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Re: Three solution designs to the first three Graphs use cases

From: Sandro Hawke <sandro@w3.org>
Date: Thu, 26 Jan 2012 22:45:56 -0500
To: Andy Seaborne <andy.seaborne@epimorphics.com>
Cc: public-rdf-wg@w3.org
Message-ID: <1327635956.2698.75.camel@waldron>
On Thu, 2012-01-05 at 11:09 +0000, Andy Seaborne wrote:
> On 04/01/12 19:23, David Wood wrote:
> > Thanks, Sandro.  That's very helpful.
> >
> > It might be useful to consider augmenting TriG syntax to support your third solution (explicitly naming relations). I'd be quite happy with that.
> 
> What would the data model be?

I think: an RDF graph which can have other RDF graphs as values of its
triples.  All these graphs would be subgraphs of some greater graph, so
they can share b-nodes.

(This is what cwm has had implemented since 2001, I think.)

> > We could also consider standardizing the existing TriG syntax to be a syntactic shorthand for TriG REST semantics; that is, a lack of explicitly declared relation infers log:semantics.
> 
> I think we should not fix a semantics for undeclared relationships.
> 
> Otherwise, it invalidates existing TriG documents which don't exactly 
> follow the TriG/ABC definition.
> 
> Ditto N-Quads - in a quadstore/database dump or extract you don't 
> necessary know the semantics.

Agreed.   If we settle on a syntax that's compatible with TriG, I think
we probable need the TriG subset to have the current TriG semantics --
roughly none.

> - - - - - - - - - -
> 
> I find the name TriG/REST confusing because, for me, identifying the 
> dereference action is modelling REST which is the other
> 
> It's more like "TriG/WebCache" -- only one instance of the graph 
> containers state is possible.

I don't follow your logic.   My thinking in picking the name "TriG/REST"
is that the implied relation is the relation that's kind of at the core
of REST, the relationship between a thing and its 'state'.

