Encoding an OWL Ontology: Guidelines and Practices

1. OWL Representing Information Using the Web Ontology Language

2. Section 3

3. Section 3 • Chapter 11: Encoding an OWL Ontology • Organization of OWL files • Chapter 12: Defining OWL Lite Classes & Properties • Definition of simple classes; pre-defined OWL classes • Chapter 13: Describing OWL Lite Property Characteristics • Global & local property restrictions; inference shortcuts • Chapter 14: Deriving OWL Lite Classes • Class descriptions, subclasses, equivalency, intersections • Chapter 15: Describing Individuals • Unique names assumption; instantiate, relate individuals • Chapter 16: OWL Lite Summary • Summary of OWL Lite constructs

4. Chapter 11

5. 11 Encoding an OWL Ontology • OWL Ontology encodings • Defined in web documents • Referenced using URI • Developed like software (requirements, design) • Use one of the OWL dialects • Use RDF/XML syntax • Describe classes & properties • OWL knowledge base • Ontology • Instance data • Datatypes • Chapter describes OWL ontology file structure • Header • Body • Footer

6. 11.1 OWL Ontology File Structure • Figure shows • Relationship between ontology files, namespaces, & related files • Distribution of ontology files on web servers • Ontologies can extend other ontologies hosted across network • Instances • Reference ontologies • Ontologies • Extend/reuse ontologies • Built on XMLS datatypes

7. 11.1 Ontology File Structure cont’d • OWL documents • Best practice: use .owl extension • Include • owl:Ontology element • Class definitions • Property definitions • Typical format • Header • Body • Footer

8. 11.2 OWL Header • OWL ontology file begins with header • Specifies RDF start tag <rdf> • Includes namespace attributes • Includes ontology element • Contains versioning & imports

9. 11.2.1 XML Declaration & RDF Tag • OWL ontologies represented using RDF/XML • Best practice: begin file with • XML declaration including version and encoding • <?xml version="1.0" encoding="UTF-8"?> • <rdf:RDF> start tag containing namespace declarations • <rdf:RDF namespaceDeclarations> • Defined in the next section

10. 11.2.2 Namespaces for OWL Ontologies • Namespace declarations • Attributes of <rdf:RDF> start tag • Identify abbreviated references in ontology file • Make file more readable for humans • Can be used in tag & attribute names, not values • Use XML entities for values, if desired • 3 types of namespaces • Standard namespace references • Part of OWL standard: XMLS, RDF, RDFS, OWL • Namespaces associated with imported ontologies • Setup shorthand abbreviations to reference URIs • Namespaces identifying ontology being defined • Defined implicitly by document URI, explicitly by “xml:base”, or “xmlns” attribute

11. 11.2.2 Ontology Namespaces cont’d • Best practice • Order namespaces from bottom to top • XMLS (for datatypes) • RDF • RDFS • OWL • Reused ontologies • Extended ontologies • Current ontology

12. 11.2.2 Ontology Namespaces cont’d • XML Schema Datatypes namespace (xsd prefix) • xmlns:xsd="http://www.w3.org/XMLSchema#" • RDF namespace (rdf prefix) • xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" • RDFS namespace (rdfs prefix) • xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#" • OWL namespace (owl prefix) • xmlns:owl="http://www.w3.org/2002/07/owl#" • Imported namespace (user-defined prefix) • xmlns:pasta="http://www.food.org/pastaDishes#" • Ontology namespace (user-defined prefix) • xmlns="http://www.food.org/food-ex#" or • URI of containing document or • Value of xml:base attribute

13. 11.2.2 Ontology Namespaces cont’d • xml:base attribute used to expand RDF URIrefs • Best practice: Either • Include xml:base in ontology file or • Use fully-resolved URIs in rdf:about attribute values • Example • <rdf:RDF xmlns:xsd="http://www.w3.org/XMLSchema#" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#" xmlns:owl="http://www.w3.org/2002/07/owl#" xmlns:pasta="http://www.food.org/pastaDishes#" xmlns="http://www.food.org/food-ex#" xml:base="http://www.food.org/food-ex#">

