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Re: Shapes Constraint Language (SHACL) Working Draft of 2017-02-02

From: Irene Polikoff <irene@topquadrant.com>
Date: Sat, 4 Feb 2017 17:24:50 -0500
Cc: public-rdf-shapes@w3.org
Message-Id: <ED3F52FF-854A-4ACF-8ABF-65B6E48F686C@topquadrant.com>
To: "Peter F. Patel-Schneider" <pfpschneider@gmail.com>
Peter,

The expectation of multiple CR publications as part of the new process was verbally communicated by the W3 management to myself and Ted Thibodeau in a phone call. I can not provide a link to a document that describes the process of multiple CRs (e.g., CR-1, CR-2, etc.). Please feel free to contact W3M with this request.

They have also explained that while the WG must discuss all comments and make a decision on how and whether to address them, having an explicit expression of satisfaction from a commenter was not a requirement for proceeding to CR or TR.

I agree that annotating the document with the information about substantive open issues is a good practice to follow.

Regards,

Irene Polikoff

> On Feb 4, 2017, at 4:50 PM, Peter F. Patel-Schneider <pfpschneider@gmail.com> wrote:
> 
> I don't see anywhere that the expectation is that more that one Candidate
> Recommendation will be published before a W3C technical report proceeds to
> Proposed Recommendation or Recommendation status.  The W3C process document of
> 1 September 2015 states that the expected next step of after Candidate
> Recommendation publication is Proposed Recommendation.  Please provide support
> for your claim.
> 
> Having a link to an issues pages from an email announcement is not a
> substitute for having a note in the document.  It is particularly not an
> effective replacement when the problem is substantial, technical, and
> long-running.
> 
> Most of the comments on previous versions of the document did not give rise to
> tracked working group issues.  Many of these comments have not had an explicit
> expression of satisfaction from the commenter.  Some of these comments have
> not received any response from the working group at all.
> 
> Peter F. Patel-Schneider
> Nuance Communications
> 
> 
> 
> On 02/04/2017 01:03 PM, Irene Polikoff wrote:
>> Peter,
>> 
>> Thank you for your feedback.
>> 
>> Current W3C process treats publishing a CR in a way that is different from the previous practices. Specifically, it is expected that more than one CR spec will be published before a spec proceeds to the FR status. With this, reviewers can continue to submit their comments and the WG will continue to discuss how to address them, even after a CR is published. There is no longer a formal Last Call.
>> 
>> The e-mail that announced the availability of the current WG draft provided a link to the issues page. As you can see from the link, your recent e-mail about pre-binding was recorded as an issue. Thus, indicating to the readers that there is a known issue in this area. All previous feedback was also recorded in a form of issues and resolution of these issues has been documented.
>> 
>> Regards,
>> 
>> Irene Polikoff
>> 
>> 
>>> On Feb 3, 2017, at 11:10 PM, Peter F. Patel-Schneider <pfpschneider@gmail.com> wrote:
>>> 
>>> I took a quick look at the recent working draft of
>>> https://www.w3.org/TR/shacl/ dated 02 February 2017.
>>> 
>>> The document says that the next version of the document is planned to be a
>>> Candidate Recommendation but does not provide a schedule for comments for
>>> this version of the document.  Nor does the document state a schedule for
>>> responses to comments on previous working drafts of this document that have
>>> not yet received substantive responses from the working group.
>>> 
>>> In this quick look I examined the document to see if some of the major
>>> problems with the document have been solved.  What I found is that the three
>>> major problems I first looked at remain unsolved.  Each of them still needs
>>> significant work.  Each of them prevents reviewers of the document from
>>> providing fully informed reviews of the definition of SHACL.  Given that
>>> there are at least these three major, pervasive problems in the document, I
>>> don't see that detailed comments on the rest of the document will be very
>>> worthwhile at this time.
>>> 
>>> 
>>> Pre-binding:
>>> 
>>> There has never been a definition of pre-binding that meets the needs of
>>> SHACL.  The definition of pre-binding in this version of the document is no
>>> different.  Pre-binding is only defined for a solution mapping and a graph
>>> pattern.  However, all uses of pre-binding in SHACL are for a solution
>>> mapping and a query so, in effect, there is no definition of pre-binding at
>>> all in this document.
>>> 
>>> As well, there is no demonstration that the current definition of
>>> pre-binding is well-behaved even where it is defined.
