An Introduction to Object-Oriented RuleML

Semantic Web languages such as RDF (Schema) and OWL permit object-centered, taxonomic, and description-logic modeling in the URI-based distributed information system of the Web. Rule efforts of the Semantic Web have thus spurred interest in deductive object-oriented databases and object-extended LP approaches such as F-Logic, TRIPLE, and FORUM. Object-Oriented RuleML (OO RuleML) is an extension of the proposed standard Rule Markup Language that employs object-centered, role-keyed atoms / complex terms (ROLED), allows URI-grounded, 'webized' facts and rules (GROUNDED), and permits typed variables via URI links into Web-based taxonomies (TYPED).

ROLED: RuleML's XML/RDF-integrating system syntax augments 'type tags' by 'role tags' that represent the 'slots' or 'features' of type-tagged objects. OO RuleML makes such role tags available as a user syntax in atomic formulas and complex terms: the atom and cterm elements can now contain -- before/after positional argument children -- non-positional argument children '_r' (for the metarole 'role') with a required CDATA attribute 'n' (for the user-defined role 'name'). This also allows for mixed positional and object-centered representations. OO RuleML has been extended by role weights specifying the relative importance of slots. A surprisingly small change to the DTDs/Schemas was needed for permitting such user-level roles.

GROUNDED: Facts and rules in OO RuleML can be 'URI-grounded' via labels containing a Web-identifier attribute, 'wid', which has a URI value that can be referred to from a complementary 'widref' attribute. The 'wid'/'widref' pair was inspired both by XML's 'id'/'idref' and RDF's 'about'/'resource' pairs.

TYPED: OO RuleML variables are optionally typed/sorted via URIs referring to classes of predefined RDF Schema or OWL taxonomies. XML namespace declarations can use fragments (#) to point into the RDF Schema documents containing the required class definitions. A 'var' element can then be sorted via a 'type' attribute whose value augments the namespace prefix by the local name identifying the class.

OO RuleML will be introduced using the Positional-Roled (PR) syntax, which will also be compared to Notation 3 (N3). The status of the Java-based OO jDREW implementation of OO RuleML will be reviewed. Finally, two running applications of OO RuleML will be sketched: Rule-applying collaborative filtering (RACOFI) and weighted similarity matching (Treesim).