The Semantic Web includes technologies (e.g., RDF) that allow metadata to be published in machine-readable form. Currently, this information is mostly enumerated as a set of facts. It is often desirable, however, to supplement such facts with rules that capture implicit knowledge. To maximize the usefulness of such published rules, a standard rule format such as RIF is necessary.
One case involves extending current standards for metadata publication with rules in order to express implicit knowledge. Suppose that the International Movie Database (IMD) publishes its metadata and rules in a machine readable format at http://imd.example.org. Besides the ground facts, which can be expressed in RDF, the metadata might also have general rules like the following:
Every science fiction movie is a movie. Every movie produced before 1930 is black and white.
Such rules allow data to be published more concisely by expressing knowledge that, without these rules, is implicit. This can greatly simplify the maintenance of data, guard against inadvertently introduced inconsistencies, and reduce storage requirements.
Published rules also allow combining data from different sources to exploit this knowledge. Consider an alternative database of movies published at http://altmd.example.org. In addition to metadata, it again publishes its own rules:
All movies listed at http://altmd.example.org but not listed at http://imd.example.org are independent movies. All movies with budgets below 5 million USD are low-budget movies.
Publication of rules with explicit references to other rulesets allows the definition of knowledge dependent on explicitly specified remote sources. Such explicitly specified scope is important in the Web environment, since it can reduce the danger of unintended interference from rules published at other remote sources, which may be exporting their own predicates.
Another example of such explicit referencing, which also illustrates implicit person-centric metadata, involves published rules being used to specify how to use other metadata, e.g. in the form of a widespread vocabulary such as FOAF or a standard exchange format like iCalendar. For example, FOAF user Charlie might choose to complement his normal FOAF profile with his preferences about which of his phone numbers should be used depending on his iCalendar schedule:
If Charlie is currently attending a public talk according to http://charlie.example.org/calender.ical then leave him a voicemail message If Charlie is currently in a meeting according to http://charlie.example.org/calender.ical and the importance is high then call his cell number If Charlie currently has no appointments according to http://charlie.example.org/calender.ical then call his office number
RIF should allow extending current standards for metadata publication by enabling such implicit knowledge to be captured via rules and allowing metadata and rules distributed over different sources to be interlinked. In a manner similar to how HTML links human-readable Web pages, RIF should permit linking metadata on the Web to support new kinds of "intelligent" crawling and search.
The following discriminators in the Rule Arrangement Framework are particularly relevant for this use case:
Syn 1.4 Explicit vs. Implicit Rule Scope
- Scoped inference and Scoped NAF are needed to model the rules in this use case
Syn1.6 Webized Names:
- HTML and RDF use IRIs to link data and metadata on the web, and so rules which define implicit interlinked metadata.
Syn1.7 Means to access data in Web formats:
- access of data in RDF, and other XML based data, respecting ontologies in OWL are essential for this use case.
SeS2.2 Fact-and-Rule Bases:
- This use case needs mixed fact and rule bases.
SeS2.8 Certain vs. Uncertain Clauses and Atoms:
- Default rules to specify default, implicitly defined metadata which may be overridden by more specific facts are an issue in this use case.
Sem3.2 Finite-Model vs. Infinite-Model Rulebases:
- The models of RDF alone are infinite already by the infinite number of axiomatic triples. On the contrary finite models are desirable for the evaluation of the consequences of rule sets.
The described rules shall take metadata in the form of RDF facts as a basis. As a possible extension additional inferences over OWL ontologies shall be taken into account, see also Semantic tagging below.
A layered approach such as taken in SPARQL (or allowing SPARQL queries in rule bodies) which decouples the adopted entailment regime (simple, RDF(S), OWL, etc. entaiment) from the semantics of such rules is possible, which requires semantic tagging of rulesets.