>>> 
>>> The document that is stated to be the source of the definition of
>>> pre-binding for SHACL is a document that has not been accepted by anyone
>>> other than the author of the document as far as I can tell.  Saying that it
>>> is the draft of a WG CG report is giving a false impression of its effective
>>> status.
>>> 
>>> The unsuitability of this definition of pre-binding has been already reported
>>> in https://lists.w3.org/Archives/Public/public-rdf-shapes/2017Jan/0010.html
>>> but there is no indication in the working draft that there are any problems
>>> with pre-binding.  The lack of such an indication in the document means that
>>> reviewers may miss the fact that much of the document has fundamental
>>> problems.
>>> 
>>> As pre-binding is a central part of SPARQL-SHACL and is also used to
>>> describe much of SHACL Core it is not possible for reviewers to provide
>>> fully informed comments on large parts of SHACL at this time.  As there is
>>> as of yet no suitable definition provided for pre-binding even though the
>>> problems with it have been known since at least June of 2015 it will be
>>> better at this late stage to simply remove all parts of SHACL and the SHACL
>>> document that depend on pre-binding.
>>> 
>>> 
>>> Shapes:
>>> 
>>> The way that shapes are formed and used in SHACL remains a severe problem.
>>> 
>>> There are shapes, node shapes, and property shapes.  There are also three
>>> RDF terms that are related to shapes: sh:Shape, sh:NodeShape, and
>>> sh:PropertyShape.
>>> 
>>> There is much confusing wording on how these all work together.
>>> 
>>> First, there is "sh:NodeShape and sh:PropertyShape can be used to represent
>>> node and property shapes".  How do these RDF terms represent anything?
>>> 
>>> Second, there are what appear to be the main definitions of node shapes and
>>> property shapes.
>>> "A node shape is a shape in the shapes graph that is not the subject of a
>>> triple with sh:path as its predicate."
>>> "A property shape is a shape in the shapes graph that is the subject of a
>>> triple that has sh:path as its predicate."
>>> What is the role of sh:NodeShape and sh:PropertyShape if the definition
>>> of node shapes and property shapes doesn't even refer to them?
>>> This is only reinforced by
>>> "However, the presence of any rdf:type triple does not determine whether a
>>> node is treated as a node shape or not."
>>> "However, the presence of any rdf:type triple does not determine whether a
>>> node is treated as a property shape or not."
>>> 
>>> Third, there are what appear to be alternative definitions of node shapes and
>>> property shapes.
>>> "sh:NodeShape is the class of node shapes and should be declared as a type
>>> for shapes that are IRIs."
>>> "sh:PropertyShape is the class of property shapes and should be declared as a
>>> type for shapes that are IRIs."
>>> There are multiple problems with these alternative definitions.  For
>>> starters, there is no description in SHACL of what it means to be the class
>>> of anything.  Next, there is no description in SHACL of how to declare a
>>> type for anything.  Further, there is the strong suggestion here that shapes
>>> that are IRIs should somehow have both sh:NodeShape and sh:PropertyShape
>>> declared as their type, which doesn't make sense at all.
>>> 
>>> Fourth, the conditions to be a shape include being a SHACL instance of
>>> sh:NodeShape or sh:PropertyShape, but not sh:Shape.  This contradicts the
>>> normative statements that rdf:type triples are irrelevant for determining
>>> whether a node is a node or property shape.  It is also exceedingly weird as
>>> sh:Shape is previously indicated to be somehow related to shapes, but being
>>> a SHACL instance of sh:Shape in an RDF graph doesn't make a node a shape in
>>> the graph.  As sh:Shape is the natural RDF term for the type of shapes,
>>> users will use it over sh:NodeShape and sh:PropertyShape.
>>> 
>>> Aside from these problems with node shapes and property shapes, there are
>>> problems with the definitions that shapes depend on.  For example, shapes
>>> graphs are defined too narrowly.  SHACL validation processes don't always
>>> validate a data graph against the shapes in another graph, but shapes graphs
>>> are not defined for these other situations.
>>> 
>>> All this ends up with a big mess.  It appears that it is possible to use
>>> sh:NodeShape and sh:PropertyShape in ways counter to what appears to be
>>> their intended meaning.  For example,
>>> ex:s1 rdf:type sh:NodeShape ;
>>>   sh:targetClass ex:Person ;
>>>   sh:path ex:child ;
>>>   sh:nodeKind sh:IRI .
>>> appears to be form a constraint on the children of people even though the
>>> type of the shape is sh:NodeShape.
>>> 
>>> What needs to be done is to get rid of sh:NodeShape and sh:PropertyShape.
