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Re: shapes-ISSUE-130 (rdf dataset assumption): SHACL should not assume that the data graph is in an RDF dataset [SHACL Spec]

From: Tom Johnson <johnson.tom@gmail.com>
Date: Thu, 17 Mar 2016 16:50:03 -0700
Message-ID: <CAJeHiNGJyRQaNLynD1AhghH6o=bSHppM97dLfPLTZueD4OjX+A@mail.gmail.com>
To: Holger Knublauch <holger@topquadrant.com>
Cc: Dimitris Kontokostas <kontokostas@informatik.uni-leipzig.de>, "Peter F. Patel-Schneider" <pfpschneider@gmail.com>, RDF Data Shapes Working Group <public-data-shapes-wg@w3.org>
That seems okay to me. In this case, wherever the SHACL spec refers to a
Dataset, the main requirement should be that it is clear how the referenced
Dataset is constructed from the Shapes graph and the Data graph. The main
point of the specs use, then is that a SHACL validation requires two
graphs: shapes & data. All of the rest of the handling re: SPARQL Datasets
can then be abstracted away by implementers if desired.

- Tom

On Thu, Mar 17, 2016 at 4:41 PM, Holger Knublauch <holger@topquadrant.com>
wrote:

> On 18/03/2016 9:23, Tom Johnson wrote:
>
> This brings us back to a week ago up-thread:
> <https://lists.w3.org/Archives/Public/public-data-shapes-wg/2016Mar/0082.html>
> https://lists.w3.org/Archives/Public/public-data-shapes-wg/2016Mar/0082.html
>
> Per this comment, I don't think this is a purely conceptual problem. If a
> dataset is a Fundamental concept that can't be ignored in defining SHACL,
> no problem. I'm somewhat skeptical of this, but I agree with all your
> points that this isn't a hard implementation, it's easily supported in all
> major existing RDF environments, etc...
>
> The problem I see exists closer to the SHACL language. When authoring a
> shapes graph, how can I ensure that I will be able to run it against the
> data graphs of my choice?
>
>
> I don't see the connection. Shapes graphs are typically authored
> independently of any specific data graph. The SHACL engine is invoked with
> certain parameters. These would be
> - dataset
> - IRI of data graph (or empty) for default graph in the dataset
> - shapes graph (hopefully in the same dataset)
>
> How these parameters are collected before the engine is invoked is outside
> of the spec. Some applications may look at sh:shapesGraph triples in the
> data graph, others may have the shapes graph hard-coded.
>
> Holger
>
>
>
>
>
> - Tom
>
> On Thu, Mar 17, 2016 at 4:16 PM, Holger Knublauch <holger@topquadrant.com>
> wrote:
>
>> A dataset is a conceptual entity that can be implemented in many ways.
>> One implementation may retrieve certain graphs on-the-fly via an LDP
>> server. That's an implementation detail to me. But SPARQL requires
>> datasets, so how can we pretend they don't exist?
>>
>> Holger
>>
>>
>>
>> On 18/03/2016 9:06, Tom Johnson wrote:
>>
>> Datasets seem fine to me, as an implementation detail, but my main
>> context for using SHACL would be to validate graphs from one of two
>> contexts:
>>
>>   - Linked Data Platform RDFSources; i.e. I have an existing shapes
>> graph, and I want to validate a number of graphs grabbed via HTTP GET.
>>   - Existing in-memory graphs. (in this case, serializing to a file is
>> always an option).
>>
>> In either of these cases specifying the Dataset construct at the SHACL
>> level seems unwieldy. When writing a shapes graph, I need to know how the
>> specific engine will handle my URI/File/native-graph input.
>>
>> I don't have a specific proposal to improve things, but the idea that
>> SHACL's behavior is undefined except when my shapes graph and data graph
>> already exist within the same SPARQL Dataset is concerning.
>>
>> - Tom
>>
>> On Thu, Mar 17, 2016 at 3:49 PM, Holger Knublauch <
>> <holger@topquadrant.com>holger@topquadrant.com> wrote:
>>
>>> On 18/03/2016 1:49, Dimitris Kontokostas wrote:
>>>
>>>
>>>
>>> On Thu, Mar 17, 2016 at 3:20 PM, Peter F. Patel-Schneider <
>>> <pfpschneider@gmail.com>pfpschneider@gmail.com> wrote:
>>>
>>>> The diff seems to indicate that functions still work only over datasets,
>>>> although there is a TODO indicated.
>>>
>>>
>>> I was waiting for Holger to wake up and clarify this but you are right
