Ontology-Based Data Access (OBDA) enables automated reasoning over an ontology as a generalized global schema for the data in local (e.g., relational or graph) databases reachable through mappings. OBDA can semantically validate, enrich, and integrate heterogeneous data sources. Motivated by rule-ontology synergies, this talk discusses key concepts of OBDA and their foundation in three kinds of (logical) 'if-then' rules, using examples from the DeltaForest case study on the susceptibility of forests to climate change. (1) A query is a special Datalog rule whose conjunctive body can be rewritten (see 2) and unfolded (see 3), and whose n-ary head predicate instantiates the distinguished answer variables of the body predicates. OBDA ontologies beyond RDF Schema (RDFS) expressivity usually permit negative constraints for data validation, which are translated to Boolean conjunctive queries corresponding to integrity rules. (2) The OBDA ontology supports query rewriting and database materialization through global-schema-level reasoning. It usually includes the expressivity of RDFS, whose class and property subsumptions can be seen as single-premise Datalog rules with, respectively, unary and binary predicates, and whose remaining axioms are also definable by rules. OBDA ontologies often extend RDFS to the description logic DL-Lite (as in OWL 2 QL), including subsumption axioms that correspond to (head-)existential rules. Recent work has also explored Rule-Based Data Access, e.g. via Description Logic Programs (as in OWL 2 RL, definable in RIF-Core), Datalog+/-, and Disjunctive Datalog. (3) OBDA data integration is centered on Global-As-View (GAV) mappings, which are safe Datalog rules allowing query unfolding of each global head predicate into a conjunction of local body predicates. These (heterogeneous) conjunctive queries can be further mapped to the database languages of the sources (e.g., to SQL or SPARQL). Conversely, the same GAV mappings allow database folding. The talk develops a unified architecture for (1) to (3).
Dr. Harold Boley is adjunct professor at the Faculty of Computer Science, University of New Brunswick, Canada, and chair of RuleML Inc. His work on the specification of Web rules through RuleML has found broad uptake. Deliberation RuleML 1.01 is now being released, used for Datalog+/-, and applied to Rule-Based Data Access. RuleML has been combined with OWL to SWRL, has become the main input to the W3C Recommendation RIF, and has provided the foundation for OASIS LegalRuleML. His Rule Responder projects have enabled deployed distributed applications for the Social Semantic Web. His recent innovations in data-plus-knowledge representation include the object-relational PSOA RuleML and the visualization framework Grailog.
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