• RIF RuleML FOAF

This is one of the possible Use Cases.

1. Abstract

RIF RuleML FOAF combines RDF-based FOAF with RIF RuleML, extending the factual FOAF vocabulary by RIF RuleML rules. Users can derive FOAF data by employing person-centric rules, either before (RDF) FOAF publication or, on demand, from published (RIF RuleML) FOAF pages.

2. Status

• Originally proposed by: (HaroldBoley). Later proposed to the First F2F WG meeting as Use Case NRC-2 http://lists.w3.org/Archives/Public/public-rif-wg/2005Dec/0100.html. Currently a collaboration of NRC with UNB as well as DERI and DFKI.

• Based on existing RuleML FOAF project.

• Based on perceived business and organizational need for expert finders, which are proposed to be FOAF-based.

• An implementation effort is underway http://www.ruleml.org/usecases/foaf.

3. Links to Related Use Cases

• Information Integration with Rules and Taxonomies: Information from various FOAF pages needs to be integrated by Harvester. XML serialization done uniformly in RuleML, especially for the hornlog sublanguage.

4. Relationship to OWL/RDF Compatibility

OWL Compatiblity can be achieved as in SWRL or, with Hybrid Rules, as in the Realistic Architecture. RDF Compatiblity is achieved via the Fact-oriented Normal Form (see below).

5. Examples of Rule Platforms Supporting this Use Case

• The rule engine OO jDREW http://www.jdrew.org/oojdrew has been employed to run RIF RuleML FOAF: Permits bottom-up as well as top-down execution, supporting both Normal Forms.

6. Benefits of Interchange

• Adapting FOAF rules from existing repositories saves FOAF users from re-creating those rules
• Shared rules avoid the need for multiple copies in a group
• From a few RIF RuleML FOAF rules, many RDF FOAF facts can be generated using an (XSLT-)translator for interchange
• Automatic rule crawling via harvesting service simplified by uniform RIF RuleML format

7. Requirements on the RIF

• Person-centric, local rules imply the requirement of a scoping construct also for positive queries. For more and more global rules, an increasing number of such scopes need to be merged, so there is the requirement of unlimited import of local rulebases into a new scope.
• Since the current FOAF is based on RDF, it is required that the FOAF subset of RDF be also expressible as facts of the rule language, that the rules are able to cope with such facts, and that the rule-derived facts can be (XSLT-)translated back to RDF.
• Because person-centric rules need closed negative conditions, which cannot be proved in the open web, (scoped) negation as failure is required.
• As information/knowledge can sometimes be better represented in a (shared) ontology and sometimes better be realised in (person-centric) rules, integrating ontologies and rules via hybrid rules is a main requirement.

8. Breakdown

8.1. Actors and their Goals

• Publisher - wants his/her FOAF data be seen by FOAF Readers and Subscribers
• Reader ('pull' service) - wants to browse the page of a FOAF Publisher
• Subscriber ('push' service) - wants to receive updates from a FOAF Publisher
• Harvester - wants to collect information from FOAF pages from multiple Publishers

8.2. Main Sequence

1. Publishers create FOAF data in either of two ways:
2. Rule-oriented Normal Form (RNF)
   • - The RNF includes rules as well as the (elementary) facts that are needed by the premises of the rules, omitting derivable facts - Advantage: the RNF is more compact
3. Fact-oriented Normal Form (FNF)
   • - The FNF includes elementary facts and derived facts, but omits the rules - Advantage: the FNF (XSLT-)corresponds to RDF FOAF facts
4. Readers come in corresponding variants:
5. RNF Readers apply a rule engine to generate relevant FOAF facts on demand
6. FNF Readers can retrieve raw facts or use any FOAF explorer on (XSLT-)translated RDF facts
7. Subscribers are served much like returning, incremental Readers
8. Harvesters periodically crawl Publishers' FOAF data according to specific collection criteria

8.3. Alternate Sequences

• Publishers can copy & edit FOAF rules from another (RNF) Publisher for his/her own purposes

• A group of Publishers can interchange their FOAF rules for sharing them among the whole group
• Publishers can translate RIF RuleML FOAF into RDF FOAF for interoperation
• Harvesters can target their crawls at FOAF rules for reuse by their owners

9. Narratives

Ben has created his person-centric metadata in the form of a FOAF document as has become popular, covering unconditional properties about his personal interests, friends, etc. However, he also needs to specify properties conditional on other persons, the time, the location, etc. For example, Ben prefers to specify his work time and free time s.t. his colleagues can contact him when at work, while his friends can reach him when off work. Consequently, with rules, his schedule might look as follows:

Monday - Friday

• IF 09:00 AM - 12:00 AM, THEN call my office number IF 13:00 AM - 17:00 AM, THEN call my office number IF 18:00 AM - 21:00 AM, THEN call my home number IF 12:00 AM - 13:00 AM, THEN call my cell number IF 17:00 AM - 18:00 AM, THEN call my cell number IF 21:00 AM - 09:00 AM, THEN do not call me

Saturday, Sunday, Holidays

• IF 09:00 AM - 21:00 AM, THEN call my cell number IF 21:00 AM - 09:00 AM, THEN do not call me

10. Commentary

Given the importance of FOAF and RDF for the Semantic Web, this should become a central Use Case for a Semantic Web RIF. It will also permit the fact and rule based description of our own community of experts.