>>>
>>>
>>> SPARQL in general relies on datasets, e.g. in the GRAPH keyword. As soon
>>> as we talk about arbitrary SPARQL queries (e.g. in sh:sparql constraints or
>>> functions), datasets need to be in the picture.
>>>
>>> I maintain my position that we would be making our lives easier if we
>>> were simply talking about datasets, acknowledging that these datasets may
>>> only exist for the duration of a SHACL execution and contain the data graph
>>> (and the shapes graph) only. There is nothing conceptually difficult here,
>>> neither difficult to implement.
>>>
>>> Holger
>>>
>>>
>>>
>>>
>>>
>>>> Two "or dataset"s were also removed -
>>>> making the dataset optional is probably benign.
>>>>
>>>> As far as $shapesGraph goes, wording along the lines of "can be used to
>>>> access
>>>> the shapes graph" would seem to work, but would need some explanation
>>>> along
>>>> the lines of "If the shapes graph is a named graph in the same dataset
>>>> as the
>>>> data graph then it can be accessed using its name in the dataset.
>>>> Otherwise a
>>>> SHACL engine would need to provide an alternative way to access the
>>>> shapes graph."
>>>>
>>>
>>> tried to fix all your comments in a second commit here
>>>
>>> https://github.com/w3c/data-shapes/commit/a0a52a4472d807683abf7cf104079f35272f27df
>>>
>>>
>>> this is a merge of both commits but also includes a few other editorial
>>> fixes
>>> https://github.com/w3c/data-shapes/compare/editorial-dk
>>>
>>> apologies for sending commits but I am not yet comfortable to commit to
>>> the main branch
>>>
>>> Best,
>>> Dimitris
>>>
>>>
>>>
>>>>
>>>>
>>>> peter
>>>>
>>>>
>>>> On 03/17/2016 02:04 AM, Dimitris Kontokostas wrote:
>>>> > I removed all mentions of dataset in the text that imply that
>>>> validation works
>>>> > on RDF datasets
>>>> >
>>>> <https://github.com/w3c/data-shapes/commit/fa2d99fd61a473d14eef9cee57c0db1c61e03684>
>>>> https://github.com/w3c/data-shapes/commit/fa2d99fd61a473d14eef9cee57c0db1c61e03684
>>>> >
>>>> > One thing left to decide and close this issue  is how to refer to the
>>>> > '$shapesGraph" variable. right now we have variations of the
>>>> following in the spec
>>>> > "$shapesGraph ... a named graph IRI that contains the the shapes
>>>> graph"
>>>> >
>>>> > reading the sparql spec, named graphs imply an RDF dataset:
>>>> > <https://www.w3.org/TR/sparql11-query/#namedGraphs>
>>>> https://www.w3.org/TR/sparql11-query/#namedGraphs
>>>> >
>>>> > even if we remove the "named" and refer to them as just graphs,
>>>> SPARQL uses
>>>> > the "GRAPH" keyword in the "Querying the Dataset" section
>>>> > <https://www.w3.org/TR/sparql11-query/#queryDataset>
>>>> https://www.w3.org/TR/sparql11-query/#queryDataset
>>>> >
>>>> > so, do we still have an implicit assumption that validation works on
>>>> RDF datasets?
>>>> >
>>>> > Since we already have a resolution on $shapesGraph I see the
>>>> following options:
>>>> > a) we accept the edits as they are now in the spec and close this
>>>> issue
>>>> > b) we try to weaken further the connection and change occurrences of
>>>> "named
>>>> > graph" to "graph"
>>>> > c) since this is  sparql-specific issue, we can add a disclaimer in
>>>> section 1.2
>>>> >
>>>> > regarding how a SHACL validation engine wraps graphs to perform
>>>> validation, it
>>>> > can be an implementation detail
>>>> >
>>>> > Dimitris
>>>> >
>>>> >
>>>> > On Wed, Mar 9, 2016 at 9:35 AM, Holger Knublauch <
>>>> <holger@topquadrant.com>holger@topquadrant.com
>>>> > <mailto: <holger@topquadrant.com>holger@topquadrant.com>> wrote:
>>>> >
>>>> >     Yes, you may have a point there - in cases like the default graph
>>>> we need
>>>> >     to make sure that the system knows which subject to look for the
>>>> >     sh:shapesGraph triples. This is probably just a URI parameter.
>>>> >
>>>> >     (There are so many edit suggestions open right now that I am
>>>> looking
>>>> >     forward to sharing the workload with a second editor, now that
>>>> Arthur has
>>>> >     left; yes I have a day job too.)
>>>> >
>>>> >     Holger
>>>> >
>>>> >
>>>> >     On 8/03/2016 11:02, Tom Johnson wrote:
>>>> >>     > An RDF dataset is a purely conceptual entity. Many APIs
