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Re: Draft for a "minimal dataset semantics"

From: Richard Cyganiak <richard@cyganiak.de>
Date: Thu, 13 Sep 2012 21:45:27 +0100
Cc: Antoine Zimmermann <antoine.zimmermann@emse.fr>, RDF WG <public-rdf-wg@w3.org>
Message-Id: <4C276764-520A-4178-8E4F-34C5BE986197@cyganiak.de>
To: Peter F. Patel-Schneider <pfpschneider@gmail.com>
Hi Peter,

On 13 Sep 2012, at 19:13, Peter F. Patel-Schneider wrote:
> Why should an inconsistent default graph make the named graphs irrelevant?  Why should the default graph situation bleed into the named graphs at all?

The most frequent usage scenarios for the default graph are:

1. Store metadata about the named graphs
2. Present a union of all named graphs

In case 1, the default graph is privileged and the application relies on its contents actually being accurate. Applications decide which graphs to use or not to use based on metadata in the default graph.

Case 2 is only useful if the named graphs come from “compatible contexts”, and obviously the default graph is not independent from the named graphs in that case.

A case that might become more important in the future is a variation of 3 -- where we store configuration, vocabulary mappings and the like in the default graph.

To me, it makes sense that the interpretation of the default graph can affect the named graphs.

Admittedly, whether that's required for a minimal useful semantics is a different question.

>> I think you mean the former (if it's the latter, I don't see why). Do you think that, if the graphs--named and default--were independent, it would be acceptable?  That's the alternative proposal where IGEXT maps IRIs to graphs, instead of resources to graphs.

The IRI-IGEXT design is not my preferred one but I can live with it.

> PS:  Here is a reiteration of the old proposals.
> 
> There is no independent notion of interpretations of RDF datasets.    If you want to do something like entailment between RDF datasets you can either look at one component of the dataset, so you ask whether the default graphs sit in an entailment relationship or ask whether the graphs with a particular name sit in an entailment relationship, or you can look at the entire dataset, so you ask whether the default graphs sit in an entailment relationship and all the similarly-named graphs sit in an entailment relationship.  In each case you probably want to consider a missing named graph to be the empty graph.  This ends up being more flexible and considerably simpler than the minimal semantics currently being proposed, as well as requiring no new reasoning machinery.

The downside is that this design provides no foundation for these kinds of extensions:
http://www.w3.org/2011/rdf-wg/wiki/TF-Graphs/Minimal-dataset-semantics#Possible_Semantic_Extensions

Best,
Richard
Received on Thursday, 13 September 2012 20:45:52 GMT

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