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Re: New Proposal (6.1) for GRAPHS

From: Arnaud Le Hors <lehors@us.ibm.com>
Date: Thu, 5 Apr 2012 15:24:26 -0700
To: Sandro Hawke <sandro@w3.org>
Cc: public-rdf-wg <public-rdf-wg@w3.org>
Message-ID: <OF2678641B.AE22F3FB-ON882579D7.00783C89-882579D7.007B1768@us.ibm.com>
Sandro Hawke <sandro@w3.org> wrote on 04/05/2012 04:31:48 AM:

> From: Sandro Hawke <sandro@w3.org>
> To: Arnaud Le Hors/Cupertino/IBM@IBMUS, 
> Cc: public-rdf-wg <public-rdf-wg@w3.org>
> Date: 04/05/2012 04:33 AM
> Subject: Re: New Proposal (6.1) for GRAPHS
> 
> On Wed, 2012-04-04 at 15:05 -0700, Arnaud Le Hors wrote:
> > Hearing people arguing over whether the statement  <u> {<a> <b> <c>}
> > defines a complete graph or not leads me to wonder whether we
> > shouldn't recognize that the answer ought to be: "it depends". :-) 
> > 
> > I think Lee explained very effectively how the same statement can be
> > interpreted differently depending on whether you're doing a GET, PUT,
> > POST or something similar. 
> > 
> > I already noted that the response Sandro expects from his question
> > "According to this query, how many triples are in the graph known to
> > that endpoint as ' ?" is actually based on additional information he
> > is providing in his question, specifically: the query is limited to
> > the graph known to that particular endpoint. 
> > 
> > If all you had were the following triples: 
> > >>>      <a>  <b>  1.
> > >>>      <a>  <b>  2.
> > >>>      <a>  <b>  3.
> > without giving any other information about how or where you got them
> > from and you'd ask: "how many triples are associated with <a>?"  I
> > think the answer would have to be: "it depends". 
> 
> FWIW, I really think you do need to keep the notion of a graph in the
> question, since SPARQL has the keyword "GRAPH".   As in, "How many
> triples are in the graph associated with <http://g1.example.org>.
> 
> But since we're not really doing this survey anyway, it probably doesn't
> matter.

But I don't think keeping the notion of graph in the question is what 
really makes the difference. What matters is the scope which is defined by 
"known to that endpoint".

> 
> > I heard Sandro say that when he dereferences <u> he expects to get all
> > the triples in <u>. I agree but I think that's a Linked Data view of
> > the world and it comes from the meaning of GET rather than what you
> > receive. In another context, retrieved in a different way, what you
> > receive might mean something else. In the case of a SPARQL query it
> > could mean "these are all the triples in <u> that this endpoint knows
> > about". 
> > 
> > So, do we really need to choose one way or the other? Can't we just
> > leave it to the application to decide whether it defines a complete
> > graph or not?
> 
> I see three ways to do this.   Which are you suggesting?
> 
>   1.  have two syntaxes.   eg trig means complete graphs and n-quads 
>       means partial graphs.
> 
>   2.  have two constructs in trig, eg: 
>         <u1> { <a> <b> <c> }         to mean full graphs and
>         <u2> < <a> <b> <c> ... }     to means partial graphs
> 
>   3.  use the same syntax, but let the consumer decide which was meant.
> 
> I don't like 3 at all, because it doesn't solve the use cases.   For
> isntance, you couldn't have a shared crawler, if the apps using the
> crawl happened to need complete graphs.

Maybe (most likely ;-) I'm missing something but, in the web crawler use 
case for instance, isn't it the fact that you're getting it via a GET of a 
URL that returns all the data the crawler accumulated that would tell you 
the graph you're getting is complete? Why does that need to be embedded in 
the syntax/data set?

Is it because you don't want to rely on out of band information?

If you know that http://example.com/all returns all data that has been 
accumulated, you don't need the dataset to tell you that, do you?

Just like Lee doesn't rely on the dataset to decide whether to replace or 
merge a graph in anzo. In his case, the command line provides the out of 
band information that drives the interpretation one way or the other.