14. 11.2.3 Ontology Element • owl:Ontology element • Built-in class used to instantiate ontologies • Header only, not classes, properties, instances • Optional but recommended • Provides metadata including • Version information • Imports • RDF comments • Should include rdf:about attribute • Empty value ("") means current ontology • Default name: URI of document, unless xml:base specified

15. 11.2.3 Ontology Element cont’d • owl:Ontology element • Recommended practices • Give name using rdfs:label property • Comment using rdfs:comment property • Syntax • <owl:Ontology rdf:about="optionalOntologyName"></owl:Ontology> • Example • <owl:Ontology rdf:about=""> <rdfs:label>Restaurant Menu</rdfs:label> <rdfs:comment>Describes basic concepts representing a restaurant’s menu</rdfs:comment></owl:Ontology>

16. 11.2.3.1 Versioning Information • OWL provides explicit versioning info including • Ontology-level versioning • Compatibility statements about previous versions • Deprecation declarations • Classes & properties for backward compatibility

17. 11.2.3.1.1 Version Information • owl:versionInfo property • String representing ontology’s version • Can use RCS, CVS, Subversion keywords (see example) • Documentation comment; no logical meaning • Normally used for ontologies but can be applied to any owl:Thing • Best practice: use owl:versionInfo in each OWL document • May eventually evolve into parseable “Dublin Core” items • Domain: Instance of owl:Thing, Range: Instance of rdfs:Literal • Syntax • <owl:versionInfo>versionText</owl:versionInfo> • Example • <owl:Ontology> <owl:versionInfo>$Id: restaurantMenu-ont.owl,v 1.2 2004/08/30 12:34:56 llacy Exp$</owl:VersionInfo></owl:Ontology>

18. 11.2.3.1.1.1 Indicating Earlier Version • owl:priorVersion property • Relates newer version to previous version • Tracks ontology version history • Domain & Range: Instance of owl:Ontology • Syntax • <owl:priorVersion rdf:resource="priorOntURI"/> • Example • <owl:Ontology rdf:about=""> <owl:priorVersion rdf:resource="http://www.food.org/pasta0603-ont"/></owl:Ontology>

19. 11.2.3.1.1.2 Backward-Compatibility • owl:backwardCompatibleWith property • Explicitly states that all constructs in this version have the same meaning as in the prior version • Subproperty of owl:priorVersion • If not explicitly stated, incompatibility assumed • Domain & Range: Instance of owl:Ontology • Syntax • <owl:backwardCompatibleWith rdf:resource="olderOntURI"/> • Example • <owl:Ontology rdf:about=""> <owl:backwardCompatibleWith rdf:resource="&kor;Menu062204-ont"/> <rdfs:comment>added new menu items since 06/22/04</rdfs:comment></owl:Ontology>

20. 11.2.3.1.1.3 Incompatible Ontologies • owl:incompatibleWith property • Identifies prior versions that this ontology supersedes and is inconsistent with • Subproperty of owl:PriorVersion • Best practice: include incompatibility comments • Domain & Range: Instance of owl:Ontology • Syntax • <owl:incompatibleWith rdf:resource="olderOntURI"/> • Example • <owl:Ontology rdf:about=""> <owl:incompatibleWith rdf:resource="&kor;Menu062204-ont"/> <rdfs:comment>added new menu items</rdfs:comment></owl:Ontology>

21. 11.2.3.1.1.4 Deprecating Classes • owl:DeprecatedClass class • Identifies classes no longer supported in future versions • Supports short-term compatibility • Existing ontologies should upgrade to new concepts as soon as possible • Signals long-term incompatibility • New ontologies should not reference deprecated classes • No semantics; treated as comment by parser • Best practice: provide comment indicating replacement class • Syntax • <owl:DeprecatedClass rdf:about="OldClassName"> • Example • <owl:DeprecatedClass rdf:about="#Pie"> <rdfs:comment>Use DessertPie instead</rdfs:comment></owl:DeprecatedClass>

