- 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
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 UTC