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Re: [Graphs] Proposal for Named Graph Semantics

From: Richard Cyganiak <richard@cyganiak.de>
Date: Thu, 7 Apr 2011 20:33:32 +0100
Cc: RDF WG <public-rdf-wg@w3.org>
Message-Id: <198BFE36-EB72-4AA3-A4A0-7120CBABC80B@cyganiak.de>
To: Alex Hall <alexhall@revelytix.com>
Hi Alex,

Thanks, that's quite useful. Before commenting on it, I'd like to ask though what the motivation is.

On 7 Apr 2011, at 09:47, Alex Hall wrote:
> Here is a proposal of a semantics for named graphs in RDF.  My goals here are to:
> - Extend http://www.w3.org/2011/rdf-wg/wiki/TF-Graphs/RDF-Datasets-Proposal to go beyond (IRI, graph) tuples.

Why do you want to go beyond (IRI, graph) tuples? What's the advantage?

> - Give something that is formally defined enough to serve as a starting point for discussion.

Do you find that the RDF-Datasets-Proposal isn't sufficiently formal to serve as a starting point?

> - Specify common semantics for multi-graph serialization formats, or at least a starting point.

Do you find that the RDF-Datasets-Proposal isn't sufficient for that?

> - Specify something that is flexible enough to satisfy applications that want to treat named graphs as either g-snaps or g-boxes.

In the simplest form of the RDF-Datasets-Proposal, one could either leave that open, or see (IRI, g-snap) pairs as snapshots of an assumed underlying named g-box that isn't explicitly modeled. So I'd say that the RDF-Datasets-Proposal is flexible enough to satisfy applications that want to tread named graphs as either g-snaps or g-boxes.

So again, I don't understand the advantage that this proposal offers over the simpler (IRI, graph) pair proposal.

Can you speak to that?


> Regarding g-boxes, I specifically want to avoid incorporating anything that suggests time variance into the semantics, because specifying semantics for temporal changes is explicitly out of scope for the existing RDF Semantics document.
> Here goes...
> 1. Graph Identification
> Let I be an IRI.  Define Graph(I) as a unary predicate such that Graph(I) implies that the resource identified by I is an RDF graph.  If desired, this can be described easily enough in RDF by defining a new class rdfs:Graph and mapping Graph(I) to the triple I rdf:type rdfs:Graph.
> Define G(I) as a function that returns the RDF graph identified by I.  In our parlance, G(I) is a g-snap, invariant over time.  Due to the nature of RDF, it is difficult to express the relationship between I and G(I) natively in RDF.  Graph literals, which I understand to be the encoding of some set of triples as a single node in a graph, are one possible approach but this proposal does not attempt to define graph literals.  Furthermore, in the open world it's not possible to have complete knowledge of all the triples in G(I) for any given I.
> 2. Graph Assertion
> Let I be an IRI and G be an RDF graph.  Define GA(I, G) as a binary predicate such that GA(I, G) implies (a) Graph(I) and (b) G(I) entails G.
> The notion of graph assertion attempts to capture the semantics of what happens when some set of triples is associated with a graph IRI in a multi-graph serialization such as TriG.  So the TriG fragment:
> :G1 { :a :b :c } .
> would be understood to construct a graph G with a single triple :a :b :c and then make the assertion GA(:G1 G).
> The use of "entails" as opposed to "equals" here is what gives us our flexibility.  Applications that want to treat named graphs as g-snaps, completely described by the triples associated with the graph IRI, can do so by extending (b) to say G(I) equals G instead of entails.  Because every graph entails itself, this extension is supported by these semantics, but this would not be required behavior.  Indeed, this could lead to trouble in the open world where you can have GA(I, G1) and GA(I, G2) with G1 != G2.
> Applications that want to treat named graphs as g-boxes would to so by essentially maintaining a (time-sensitive) mapping of IRI I to graph G.  This aligns pretty closely with my understanding of the notion of graph store from SPARQL 1.1 Update.  Poking the g-box to obtain content (either a g-text serialization or query results) amounts to asserting GA(I, G) for the current value of G at some point in time.  Given a new graph assertion for an IRI that is already mapped in the store, an implementation could replace the currently mapped graph with the new one (effectively discarding all prior graph assertions) or merge them at its discretion; either approach would be supported by these semantics.
> Any vocabulary for specifying graph literals and attaching them to a graph IRI in RDF would be defined as making a graph assertion, not setting the value of the identified graph.
> 3. RDF Datasets
> I haven't thought this part through entirely, but I think these semantics could be aligned with the existing notion of RDF datasets from SPARQL (and as proposed on the wiki) by simply mapping the (IRI, graph) tuples in the dataset to the appropriate graph assertions.
> 4. Graph Equality
> Because it is not the case that (G1 entails G and G2 entails G) implies G1 = G2, it is also not the case that (GA(I1, G) and GA(I2, G)) implies I1 and I2 are the same graph.  Such a conclusion could be reached if you extend the definition of GA to mean equals instead of entails as discussed before, but again that is an extension and not part of the proposed semantics.
> 5. Empty Graphs
> Because every graph trivially entails the empty graph E, the assertion GA(I, E) is trivially true for every graph IRI I.  Making that assertion doesn't do anything beyond identify the resource denoted by I as a graph.
> 6. Graph Merges
> It follows from the definition of GA (and the definition of entails) that (GA(I, G1) and GA(I, G2)) implies GA(I, Merge(G1, G2)).  I think this gives us a pretty straightforward approach to merging of RDF datasets if this is required of the spec.
> Hope you find this useful...  or at least that this stirs up some interesting debate.
> Regards,
> Alex
Received on Thursday, 7 April 2011 19:34:02 UTC

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