> 
> The second one is cute, but I think would be very hard to implement; it
> would force every consumer to deal with both complete and incomplete
> graphs, at least a bit.
> 
> Both 1 and 2 raise the issue of how you reflect this difference in the
> dataset, or in SPARQL.   How could you do that?
> 
>     -- Sandro

I would have said:
4. use the same syntax, let the context decide which it is

Regards.
--
Arnaud  Le Hors - Software Standards Architect - IBM Software Group


> 
> 
> > --
> > Arnaud  Le Hors - Software Standards Architect - IBM Software Group
> > 
> > 
> > 
> > 
> > From:        Lee Feigenbaum <lee@thefigtrees.net> 
> > To:        Ivan Herman <ivan@w3.org>, 
> > Cc:        Arnaud Le Hors/Cupertino/IBM@IBMUS, Sandro Hawke
> > <sandro@w3.org>, public-rdf-wg <public-rdf-wg@w3.org> 
> > Date:        04/04/2012 05:16 AM 
> > Subject:        Re: New Proposal (6.1) for GRAPHS 
> > Sent by:        Lee Feigenbaum <figtree@gmail.com> 
> > 
> > ______________________________________________________________________
> > 
> > 
> > 
> > How do people use TriG in practice today? For us, the choice between 
> > these semantics today is determined externally to a TriG file. That
> > is, 
> > given foo.trig which contains u1 { a b c }, whether this is all of u1
> > or 
> > a subgraph of u1 is determined based on which API or which
> > command-line 
> > command is used. For example:
> > 
> > > anzo import foo.trig
> > 
> > ...interprets what's in the the trig file as subgraphs that get added
> > to 
> > any existing contents of the graphs. So "a b c" would be added to u1.
> > 
> > > anzo replace foo.trig
> > 
> > ...interprets what's in the trig file as complete graphs, and sets
> > the 
> > contents of the graphs in the repository, overwriting whatever might 
> > already be in the repository as the contents of the graphs. So after 
> > this operation u1 ends up with exactly { a b c } as its contents.
> > 
> > (Aside: So for us, "import" is basically like doing a POST of the 
> > triples in the trig file to the associated graphs via the SPARQL
> > Graph 
> > Store Protocol, and "replace" is like doing a PUT.)
> > 
> > (Aside 2: there are other operations as well, such as "anzo update 
> > --remove" which uses the subgraph semantics and also means that the 
> > triples in question should be removed from the associated graphs in
> > the 
> > repository.)
> > 
> > All of which is to say, there are plenty of use cases in our
> > experience 
> > for both of these semantics. If the standard supported a way to make 
> > these semantics explicit, we would probably support that via some
> > sort 
> > of generic command ("anzo process"? who knows), but would still let 
> > these existing command line commands override the semantics. We have 
> > plenty of cases in which we export some bit of trig, and then later
> > on 
> > either use "anzo import" or "anzo replace" based on the situation --
> > and 
> > we wouldn't want to have to produce two different trig files for that 
> > situation! (This would be roughly analogous to the way in which the 
> > SPARQL Protocol lets the RDF dataset definition override what's in
> > the 
> > query, so that queries can easily be reused in different contexts.)
> > 
> > Lee
> > 
> > On 4/4/2012 3:18 AM, Ivan Herman wrote:
> > >
> > > On Apr 4, 2012, at 07:09 , Arnaud Le Hors wrote:
> > >
> > >> Hi Sandro,
> > >> I have to say that my expectation was similar to Charles's. I guess
> > it's a matter of deciding whether<u1>  {<a>  <b>  <c>   } defines
> > the<u1>  graph in its entirety, as containing one triple, or merely
> > states that the triple<a>  <b>  <c>   is part of graph<u1>.
> > >>
> > >> I'm not saying it should be the latter rather than the former, just
> > that it's not obvious.
> > >> See below for more on that.
> > >
> > > So let me give my typical W3C answer, ie, trying to find a
> > compromise:-)
> > >
