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Re: Graph store protocol editor's draft updated

From: Andy Seaborne <andy.seaborne@epimorphics.com>
Date: Wed, 15 Feb 2012 18:24:24 +0000
Message-ID: <4F3BF858.40007@epimorphics.com>
To: Sandro Hawke <sandro@w3.org>
CC: public-rdf-dawg@w3.org

Thanks for the description.

http://northeast-region.example.gov/wqdat/sparql is covered by the 
SPARQL Protocol, not the Graph Store Protocol.  GET on that URL will 
return the service description as you describe.

Service Description / sec 2:
SPARQL services made available via the SPARQL Protocol SHOULD return a 
service description document at the service endpoint when dereferenced 
using the HTTP GET operation.

Would it help to put the same point in the SPARQL Protocol document?


On 15/02/12 04:05, Sandro Hawke wrote:
> On Tue, 2012-02-14 at 22:19 +0000, Andy Seaborne wrote:
>> On 14/02/12 15:56, Sandro Hawke wrote:
>>> Looking more closely, it's not 5.8 that I want back, it's this sentence:
>>>           Within a service description document for an implementation of
>>>           this protocol, the object of an sd:defaultDataset statement is
>>>           understood to be the identifier of the Graph Store
>> Where do you expect to read the service description from?
>> Could you write out a concrete example, with URIs and actions, so I can
>> understand the process you are envisaging that is behind your comments?
>>    I'm quite confused as to the information flow you are looking for.
> Imagine a national government wants data feeds about water quality from
> each of its regional governments.    Each region is responsible for
> running a SPARQL endpoint serving the data, broken up into a different
> graph for each km^2 and month.  In the default graph is to be metadata
> about each of those other graphs, saying when it's valid and what area
> it covers.
> Now, the national government collects the endpoint addresses, one per
> region, looking something like this:
>     http://northeast-region.example.gov/wqdat/sparql
>     http://northwest-region.example.gov/~smith/fed/sparql
>     http://northcentral.example.gov/water/sparql
> Following normal SPARQL practice, some of the regions pick graph names
> which are not actually working URLs which can be used to fetch the
> associated data.  Instead one region use tag URIs, one uses UUIDs, one
> uses the URI of the most prominent geographic feature in the block, and
> another uses a homegrown URI scheme which produces URIs like this:
>          block:34.2234-34.2547,81.3331,80.9830:2010-01-01
> This all works.    Given this list of SPARQL endpoints, the nation govt
> can write various clients which query each region's data as necessary.
> They can also publish this list of endpoint addresses, and let the
> general public query as they will.
> But there are some things we'd like to be able to do that we can't:
> * Alice wants to download all the graphs concerning a certain area and
> time-range, crossing several regions, without knowing SPARQL.  She just
> wants a REST interface for GET'ing the default graphs and then the other
> data graphs.
> * Bob is doing analysis for which he needs to provide provenance.  He
> wants a single URI for each of the graphs he's using, so he can put it
> into the "source" field for that part of the analysis.
> * Charlie is on a data-quality crusade.   He's getting people to double
> check the data against other private data sources and their own
> experience.  He's built a system for flagging questionable blocks of
> data, and even submitting corrections (patches).  For this system, he
> needs some way to refer to each graph which has been flagged for
> correction.
> I think the simplest solution would be to just let everyone know they
> can always use:
>      ${endpoint_addr}?graph=${graph_name}
> or
>      ${endpoint_addr}?default
> as a URI for the indicated graph.   I'd hope most endpoints would
> implement at least HTTP GET on those addresses, if not the whole GSP.
> Even if they just having this convention -- with no code changes -- it
> would address Bob and Charlie's problems.   And Alice will know what to
> try, in case GSP happens to be implemented.
> Alternatively, if for some reason the SPARQL WG is not okay with using
> the endpoint address this way, we could use Service Description, as was
> in GSP until the most recent change [1].   With this, to get the URI of
> the default graph for the first region, Alice would:
> 1.  GET http://northeast-region.example.gov/wqdat/sparql
> and get back a SPARQL service description that includes triples like
> this:
>          @prefix sd:<http://www.w3.org/ns/sparql-service-description#>  .
>          <>  a sd:Service;
>               sd:defaultDataset<http://northeast-region.example.gov/wqdat/dataset>  .
> 2.  Given this, and the text that used to be GSP, plus what's still there,
> Alice knows the URL of the default graph for the northeast region is:
>          http://northeast-region.example.gov/wqdat/dataset?default
> She can do a GET on this to get the contents of the default graph, which
> has something like this:
>          <urn:uuid:eee02beb-eca7-4cb7-839c-9fc6206caae0>  geo:lon0 34.2234;
>                                                          geo:lon1 34.2547;
>                                                          geo:lat1 81.3331;
>                                                          geo:lat0 80.9830;
>                                                          dc:temporal "2010-01-01"^xs:datetime.
> 3.  Now she can construct a URL from which she can fetch the data for
> that region and that time, like this:
>          http://northeast-region.example.gov/wqdat/dataset?graph=urn:uuid:eee02beb-eca7-4cb7-839c-9fc6206caae0
> And that's about it.  Repeat 3 for each block in the region; repeat 1-2
> for each region.
>      -- Sandro
> [1]
> https://cvs.w3.org/Team/~checkout~/WWW/2009/sparql/docs/http-rdf-update/Overview.html?rev=1.81;content-type=text%2Fhtml#http-post and scroll down toward the end of that POST section.
Received on Wednesday, 15 February 2012 18:24:53 UTC

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