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Re: Web Semantics for Datasets

From: Sandro Hawke <sandro@w3.org>
Date: Fri, 07 Oct 2011 10:35:26 -0400
To: Andy Seaborne <andy.seaborne@epimorphics.com>
Cc: public-rdf-wg@w3.org
Message-ID: <1317998126.12194.128.camel@waldron>
On Fri, 2011-10-07 at 13:48 +0100, Andy Seaborne wrote:
> > Okay, that's enough for now.  Give me a +1 if you think this is headed
>  > in a useful direction.
> 
> I like something like this as a pattern of good practice (well, 2 
> patterns).  I don't agree with forcing the 4th column to have a specific 
> meaning given all the other deployed uses we have now collected.

Yeah....   There is a middle ground where some datasets use Web
semantics and some don't.  I see your point that we can't just force
people to change -- we can't say the thingsthey've been saying now means
something else.

Maybe we can have a way to flag which datasets are using Web semantics,
and allow market pressures to work?    Like, where we do a new mime type
for a multigraph syntax, we could add this.   And maybe it's something
we can flag in SPARQL service description.

> On one points:
> 
> I don't see why
> 
> <http://example.org>  { <s>  <p>  <o> . }
> 
> should mean it is ONLY that triple rather than CONTAINS that triple.  If 
> the data publisher wants to say "and that's all" then they should say so 
> as an additional fact.  The converse of "it's closed by default" is 
> harder to see how to allow it to be open sometimes.
> 
> For a large graph, and you only need to talk about a small subset, the 
> deployment issues.  Consider dbpedia.
> 
> (I also want to see the same change in TriG for concatenation of files)

It seems to me that it's easy to go from complete to incomplete, just
using a subgraph predicate.   Let's say we want to say G1 is the graph
with only <s> <p> <o> and G2 is a graph with that triple and maybe other
stuff.   I'd say:

    G1 { <s> <p> <o>. }
    { G1 r:subgraphOf G2. }      

But I don't see how to communicate G1 the way you're talking about. How
do you say "and that's all"?

    -- Sandro


> 	Andy
> 
> On 07/10/11 03:04, Sandro Hawke wrote:
> > Here's a proposal for what the fourth column should mean.  It's kind of
> > obvious, and I think it's how many of us just assumed Named Graphs were
> > supposed to work.    But I don't think it's been written down in a form
> > we can use, so here it is, in a first draft.
> >
> > I haven't really tried to motivate this, but one thing it does is allow
> > folks to refer to a graphs using just one URI.  As [1] points out rather
> > painfully, as things stand now, you need multiple URIs just to identify
> > each g-box (and thus g-snap).  (That is, you need to say which sparql
> > endpoint you're talking about, and then which graph within its
> > dataset.)
> >
> > My starting question was: what is the relationship between the IRI (the
> > "graph name") and its associated g-snap in an RDF Dataset.  This
> > applies to the dataset backing any SPARQL end point, as well as the
> > dataset serialized in any multigraph syntax, like TriG or N-Quads.
> > Another way to look at it: what does it mean to assert a TriG
> > document?  If you send me the TriG Document "<a>  {<s>  <p>  <o>  }", and
> > I trust you, what do I now know?
> >
> > Richard, I think, has been arguing for a minimalist position,
> > answering "nothing", or "it depends on out-of-band agreements".  This
> > "Web Semantics" proposal is an alternative.
> >
> > === Proposal
> >
> > The idea here is to make the relationship between the URI and the
> > graph be the standard Web naming relationship, similar to what we all
> > use for Web pages.  When you dereference the URI, you get the graph.
> >
> > This has the feature of being, to some extent, observable.  Just like
> > triples are claims about some domain of discourse, quads become claims
> > about idealized Web dereference behavior.
> >
> > Specifically: Consider a "graph naming" to be the association of a
> > graph name N with a graph G.  For the graph naming to hold, every
> > successful dereference of N yielding an RDF graph must yield G.  For a
> > dataset D to hold, its default graph must hold (as normal in RDF) and
> > every graph naming pair in D must hold.
> >
> > Example 1:  This dataset
> >
> >     <http://example.org>  {<s>  <p>  <o>. }
> >
> > means that if anyone is able to dereference "http://example.org"
> > and obtain an RDF graph serialization, the serialized graph will
> > consist of the single triple,<s>  <p>  <o>.  Failure to dereference
> > does not make the graph naming untrue, but a successful dereference
> > yielding a different graph does.
> >
> > Example 2:  This dataset can never be true:
> >
> >     <http://example.org>  {<s>  <p>  1. }
> >     <HTTP://example.org>  {<s>  <p>  2. }
> >
> > ... since one cannot get different results dereferencing URIs that
> > differ only in the case of the scheme component (as per RFC 3986).
> >
> > Example 3:  This dataset:
> >
> >    <tag:hawke.org,2010-10-06:eg1>  {<s>  <p>  <o>. }
> >
> > cannot be tested using Web protocols, since the "tag" URI scheme is
> > (by design) not dereferenceable.  Whether it is true or false cannot
> > be determined experimentally.
> >
> > ==== Temporal Context
> >
> > How can we say:
> >
> >     <http://example.org>  {<s>  <p>  <o>. }
> >
> > if we suspect that "http://example.org" might serve some other content
> > tomorrow?
> >
> > The answer is that datasets often need temporal qualification just
> > like RDF graphs do.  It's just like saying in RDF:
> >
> >     <http://example.org/Alice>  foaf:age 25.
> >
> > One solution for foaf:age triples is to include triples like:
> >     <>  dc:temporal "2011-10-06"^^xs:dateTime.
> >
> > and that can be done in datasets as well, using the default graph.
> > More work is needed on this, but I'm pretty sure this proposal can use
> > whatever solution people come up with for RDF and doesn't make matters
> > much worse than they are already.
> >
> > ==== Practical Deployment Choices
> >
> > Any system which maintains a dataset (eg a sparql endpoint) or
> > generates multigraph documents like TriG has to do one (or more) of
> > the following:
> >
> > 1.  Use new non-dereferenceable graph names.  These could be tag or
> >      uuid URIs, or http URIs in your own name space which you choose to
> >      leave 404.
> >
> > 2.  Use your own dereferenceable graph names, perhaps relative to the
> >      endpoint or TriG document URI.  If you do serve RDF content at
> >      those URIs, it MUST be the same content (give or take stated time
> >      lag).
> >
> > 3.  Use someone else's graph names.  Here, the key thing is temporal
> >      metadata.  You have to decide what you want (copy once vs
> >      synchronize with what accuracy) and (somehow) share that temporal
> >      metadata.
> >
> >
> > ...
> >
> > Okay, that's enough for now.  Give me a +1 if you think this is headed
> > in a useful direction.
> >
> >      -- Sandro
> >
> > [1] http://www.w3.org/2011/prov/wiki/Using_named_graphs_to_model_Accounts
> >
> >
> 
> 
Received on Friday, 7 October 2011 14:35:36 GMT

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