> > > More seriously. The structure offered by Sandro relies on the fact
> > that the
> > >
> > > <u>  {<a>  <b>  <c>  }
> > >
> > > syntax gets its more precise meaning through a possible
> > >
> > > <u>  rdf:type rdf:SOMECLASSHERE .
> > >
> > > Sandro offered two such classes; isn't possible to have three, one
> > that makes the graph THE graph, the other that makes it PART OF the
> > graph?
> > >
> > > We can of course have long discussions on which the default is. But
> > that is a lighter discussion I believe.
> > >
> > > Ivan
> > >
> > >
> > >
> > >>
> > >> Sandro Hawke<sandro@w3.org>  wrote on 04/02/2012 05:57:13 PM:
> > >>
> > >>> From: Sandro Hawke<sandro@w3.org>
> > >>> To: Charles Greer<cgreer@marklogic.com>,
> > >>> Cc: Charles Greer<Charles.Greer@marklogic.com>, public-rdf-wg
> > >>> <public-rdf-wg@w3.org>
> > >>> Date: 04/02/2012 05:57 PM
> > >>> Subject: Re: New Proposal (6.1) for GRAPHS
> > >>>
> > >>> On Mon, 2012-04-02 at 14:00 -0700, Charles Greer wrote:
> > >>>> Thanks for responding Sandro.  I think that what I'm finding
> > difficult,
> > >>>> or at least a significant departure from RDF as I have understood
> > it in
> > >>>> the past, is that this TRIG document
> > >>>>
> > >>>> <u1>  {<a>  <b>  <c>  .<d>  <e>  <f>  }
> > >>>>
> > >>>> is not equivalent to these n-quads:
> > >>>>
> > >>>> <a>  <b>  <c>  <u1>.
> > >>>> <d>  <e>  <f>  <u1>.
> > >>>>
> > >>>> Or rather, you now need a document structure around n-quads as
> > well in
> > >>>> order to provide the context in which rdf knows that these
> > triples, and
> > >>>> only these triples, constitute the graph<u1>.
> > >>>>
> > >>>> I had previously thought that RDF was a data model that didn't
> > need any
> > >>>> notion of 'document' to work.  I'm not sure how another assertion
> > that
> > >>>>
> > >>>> {<u1>  a rdf:Graph }
> > >>>>
> > >>>> can assert the boundaries of<u1>  unless either the { } syntax
> > does more
> > >>>> than it appears to, or the document is a harder scope boundary
> > than I
> > >>>> would have expected.  If the document has some relationship to
> > scope, I
> > >>>> think that should be made explicit.
> > >>>
> > >>> Two main points:
> > >>>
> > >>> 1.  That rdf:Graph declaration is different thing.  It changes
> > how<u1>
> > >>> relates to the graph, but in a semantic (not syntactic) way.  It
> > can be
> > >>> in a different document, or deduced by the use of some predicates,
> > or
> > >>> known a priori by a data consumer.  Knowing it entitles the
> > consumer to
> > >>> see that<u1>  actually identifies the graph directly, rather than
> > just
> > >>> being a label for the graph.     This might matter if we also
> > know<u1>
> > >>> dc:licence ...SomeLicensingTerms....   Is it the graph that's
> > licensed,
> > >>> or something else?     There are some use cases that suggests this
> > >>> distinction is important, but if it turns out not to be, it's not
> > bad,
> > >>> people will just not use rdf:Graph declarations much.
> > >>>
> > >>> 2.  Whether or not your trig example and your n-quads example are
> > >>> equivalent depends on your reading of n-quads.   This extends to
> > your
> > >>> reading of SPARQL as well.     My understanding is people are
> > somewhat
> > >>> informal about this, but they generally do expect that once
> > they've seen
> > >>> the whole trig file, or the whole n-quads file, or searched the
> > whole
> > >>> SPARQL end point, that they've seen all the triples in the graph
> > with
> > >>> that name/label.
> > >>>
> > >>> As a social test case, we could tell people this SPARQL query is
> > run:
> > >>>
> > >>>      SELECT ?s ?p ?o
> > >>>      WHERE GRAPH<http://g1.example.org>  { ?s ?p ?o }.
> > >>>
> > >>> and that we got three result bindings back:
> > >>>
> > >>>      ?s  ?p  ?o
> > >>>      === === ===
> > >>>      <a>  <b>  1.
> > >>>      <a>  <b>  2.
> > >>>      <a>  <b>  3.
> > >>>
> > >>> Then we ask them: "According to this query, how many triples are
