Semantic Technology Standards and Tools

A. W. Crapo, Last revised 01/10/2011.

Some Definitions

Semantic Technology -- a way of encoding meaning in formal, logical models (called ontologies). The formal semantics allow inferences to be drawn. Rules may also be expressed in terms of the model concepts.

Ontology -- a formal model that defines what does or can exist in a domain, including

  1. Classes (concepts) -- abstract groups or sets
    1. defined intensionally by specifying the conditions of set membership, e.g., Woman ≡ a Person whose age >= 18 and whose gender is Female
    2. defined extensionally by specifying the members of the set, e.g., Season {Spring, Summer, Fall, Winter}
  2. Individuals (instances) -- proxies for "real things" in the modeled domain, e.g., a Dog "Lassie"
  3. Properties -- [generalization of] characteristics or relationships of individuals
    1. [simple] attributes (data type properties) have values that do not have individual identity apart from the individual they describe: "Lassie age 5" ("5" does not identify an instance in the domain)
    2. relationships associate an individual with another individual that has separate identity: "Lassie friend Spot"

OWL (Web Ontology Language) -- a formal (unambiguous) language that builds on RDF and RDFS and is informed by prior work including Description Logics (DL), OIL, DAML, and DAML+OIL

  1. Classes, individuals, and properties are RDF resources and are identified by URIs
  2. A model is built using statements (RDF triples) of the form <subject> <predicate> <object>
  3. A model is a [mathematical] graph
  4. Different flavors of OWL have different expressiveness and different decidability: OWL-Lite, OWL-DL, OWL-Full
  5. Models may be distributed, modular, extensible by language design

SWRL (Semantic Web Rule Language) -- A W3C proposal that combines OWL (DL and Lite) with a subset of the Rule Markup Language (RuleML)

  1. Provides a rule language that allows Horn-like rules to be combined with an OWL knowledge base
  2. An extension of the OWL model-theoretic semantics to include rules
  3. May be part of a given model or an extension of it

Note: Jena Rules are similar to SWRL.

SPARQL (RDF query language) -- A W3C proposed recommendation query language for RDF (and therefore OWL) models

  1. Defined semantics as well as syntax
  2. A language for specifying graph patterns to be matched from a model

Reasoner -- a piece of software able to infer logical consequences from a model (set of assertions)

  1. Given definition of Woman above and assertion "Jane is a Woman", reasoner infers 1) Jane age >= 18 and 2) Jane gender Female
  2. Some reasoners can also process SWRL rules
  3. Reasoners may also be integrated with SPARQL query support
  4. Interesting reasoners:
    1. Pellet -- open source Java OWL-DL reasoner with explanation (version supporting uncertainty nearing release)
    2. Jena -- open source Java framework supporting model  creation, persistence, and inference