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

From: Andy Seaborne <andy.seaborne@epimorphics.com>
Date: Thu, 05 Jan 2012 11:09:49 +0000
Message-ID: <4F0584FD.1090003@epimorphics.com>
To: public-rdf-wg@w3.org
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?

> 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.

- - - - - - - - - -

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.

	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 Thursday, 5 January 2012 11:12:50 GMT

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