> > in the
> > >>> graph known to that endpoint as 'http://g1.example.org' ?"
> > >>>
> > >>> What do you think they'll say?
> > >>>
> > >>> I think most folks will say, "Three", even if you ask them to
> > think
> > >>> again and be pedantically precise.
> > >>>
> > >>
> > >> I agree that's what they would say but primarily because you said:
> > "in the graph known to that endpoint"
> > >> This is a critical element which isn't apparent in a mere statement
> > like:
> > >>
> > >> <u1>  {<a>  <b>  <c>  .<d>  <e>  <f>  }
> > >>
> > >> Which doesn't say anything about where it comes from and whether
> > it's complete or not.
> > >>
> > >> This being said, I can get used to having it the way you suggest.
> > Especially when the graph name comes first. If we had: {<a>  <b>  <c>
> >  .<d>  <e>  <f>  }<u1>  I would think differently.
> > >> --
> > >> Arnaud  Le Hors - Software Standards Architect - IBM Software Group
> > >>
> > >>
> > >>> I think that means they're using the complete-graph semantics I'm
> > >>> suggesting.  If they were using partial-graph semantics, they'd
> > have to
> > >>> say, "Three or more".
> > >>>
> > >>> You see what I'm saying?   When we have a complete protocol
> > interaction,
> > >>> via SPARQL, or transmitting a trig or n-quad files, I think the
> > usual
> > >>> assumption is that *all* the triples in the named graph are being
> > sent,
> > >>> not just some of them.
> > >>>
> > >>> I understand sometimes it would be nice to store/transmit just
> > part of
> > >>> some named graph.   But, as I discussed in a message a couple of
> > minutes
> > >>> ago, I think we have to pick one or the other, and I think the
> > >>> complete-graph approach is better.  It's pretty easy to convey
> > partial
> > >>> graphs if we define the complete approach.
> > >>>
> > >>> (I suppose if we defined the partial-graph approach we could
> > transmit
> > >>> complete graphs by transmitting partial graphs and including a
> > >>> triple-count as metadata, so you know it's complete.   I guess
> > that
> > >>> would work, but it seems to me to be optimizing for the
> > much-less-common
> > >>> case.)
> > >>>
> > >>> Coming back to:
> > >>>
> > >>>> I had previously thought that RDF was a data model that didn't
> > need
> > >>> any
> > >>>> notion of 'document' to work.
> > >>>
> > >>> Yeah, it depends what you're doing with it.   There's a lot you
> > can do
> > >>> with RDF without paying any attention to what documents particular
> > bits
> > >>> of RDF were found in, but I think most of the Graphs use cases
> > involve
> > >>> situations where you do need to pay attention to these document
> > >>> boundaries.
> > >>>
> > >>>> Thanks for your willingness to understand my points --- I'm sure
> > that my
> > >>>> formal language will improve over time.
> > >>>
> > >>> It's a long process.   :-)    Interesting, it seems to be helped
> > by
> > >>> arguing.
> > >>>
> > >>>      -- Sandro
> > >>>
> > >>>>
> > >>>> Charles
> > >>>>
> > >>>>
> > >>>>
> > >>>> On 04/02/2012 08:36 AM, Sandro Hawke wrote:
> > >>>>> On Thu, 2012-03-29 at 09:25 -0700, Charles Greer wrote:
> > >>>>>> I really like this solution and it seems to satisfy the use
> > cases
> > >>>>>> familiar to me from when I actually worked a lot with RDF in
> > the wild.
> > >>>>>>
> > >>>>>> One thing I'm tripping over though --  The scope of a TRIG
> > document or
> > >>>>>> RDF dataset in effect 'closes the world.'  Is the idea of
> > "merge" only
> > >>>>>> within a TRIG document/dataset?
> > >>>>>>
> > >>>>>> I can only see two ways to really assert a graph literal --
> > either by
> > >>>>>> sanctifying the boundaries of  a dataset, thereby making merges
> > with
> > >>>>>> external data problematic, or by signing bytes.  Am I missing
> > something,
> > >>>>>> as usual?
> > >>>>> There's some misunderstanding here, yes.   Maybe you can talk
> > through
> > >>>>> some particular thing you imagine doing, involving merging and
