • describe the internal structure of concepts by explicating their extrinsic properties (e.g., name, duration, and use), intrinsic properties (e.g., weight), parts, and relations to other classes and individuals in those classes. • Define the facets of the slots • these are things such as the slot value type, the allowed values (domain and range), the number of values (cardinality), and other features of the values that the slot can take. • Create instances • this includes filling in the slot values for each instance created. Sept-Dec 2009 – w6d2