- From: Ralph R. Swick <swick@w3.org>
- Date: Wed, 05 Jan 2000 11:16:29 -0500
- To: w3c-rdf-schema-wg@w3.org, www-rdf-interest@w3.org
What follows is partially a question of terminology, partially a question of the RDF Schema Working Group intent, and partially a query about what the community thinks is most useful in regard to the RDF Schema specification. On the subject of constraint violation and "inconsistent models", http://www.w3.org/TR/1999/PR-rdf-schema-19990303/#constraints [Section 3, "Constraints"] defines the term "inconsistent model" to describe an RDF model that violates a constraint as specified by the RDF Schema properties. The spec explicitly leaves it open to implementations to decide how to behave in the event of a constraint violation. But the spec doesn't specify whether a missing triple is a constraint violation. After researching the WG mail archives, I am still not sure whether this omission is intentional or unintentional. Early in the WG discussion Andrew Layman and Guha observed that all the constraints can be interepreted in one of two ways: http://lists.w3.org/Archives/Member/w3c-rdf-schema-wg/1998JulSep/0083 (sorry, some of the links will be to W3C Member-only mail archives. I can't retroactively change the access permissions, though I encourage the authors of the cited messages to repost them to the IG if they wish. My summaries that follow should be adequate to understand the messages, however) 1. as rules that permit inferencing; e.g. to infer a type for the subject of a statement from a domain property of the predicate of that statement or 2. as integrity constraints; e.g. a graph that does not contain a type property where a domain constraint would require one would be regarded as being in error in some manner. This became #26 on the working group's issues list. http://www.w3.org/RDF/Group/Schema/openissues.html#c26 At the time the working group considered this question there were many more constraint properties contemplated (minimum and maximum number of occurences, value constraints, etc.). Subsequently the working group decided to limit the constraint properties in the first version of RDF Schema to the barest minimum. Issue 26 was decided by adding the definition of "inconsistent model" that is cited above. http://lists.w3.org/Archives/Member/w3c-rdf-schema-wg/1998JulSep/0130 Andrew noted that there may be interoperability issues for applications that attempt inferencing and suggested that the WG might need to further discuss this question later. The working group chose not to directly tackle the question of inferencing. Some of us had the expectation that a subsequent working group would be established to work on an "RDF Logic" layer and therefore such matters should be deferred to that working group. Thus, as Sean Luke points out in http://lists.w3.org/Archives/Public/www-rdf-interest/1999Dec/0117 RDF Schema is (intentionally) weak in specifying inferencing. As described in http://www-diglib.stanford.edu/diglib/ginf/rdf-schema-in-ginf.html (cited by Dan Brickley in http://lists.w3.org/Archives/Member/w3c-rdf-schema-wg/1999JulSep/0007 ) GINF has implemented validation for RDF Schema, checking that subjects have types according to domain constraints. However it also notes "[it] would be helpful for developers to have a standard set of inference rules for RDF schema implementation." Further comments from the GINF folk would be welcomed. Recently in a local discussion, the question was raised as to whether the language of http://www.w3.org/TR/1999/PR-rdf-schema-19990303 precludes an application from performing such inferencing. I believe that it does not. The term "inconsistent model" may be confusing; it may bring in too many other connotations that are not in the specification. In the case of violation of a constraint such as [2.3.3] "A property can never be declared to be a sub-property of itself, nor of any of its own sub-properties" this is a reasonable term, but in the case of a missing triple it may interfere with peoples' understanding. Did the Working Group intend to say that a model that does not contain a triple where one should be expected is also an "inconsistent model"? If so, is it appropriate to add words to the specification to note that in the case of a constraint violation such as this an implementation might choose to infer the triple? I think this would help clarify the intent of "inconsistent model" and help set the foundation for an "RDF Logic" layer to be specified. Comments welcome, especially as Dan Brickley is putting the final editorial changes into a new RDF Schema Proposed Recommendation. RDF Schema WG members are encouraged to include mailto:www-rdf-interest@w3.org in their replies, noting this is a public forum. -Ralph
Received on Wednesday, 5 January 2000 11:17:33 UTC