22. 11.2.3.1.1.5 Deprecating Properties • owl:DeprecatedProperty class • Identifies properties no longer supported in future versions • Same concepts as owl:DeprecatedClass • Best practice: provide comment indicating replacement property • Syntax • <owl:DeprecatedProperty rdf:about="oldPropertyName"> • Example • <owl:DeprecatedProperty rdf:about="#cost"> <rdfs:comment>Use menuPrice instead</rdfs:comment></owl:DeprecatedProperty>

23. 11.2.3.1.1.6 Versioning Summary • Versioning constructs • Documentation only • Do not provide additional semantics • Not intended for use by inferencing engines

24. 11.2.3.2 Importing Ontologies • owl:imports property • Allows an ontology to reference and extend another ontology • Supports incremental & distributed ontologies • Similar to “include”/“import” in programming languages • Introduces dependencies • Made in the ontology element of importing ontology • Domain & Range: Instance of owl:Ontology • OWL ontology always implicitly imported • Syntax • <owl:imports rdf:resource="URIofImportedOntology"/> • Example • <owl:imports rdf:resource="http://ut.edu/ont/fruit-ont"/>

25. 11.2.3.2 Namespace vs. Imports cont’d • Namespace declarations • Create prefix abbreviations for identifiers • owl:imports properties • Include all constructs from ontology referenced in URI • Use namespace declaration to create shortened names to reference ontology constructs • Example • <rdf:RDF xmlns:pasta="http://www.food.org/pastaDishes#"> • <owl:Ontology about=""> <owl:imports rdf:resource="http://www.food.org/pastaDishes"/></owl:Ontology>

26. 11.2.3.2 Imports Rules cont’d • Multiple imports permitted • owl:imports are transitive • A imports B • B imports C • Therefore A imports C • No effect if ontology imports itself (prevents cycles) • If A imports B and B imports A, A and B are equivalent • If imported ontology not accessible at specified URI during runtime, it may prevent use of importing ontology

27. 11.2.3.3 Ontology Element Summary • owl:Ontology element provides metadata • Version information (owl:versionInfo & related) • Name (rdfs:label property) • Descriptive comments (rdfs:comment property) • Imported ontologies (owl:imports property) • Example complete ontology element • <owl:Ontology rdf:about=""> <owl:versionInfo>v 1.2 2004/10/24</owl:versionInfo> <rdfs:comment>Restaurant Menu ontology</rdfs:comment> <owl:imports rdf:resource="&rest;2004/08/food-ont"/> <owl:priorVersion rdf:resource="&rest;2004/07/Menu-ont"/> <owl:incompatibleWith rdf:resource="&rest;2004/06/Menu-ont"/> <owl:backwardCompatibleWith rdf:resource="&rest;2004/07/Menu-ont"/><owl:Ontology>

28. 11.2.4 OWL Header Summary • OWL file header contains • RDF tag and namespace properties • owl:Ontology element

29. 11.3-4 OWL File Body & Footer • OWL file body • Contains class & property definitions • Classes derive from owl:Class; share similar properties • Values are XMLS datatypes or object instances • Properties define relationships between classes & values • Order is not important • Forward references are permitted • Co-occurring statements assumed to be conjunctive (AND) • “Open World Assumption” • Anything that is not explicitly stated cannot be assumed to be either true or false. Statements can only be added, not deleted. • Limits inferencing and deductive power • OWL files end with </rdf:RDF> end tag

30. 11.5 OWL Encoding Summary • OWL ontologies specified through statements in ontology files • Ontology files contain • Header contains • Namespaces • Versioning Information • Import Statements • Body contains statements about • Classes • Properties • Relationships • Footer (rdf:RDF end tag)

31. Chapter 12

32. 12 Defining Basic Classes & Properties • OWL Lite classes • Based on owl:Class, a richer subclass of rdfs:Class • 2 basic class types • Simple named classes • Predefined classes • owl:Thing, owl:Nothing • OWL Lite properties • Based on rdf:Property • 4 basic property types • owl:DatatypeProperty, owl:ObjectProperty, owl:AnnotationProperty, owl:OntologyProperty