>>> They serve no useful purpose.  They will only produce confusion.  Then the
>>> defintions underlying shapes need to be corrected.  Because of these
>>> significant and pervasive problems with shapes in SHACL, reviewers cannot
>>> provide fully informed commments on the SHACL document at this time.
>>> 
>>> 
>>> Validation results and reports:
>>> 
>>> A validation report is the result of validation.  It is an RDF graph where
>>> some nodes are validation results reporting on constraints that were not
>>> satisifed.  There are serious problems in how validation reports are
>>> generated and the form of validation reports.
>>> 
>>> The first problem is the generation of validation results.  Throughout the
>>> definitions of SHACL Core constraint components there is wording like "For
>>> each value node [...], a validation result MUST be produced with the value
>>> node as sh:value." and "If [...], a validation result MUST be produced."
>>> This means that each SHACL processor must produce these validation results
>>> to be a conforming implementation of SHACL.
>>> 
>>> The processor must produce these validation results no matter whether they
>>> are going to show up in the final validation report or not.  The processor
>>> must produce these validation results even if it not going to return a
>>> validation report at all.  This mixing of conformance requirements into the
>>> definition of validation introduces an unnecessary and problematic
>>> procedural aspect into the underlying definitions of SHACL.
>>> 
>>> Although it is mandated that a SHACL processor much produce these validation
>>> results it is completely unclear how many must be produced.  A SHACL
>>> processor may end up checking whether a particular node satisfies a
>>> particular constraint numerous times.  Must it produce a validation result
>>> for each of these times?  Must it only produce one validation result for all
>>> of these times?  Or is the number of times it produce a validation result
>>> undetermined?  This multiplicity problem can show up at top-level due to
>>> converging sh:property chains.
>>> 
>>> The second problem is the form of a validation report.  There is
>>> insufficient guidance on how multiple validation results are to be
>>> produced.  For example, can a single validation result have multiple values
>>> for sh:value, making it a validation report for multiple violations?
>>> Similarly, if a shape has two sh:ClassConstraintComponent constraints, can
>>> a single validation report be used for violations from both of them?
>>> Without better guidance on these issues it will be very difficult to
>>> determine just violations occured from a validation report.
>>> 
>>> The third problem is just what validation results are to be included in a
>>> validation report and which of these are to be values of sh:result for the
>>> single node in the graph that is a SHACL instance of sh:ValidationReport.
>>> There is "Only the validation results that are not object of any sh:details
>>> triple in the results graph are top-level results." and "The property
>>> sh:detail may link a (parent) result with one or more other (child) results
>>> that provide further details about the cause of the (parent) result."
>>> So a validation process has to produce validation results which then end up
>>> in the validation report if they are not values for sh:details triples.
>>> What happens if a validation result comes from violation of a constraint
>>> that is both directly at top level (e.g., from a property shape that is value of
>>> sh:property for a shape that has targets) and not at top level (e.g., from
>>> the same property shape as before that is linked to the shape with targets
>>> via a combination of sh:node and sh:property triples)?  Can a SHACL
>>> processor use sh:detail to collect that otherwise might be top-level
>>> validation results?
>>> 
>>> There are also some other minor problems with validation reports.  For
>>> example, there is the requirement that "A validation report has exactly one
>>> value for the property sh:conforms that is of datatype xsd:boolean."
>>> However, the result of validation is an RDF graph and RDF graphs so this
>>> requirement doesn't make sense.  The definitions underlying validation
>>> reports need to be carefully examined to eliminate problems like these.
>>> 
>>> Much of the description of how validation reports are generated and what
>>> they contain need to be rewritten to remove any procedural aspects and to
>>> suitably describe the contents of validation resports.  As this will change
>>> large portions of the document, reviewers cannot provide fully informed
>>> commments on it at this time.
>>> 
>> 
Received on Saturday, 4 February 2017 22:25:25 UTC

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