> > TriG, and
> > >>>>> I'll be able to pick it up.   From what you've written, I'm
> > confused.
> > >>>>>
> > >>>>> Maybe I can clarifying by translating this TriG document:
> > >>>>>
> > >>>>>           <u1>    {<a>    <b>    <c>   }
> > >>>>>
> > >>>>> into this English declaration:
> > >>>>>
> > >>>>>           The URI 'u1' denotes something, and that thing has
> > exactly one
> > >>>>>           associated RDF Graph.   That associated RDF graph
> > consists of
> > >>>>>           one RDF triple, which we can write in turtle as "<a>
> > <b>   <c>".
> > >>>>>
> > >>>>> So, perhaps it's more clear, now.  If you merged that with
> > another TriG
> > >>>>> document:
> > >>>>>
> > >>>>>           <u1>    {<a>    <b>    <d>   }
> > >>>>>
> > >>>>> Then, trying to accept both documents at onces, you'd be saying:
> > >>>>>
> > >>>>>           The URI 'u1' denotes something, and that thing has
> > exactly one
> > >>>>>           associated RDF graph.  In one document that associated
> > graph is
> > >>>>>           claimed to be the RDF triple "<a>   <b>   <c>", but in
> > another
> > >>>>>           document that graph is claimed to be the RDF triple
> > "<a>   <b>
> > >>>>>           <d>".
> > >>>>>
> > >>>>> So, in this case, you can try to merge the documents, but when
> > you do,
> > >>>>> you find there is a contradiction, since there is only allowed
> > to be one
> > >>>>> associated graph, but in this case there are two different ones.
> > >>>>>
> > >>>>>          -- Sandro
> > >>>>>
> > >>>>>> Charles
> > >>>>>>
> > >>>>>>
> > >>>>>> On 03/27/2012 07:23 PM, Sandro Hawke wrote:
> > >>>>>>> I've written up design 6 (originally suggested by Andy) in
> > more
> > >>>>>>> detail.  I've called in 6.1 since I've change/added a few
> > details that
> > >>>>>>> Andy might not agree with.  Eric has started writing up how
> > the use
> > >>>>>>> cases are addressed by this proposal.
> > >>>>>>>
> > >>>>>>> This proposal addresses all 15 of our old open issues
> > concerning graphs.
> > >>>>>>> (I'm sure it will have its own issues, though.)
> > >>>>>>>
> > >>>>>>> The basic idea is to use trig syntax, and to support the
> > different
> > >>>>>>> desired relationships between labels and their graphs via
> > class
> > >>>>>>> information on the labels.  In particular, according to this
> > proposal,
> > >>>>>>> in this trig document:
> > >>>>>>>
> > >>>>>>>       <u1>    {<a>    <b>    <c>    }
> > >>>>>>>
> > >>>>>>> ... we only know that<u1>    is some kind of label for the RDF
> > Graph<a>
> > >>>>>>> <b>    <c>, like today.  However, in his trig document:
> > >>>>>>>
> > >>>>>>>       {<u2>    a rdf:Graph }
> > >>>>>>>       <u2>    {<a>    <b>    <c>    }
> > >>>>>>>
> > >>>>>>> we know that<u2>    is an rdf:Graph and, what's more, we know
> > that<u2>
> > >>>>>>> actually is the RDF Graph {<a>    <b>    <c>    }.  That is,
> > in
> > >>> this case, we
> > >>>>>>> know that URL "u2" is a name we can use in RDF to refer to
> > that g-snap.
> > >>>>>>>
> > >>>>>>> Details are here:
> > http://www.w3.org/2011/rdf-wg/wiki/Graphs_Design_6.1
> > >>>>>>>
> > >>>>>>> That page includes answers to all the current GRAPHS issues,
> > including
> > >>>>>>> ISSUE-5, ISSUE-14, etc.
> > >>>>>>>
> > >>>>>>> Eric has started going through Why Graphs and adding the
> > examples as
> > >>>>>>> addressed by Proposal 6.1:
> > >>>>>>> http://www.w3.org/2011/rdf-wg/wiki/Why_Graphs_6.1
> > >>>>>>>
> > >>>>>>>         -- Sandro (with Eric nearby)
> > >>>>>>>
> > >>>>>>>
> > >>>>>>
> > >>>>>
> > >>>>
> > >>>>
> > >>>
> > >>>
> > >>>
> > >
> > >
> > > ----
> > > Ivan Herman, W3C Semantic Web Activity Lead
> > > Home: http://www.w3.org/People/Ivan/
> > > mobile: +31-641044153
> > > FOAF: http://www.ivan-herman.net/foaf.rdf
> > >
> > >
> > >
> > >
> > >
> > 
> > 
> 
> 
> 
Received on Thursday, 5 April 2012 22:25:06 UTC

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