33. 12.1 Defining a Simple Named Class • Simple named classes • Defined with owl:Class element & rdf:ID attribute • Does nothing but declare a name (URI) • Within same file, often referenced by fragment (#classname) • All statements about a class apply to its individuals • Best practices: • Name using UpperCamelCase • Use rdfs:label and rdfs:comment to document • Syntax • <owl:Class rdf:ID="className"/> • Example • <owl:Class rdf:ID="KeyLimePie"> <rdfs:label>Key Lime Pie</rdfs:label> <rdfs:comment>Custard pie made with Key lime juice</rdfs:comment></owl:Class>

34. 12.2 Predefined OWL Classes • OWL defines two “extreme” classes • owl:Thing • Used in statements that apply to all instances • owl:Nothing • Used in statements that apply to no instances

35. 12.2.1 Thing Class • owl:Thing class • Most general class in OWL • Root class for all classes • Even rdf:Resource, root class of RDF • Every individual is a member of owl:Thing • Every class is a subclass of owl:Thing • Equal to the union of any class and its complement • Set equivalent of “universe” • Can be used to make global assertions

36. 12.2.2 Nothing class • owl:Nothing class • Subclass of all classes • Empty class • No member individuals • Set equivalent of “null set”

37. 12.3 Describing OWL Lite Properties • Properties • Are binary relations from one individual (domain) to another individual or a value (range) • Enable specification of facts (attribute/value pairs) • Four disjoint property types in OWL • Datatype properties relate objects to datatype values • Object properties relate objects to other objects • Annotation properties describe objects • Ontology properties relate ontologies to other ontologies

38. 12.3.1 Datatype Properties • owl:DatatypeProperty class • Identifies a property whose value is a member of a datatype (typed or untyped literals) • Strings or simple XMLS datatypes • Reasoners use them to interpret a statement’s datatype value • Syntax • <owl:DatatypeProperty rdf:ID="propertyName"/> • Example (specification) • <owl:DatatypeProperty rdf:ID="restCloseTime"> <rdfs:domain rdf:resource="#Restaurant"/> <rdfs:range rdf:resource="&xsd;time"/></owl:DatatypeProperty> • Example (use) • <rest:Restaurant rdf:ID="JoesPizza"> <restCloseTime rdf:datatype="&xsd;time">21:00:00+01:00</restCloseTime></rest:Restaurant>

39. 12.3.2 Object Properties • owl:ObjectProperty class • Identifies property whose value is reference to another individual • Value is a resource • Points Reasoner to resource containing desired value • “What is the name of the owner of Knight Owl Restaurant?” (see below) • Syntax • <owl:ObjectProperty rdf:ID="propertyName"/> • Example (specification) • <owl:ObjectProperty rdf:ID="restOwnedBy"/> • Example (use) • <rest:Person rdf:ID="person123"> <personName>Jason Relles</personName></rest:Person> • <rest:Restaurant rdf:ID="KnightOwlRestaurant"> <ownedBy rdf:resource="#person123"/></rest:Restaurant>

40. 12.3.3 Annotation Properties • owl:AnnotationProperty class • Identifies property that describes a construct • Superclass of rdfs:label, rdfs:comment, rdfs:seeAlso, rdfs:isDefinedBy, owl:versionInfo • Domain: named class, property, individual, or ontology • Range: individual, data literal, URIref • Syntax • <owl:AnnotationProperty rdf:about="uriRef"/> • Example • <owl:AnnotationProperty rdf:about="&rest;reviewer"/>

41. 12.3.4 Ontology Properties • owl:OntologyProperty class • Relates two owl:Ontology instances • Superclass of owl:imports, owl:priorVersion, owl:backwardCompatibleWith, owl:incompatibleWith • Not often used in practice

42. 12.4 Classes & Properties Summary • Basic classes are defined using owl:Class • owl:Thing is a universal root class & contains everything • owl:Nothing is a universal subclass & contains nothing • Datatype properties relate objects to literal values • Object properties relate objects to other objects • Annotation properties provide additional information • Ontology properties relate ontologies to other ontologies