    -- Sandro

> 	Andy
> 
> >
> > Regards,
> > Dave
> >
> >
> > On Jan 4, 2012, at 1:45 PM, Sandro Hawke<sandro@w3.org>  wrote:
> >
> >> While it's fresh in my mind, let me write down the view I came to during
> >> today's telecon.   (And, carry it a bit farther.)  Guus, I don't know if
> >> you still want to write up your understanding of it, or if this obviates
> >> your action.
> >>
> >>
> >> * Use Case 1:   (presented by cygri at 21 Dec meeting)
> >>
> >> Several systems want to use the data gathered by one RDF crawler.  They
> >> don't need simultaneous access to older versions of the data.
> >>
> >> Solution A: use TriG or N-Quads with the fourth column (graph label)
> >> being the URL the content was fetched from.
> >>
> >>         <http://example.org>  { ... triples recently fetched from there }
> >>
> >> * Use Case 2:   (brought up in questions by sandro at 21 Dec meeting)
> >>
> >> Several systems want to use the data gathered by one RDF crawler.  They
> >> need simultaneous access to older versions of the data.
> >>
> >> Solution B: use TriG or N-Quads with the fourth column being some
> >> identifier created at the time the retrieval was done.  Then, some other
> >> data connects that identifier with the URL the content was fetched from.
> >>
> >>         <http://crawler.example.org/r8571>  { ... triples fetched in retrieval 8671 }
> >>         {
> >>            <http://crawler.example.org/r8571>  eg:source<http://example.org>;
> >>                                               eg:date "2011-01-04T00:03:11"^^xs:dateTime
> >>         }
> >>
> >> * Use Case 3:   (suggested by sandro at 4 Jan meeting)
> >>
> >> A system wants to convey to another system in RDF that some person
> >> agrees with or disagrees with certain RDF triples.
> >>
> >> Solution C: use TriG or N-Quads with the fourth column being an
> >> identifier for an RDF Graph (g-snap), so that it can be referred to in
> >> the default graph.
> >>
> >>         { eg:sandro eg:endorses<g1>  }
> >>         <g1>  { ... the triples I'm endorsing ... }
> >>
> >> ====
> >>
> >> So, here we have two different semantics for TriG clearly motivated and
> >> expressed.  The TriG document:
> >>
> >>         g { s p o }
> >>
> >> is understood in Solution A to mean (in N3):
> >>
> >>         g log:semantics { s p o }.       # TriG "REST" semantics
> >>
> >> but in Solution C it means (in N3):
> >>
> >>         g owl:sameAs { s p o }.          # TriG "Equality" semantics
> >>
> >> ====
> >>
> >> It looks like it's possible to solve all three uses cases with either
> >> semantics, although it gets a bit tricky.
> >>
> >> With TriG/REST:
> >>
> >>         UC1 -- as reported by Richard; the URL used by the crawler is
> >>         the fourth column URL
> >>
> >>         UC2 -- as implemented in Sandro's semwalker code; the crawler
> >>         makes a new URL in its own web space, mirrors the content there,
> >>         and puts that URL in the fourth column
> >>
> >>         UC3 -- rather than endorsing an RDF Graph, I endorse a Graph
> >>         Container on the condition that it never changes (or something
> >>         like that -- needs to be fleshed out more).
> >>
> >>                 { eg:sandro eg:endorses<g1>.
> >>                   <g1>  a rdf:StaticGraphContainer.
> >>                 }
> >>             <g1>  { ... the triples I'm endorsing ... }
> >>
> >>
> >> With TriG/Equality:
> >>
> >>         UC1 -- A layer of indirection is needed, as new URIs need to be
> >>         created for the different RDF Graphs.
> >>
> >>                 {<http://example.org>  rdf:graphState<uuid:nnnnn>  }
> >>                 uuid:nnnnn { ... triples fetched from example.org }
> >>
> >>         Maybe there's some clever way to do it without this, involving
> >>         URL mangling or something to eliminate the second lookup.
> >>
> >>         I used uuid:nnnnn as a URI for the RDF Graph, but I could just
> >>         as easily have used a hash of the graph or graph serialization.
> >>         I *could* use an http URL, I think, but that's likely to lead to
> >>         confusion and breakage, especially when someone gets the bright
> >>         idea of changing what triples are served at that address.  (I'm
> >>         sure it will have seemed like a good idea at the time.)
> >>
> >>         UC2 -- Pretty straightforward, since we already have that layer of
> >>         indirection in UC1.  We can't quite use the r8571 example as is, because graph
> >>         equality could smoosh the two retrieval operations together.  So we
> >>         need something like this:
> >>
> >>                 <uuid:nnnnn>  { ... triples fetched in operation 8671 }
> >>             {
> >>               [ a eg:Retrieval;
> >>                 eg:gotGraph<uuid:nnnnn>;
> >>                 eg:source<http://example.org>;
> >>                 eg:date "2011-01-04T00:03:11"^^xs:dateTime;
> >>               ]
> >>             }
> >>
> >>         UC3 -- easy:
> >>
> >>                 { eg:sandro eg:endorses<uuid:nnnnn>. }
> >>                 <uuid:nnnnn>  { ... the triples I'm endorsing ... }
> >>
> >>                 Here you can see why I want a blank node as the graph
> >>                 label, rather than making up uuids.
> >>
> >> Between these two, I have a preference for TriG/REST over
> >> TriG/Equality, I think.   I think people are too likely to get the
> >> semantics of TriG/Equality wrong in practice.  Of course, spelling out
> >> the semantics of TriG/REST will be a tricky given it has a some
> >> contextual qualities, as we've discussed.
> >>
> >> MEANWHILE, we have a third solution, where we name the relation
> >> explicitly.   This is the one I prefer.
> >>
> >>         UC1
> >>
> >>                 <http://example.org>  rdf:graphState { ... triples recently fetched from there }
> >>
> >>         UC2 -- either of the styles given above, depending whether the
> >>         harvester wants to publish its copies on the web or not.
> >>
> >>         UC3
> >>
> >>                 eg:sandro eg:endorses<uuid:nnnnn>.
> >>         <uuid:nnnnn>  owl:sameAs { ... the triples I'm endorsing ... }
> >>
> >>                 or, logically:
> >>
> >>                 eg:sandro eg:endorses { ... the triples I'm endorsing ... }
> >>
> >>                 (Then, I would probably get rid of the curly braces around the default
> >>                 graph, so it becomes Turtle with Nesting.)
> >>
> >> Do those three solution designs make sense?   Any strong preferences
> >> among them?  Are there more use cases that people think the group will
> >> find compelling and which cannot be solved by all three of these
> >> solutions?  (I think the next use case I'd approach would be "Tracing
> >> Inference Results", mostly because it motivates shared blank nodes.
> >> But I'm out of time for today.)
> >>
> >>     -- Sandro
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >
> 
> 
Received on Friday, 27 January 2012 03:46:11 GMT

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