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

From: Pierre-Antoine Champin <pierre-antoine.champin@liris.cnrs.fr>
Date: Thu, 05 Jan 2012 17:36:03 +0100
Message-ID: <4F05D173.8050506@liris.cnrs.fr>
To: Guus Schreiber <guus.schreiber@vu.nl>
CC: Sandro Hawke <sandro@w3.org>, public-rdf-wg <public-rdf-wg@w3.org>
On 01/05/2012 03:13 PM, Guus Schreiber wrote:
> On 04-01-2012 19:45, Sandro Hawke 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.
> 
> Sandro,
> 
> Very clear, thanks for this (and I indeed see no more need for my action).
> 
> I understand why you like third solution. However, it means we have to 
> come up with a name-to-graph relation vocabulary.

Why so? We can suggest some useful predicates, as Sandro did in his
example, but in the end, people will use whichever predicate they want.

> I don't think we're in 
> a a position to standardize that. Also, it means we have to introduce 
> new syntax and thus invalidate/deprecate/mark as archaic/...  the 
> current quad stores out there.

Not if we state how they relate to the new syntax (as syntactic sugar,
as suggested by Sandro).

> That would not be in the spirit of our 
> charter.

> 
> The first solution (your TriG/REST) has the advantage of being the most 
> conservative extension, i.e. providing a hook for explicating 
> name-to-graph semantics without enforcing it.

I sympathize with Andy's concern:
> 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.

and Trig/REST does fix a semantics.

I think the only way out would be to define a very loose predicate
(rdf:hasRelatedGraph ?). This makes it impossible to infer much from any
Trig file, but at least it does not break any of them.

  pa

>  We can write a 
> non-normative section/appendix/note with suggested practices for 
> vocabulary to be used there.
> 
> If we can get consensus on (some variant of) the first solution, I see 
> us moving on quickly. The path forward would be to continue writing down 
> further use-case examples.
> I have asked Antoine Isaac to do this for the Europeana data model [1].
> 
> Guus
> 
> [1] 
> http://www.europeana-libraries.eu/web/europeana-project/technicaldocuments/
> 
>>
>>
>> * 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 Thursday, 5 January 2012 16:39:13 GMT

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