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Re: service description vocabulary

From: Steve Harris <steve.harris@garlik.com>
Date: Tue, 29 Sep 2009 08:53:47 +0100
Cc: "public-rdf-dawg@w3.org Group" <public-rdf-dawg@w3.org>
Message-Id: <E5B1F2D4-9BAD-41A1-A6B2-09EE38F2E60B@garlik.com>
To: Gregory Williams <greg@evilfunhouse.com>
On 28 Sep 2009, at 17:16, Gregory Williams wrote:
> On Sep 26, 2009, at 5:06 AM, Steve Harris wrote:
>
>>> On the other side of things, if I look at the dataset description  
>>> and find that there's information about a single foaf:Person in  
>>> the dataset and I want to retrieve that information, how am I  
>>> meant to get to it if I don't know which of the million named  
>>> graphs it's in?
>>
>> SELECT ?g ?x WHERE { GRAPH ?g { ?x a foaf:Person } } ?
>>
>> Maybe I misunderstood the question.
>
> No, you got it right. Maybe I just gave a bad example :) I just  
> don't want to end up with the only option of having to describe the  
> whole dataset as simply the graph merge of the constituent graphs.  
> Also, I know it's hard to argue spec points based on optimization  
> stuff, but if you've got a large dataset and a not-so-intelligent  
> query engine/optimizer, this might be a really bad query to run if  
> simply knowing what ?g is ahead of time could let you sidestep the  
> whole issue.

You're right it's hard to argue on points like that. I would say that  
if you have such a system (not so unlikely) then exposing it to the  
internet at large is probably not useful.

>> Let's not fixate on Void. If Void is not sufficient then the  
>> community will come up with something more comprehensive.
>
> Well, I'm torn between saying "yes, absolutely," and thinking that  
> there are people (like the voiD folks) that are working on  
> describing RDF graphs, but that the SPARQL dataset case is specific  
> enough to SPARQL that maybe we should be providing the handful of  
> properties to allow leveraging graph description vocabularies in the  
> context of SPARQL datasets.

The chances are that the query language and associated specs will  
outlive any specific vocabulary.

>>>> while I'd think a simple way would be
>>>>
>>>> <endpoint> sd:datasetDescription <void-dataset-for-dataset> ;
>>>> 	sd:defaultGraph <graph-name> ;
>>>> 	sd:namedGraph <graph-name> .
>>>
>>> Again, I'm not sure how useful the <void-dataset-for-dataset>  
>>> would be, since by this point you've lost the ability to  
>>> discriminate between the named (and default) graphs.
>>
>> Well, it's a little murky anyway, both the protocol and query have  
>> the ability to change the contents of the default graph.
>
> In many cases you'd know that, though, right? If I provide a FROM  
> clause or a default-graph-uri parameter, then I wouldn't expect the  
> default graph description data to match up (since I've explicitly  
> overridden them). In this case, either I can get the same graph  
> description from one of the provided named graph descriptions (if  
> the endpoint has a list of available graphs that I can use in FROM  
> clauses) or I can get it from an external source if, for example,  
> the endpoint is simply dereferencing an RDF URL I give it.

Fair point.

Proxies could muddy this, though we don't override the default graph  
choice in proxies, and I don't know of anyone that does.

>> Requiring systems to return everything in one graph could be  
>> onerous for client and server, eg. in the 2M FOAF graph case, both  
>> the list of graphs, and the description of the store will be large.
>
> Agreed. I would, however, like this as an *option* for  
> implementations.

Sure, as long as it's not required/expected.

- Steve

-- 
Steve Harris
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Received on Tuesday, 29 September 2009 07:54:23 GMT

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