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Re: Text for clarification of re-use of IRIs in dataset clauses

From: Chimezie Ogbuji <ogbujic@ccf.org>
Date: Mon, 15 Oct 2007 12:36:35 -0400
To: andy.seaborne@hp.com
cc: public-rdf-dawg@w3.org
Message-ID: <1192466195.31761.31.camel@otherland>

Andy, my comments are inline below.

On Mon, 2007-10-15 at 15:32 +0100, Seaborne, Andy wrote:
> Ogbuji, Chimezie wrote:
> > [[
> > The FROM NAMED syntax suggests that the IRI identifies the corresponding 
> > graph, but the relationship between an IRI and a graph in an RDF dataset 
> > is indirect. The IRI identifies a resource, and the resource is 
> > represented by a graph (or, more precisely: by a document that 
> > serializes a graph). The relationship between the IRI and the 
> > representation is subject to time, an intermediate caching policy, the 
> > query service, and the mechanics of the underlying transport protocol.  
> > For further details see [WEBARCH]. 
> > 
> > The distinction between a surface RDF notation and the abstract RDF 
> > graph which results from parsing an instance of the surface notation is 
> > an additional indirection.  As a consequence of these things, the 
> > repeated use of an IRI in either the same dataset clause, across dataset 
> > clauses, or across whole SPARQL queries can feasibly result in either 
> > the formulation of a single canonical graph, separate but isometric 
> > graphs, or completely disjoint [2] RDF graphs for each use of the same IRI.
> > ]]
> Chimezie,
> 
> This text introduces some new terminology - I only found one reference on the 
> web to "surface notation" in the context of RDF (a note by Pat).

Yes, that was the only source for that term.  How about instead:

s/surface notation/RDF graph serialization

> Isn't the indirection due to the mention of the IRI twice and the use of a 
> graph in the dataset.

I'm not sure what you mean by the "use" of a graph in the dataset, could
you clarify the second part of that sentence? In any case, the
indirection is three-fold (it spans web architecture and the
concrete/abstract RDF syntax divide).  i.e.:

IRI -> RDF "information resource" -> RDF graph representation -> RDF
abstract graph

I was trying to be explicit about the nature of this indirection so as
to cover all cases where this is relevant not just the situation that
motivated the clarification (i.e., the dataset tests and the assumption
about distinct BNodes across graphs formed from the same IRI)

> How about: for 8.2.3:
> [[
> The actions required to construct the dataset are not determined by the 
> dataset description.  If an IRI is given twice in an dataset description, 
> either by using two FROM clauses, or a FROM clause and a FROM NAMED clause, 
> then it does not assume that exactly one or exactly two attempts are made to 
> obtain an RDF graph associated with the IRI.  Therefore, no assumptions can be 
> made about blank node identity in triples obtained from the two occurrences in 
> the dataset description.
> ]]
> 	Andy

I was hoping that we could be a little more specific than that - without
risking the introduction of concepts that are not already covered by our
normative dependencies.  The interplay between web architecture and the
formulation of the dataset (in this case) is the crucial bit.  In
addition, I didn't call out the blank node identity scenario because I
got the impression that you were concerned about covering the general
case.  I'm not sure how to reconcile the larger picture with your
suggested text above, but below is an attempt:

[[
The actions required to construct the dataset are not determined by the
dataset description alone.  If an IRI is given twice in an dataset
description, either by using two FROM clauses, or a FROM clause and a
FROM NAMED clause, then it does not assume that exactly one or exactly
two attempts are made to obtain an RDF graph associated with the IRI.
Therefore, no assumptions can be made about blank node identity in
triples obtained from the two occurrences in the dataset description.
In general, no assumptions can be made about the isomorphism of the
formulated graph.
]]

-- 
Chimezie Ogbuji
Lead Systems Analyst
Thoracic and Cardiovascular Surgery
Cleveland Clinic Foundation
9500 Euclid Avenue/ W26
Cleveland, Ohio 44195
Office: (216)444-8593
ogbujic@ccf.org


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Received on Monday, 15 October 2007 16:36:57 GMT

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