>>>> implement
>>>> >>     Dataset. Any Graph can be wrapped into a Dataset for execution,
>>>> even if
>>>> >>     that Dataset is just virtual and only has a single graph in it.
>>>> >>
>>>> >>     Reading the quoted text, this doesn't seem to hold. The "data
>>>> graph"
>>>> >>     links to the "shapes graph" via a triple with its graph name as
>>>> the
>>>> >>     subject. Many graphs do not have such a name (even those that
>>>> are within
>>>> >>     Datasets; i.e. default graphs).
>>>> >>
>>>> >>     Does SHACL provide a mechanism for connecting such a graph to a
>>>> shapes
>>>> >>     graph? If not, how does wrapping the graph in a dataset within
>>>> the
>>>> >>     implementation help a SHACL user make this connection?
>>>> >>
>>>> >>     - Tom
>>>> >>
>>>> >>     On Mon, Mar 7, 2016 at 3:00 PM, Holger Knublauch <
>>>> <holger@topquadrant.com>holger@topquadrant.com
>>>> >>     <mailto: <holger@topquadrant.com>holger@topquadrant.com>> wrote:
>>>> >>
>>>> >>         An RDF dataset is a purely conceptual entity. Many APIs
>>>> implement
>>>> >>         Dataset. Any Graph can be wrapped into a Dataset for
>>>> execution, even
>>>> >>         if that Dataset is just virtual and only has a single graph
>>>> in it.
>>>> >>
>>>> >>         Holger
>>>> >>
>>>> >>
>>>> >>
>>>> >>         On 8/03/2016 2:45, RDF Data Shapes Working Group Issue
>>>> Tracker wrote:
>>>> >>
>>>> >>             shapes-ISSUE-130 (rdf dataset assumption): SHACL should
>>>> not
>>>> >>             assume that the data graph is in an RDF dataset [SHACL
>>>> Spec]
>>>> >>
>>>> >>              <http://www.w3.org/2014/data-shapes/track/issues/130>
>>>> http://www.w3.org/2014/data-shapes/track/issues/130
>>>> >>
>>>> >>             Raised by: Peter Patel-Schneider
>>>> >>             On product: SHACL Spec
>>>> >>
>>>> >>             """4. Declaring the Shapes Graph
>>>> >>
>>>> >>             A data graph MAY link to one or more shapes graphs via
>>>> the
>>>> >>             property sh:shapesGraph. The subject of this predicate
>>>> must be
>>>> >>             the graph resource, i.e. the name of the data graph in
>>>> the
>>>> >>             dataset. The objects of this predicate must be IRI nodes,
>>>> >>             pointing at a named graph in the dataset. Tools may use
>>>> this
>>>> >>             information to determine which shapes graph to use for
>>>> >>             validation. If present, tools SHOULD transitively follow
>>>> any
>>>> >>             links from the shapes graph via the predicate
>>>> owl:imports to
>>>> >>             other graphs and use the resulting union graph as
>>>> parameter to
>>>> >>             the validation process."""
>>>> >>
>>>> >>             This assumes that the data graph is in an RDF dataset.
>>>> SHACL
>>>> >>             validation should work on data graphs that are not in an
>>>> RDF
>>>> >>             dataset.
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >>     --
>>>> >>     -Tom Johnson
>>>> >
>>>> >
>>>> >
>>>> >
>>>> > --
>>>> > Dimitris Kontokostas
>>>> > Department of Computer Science, University of Leipzig & DBpedia
>>>> Association
>>>> > Projects: <http://dbpedia.org>http://dbpedia.org,
>>>> http://rdfunit.aksw.org,
>>>> > http:// <http://aligned-project.eu>http://aligned-project.eu <
>>>> http://aligned-project.eu/>
>>>> > Homepage: <http://aksw.org/DimitrisKontokostas>
>>>> http://aksw.org/DimitrisKontokostas
>>>> > Research Group: AKSW/KILT <http://aksw.org/Groups/KILT>
>>>> http://aksw.org/Groups/KILT
>>>> >
>>>>
>>>>
>>>
>>>
>>> --
>>> Dimitris Kontokostas
>>> Department of Computer Science, University of Leipzig & DBpedia
>>> Association
>>> Projects: http://dbpedia.org, http://rdfunit.aksw.org, http://
>>> <http://aligned-project.eu>http://aligned-project.eu
>>> Homepage: <http://aksw.org/DimitrisKontokostas>
>>> http://aksw.org/DimitrisKontokostas
>>> Research Group: AKSW/KILT  <http://aksw.org/Groups/KILT>
>>> http://aksw.org/Groups/KILT
>>>
>>>
>>>
>>
>>
>> --
>> -Tom Johnson
>>
>>
>>
>
>
> --
> -Tom Johnson
>
>
>


-- 
-Tom Johnson
Received on Thursday, 17 March 2016 23:51:11 UTC

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