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some more comments on Fed query doc... (was: Re: Review of Query document (basic federated query))

From: Axel Polleres <axel.polleres@deri.org>
Date: Tue, 5 Oct 2010 13:31:30 -0400
Cc: Andy Seaborne <andy.seaborne@epimorphics.com>, Eric Prud'hommeaux <eric@w3.org>
Message-Id: <A917E080-8CBA-4980-AFF1-957C6C413022@deri.org>
To: SPARQL Working Group <public-rdf-dawg@w3.org>
Those comments are likely too late for this pub cycle since we resolved to publish, but 
just not to forget about them:

I have 2 minor ( 1) and 2) ) and one major comment 3) ...


1) In the example in section 4.1 of [1]

Join( Join( BGP(?s :p1 ?v1),
            Service(?s :p2 ?v2) ),
      BGP (?s :p3 ?v2) )

should be 
 

Join( Join( BGP(?s :p1 ?v1),
            Service(<srvc>, (?s :p2 ?v2) ),
      BGP (?s :p3 ?v2) )

right?

2) Another minor comment (editorial): the href http://www.w3.org/TR/sparql11-federated-query/#update
  is a bit strange, it should be 
   http://www.w3.org/TR/sparql11-federated-query/#def-bindings
  or alike.

3) As for the editorial note
"Editorial note This notion of "already bound" (note the related constraint in the grammar) is still an issue for the SPARQL Working Group, as it the question of having variables in SERVICE calls at all. Feedback from the community is encouraged."

 and further in   http://www.w3.org/TR/sparql11-federated-query/#pre-bound 

 "if that variable is not bound (at least optionally)"

 I think we should refer to the definition of "potentially bound" as already noted by Andy in [2].
The draft definition is here [3]. Shall we mention [3] at least in an editor's note in Query?

However, further regarding this point 3), I think we have various options for *evaluating* 
variables in SERVICE clauses, let me try to summarise them:


  a) FILTER-style: SERVICE patterns are always assumed to be evaluated *last* in a qroup, depending on the bindings provided within the 
     group, one SERVICE pattern can't provide bindings for another SERVICE pattern within the same group

     i.e. 
      - { ... ?X ... SERIVCE ?X {...} } is the same as { SERVICE ?X {...} ... ?X ...  } 
      - { ... SERVICE <a> {... ?X ...}  ... SERVICE ?X {...} } would not work unless ?X also appears outside a SERVICE pattern
  
  b) order-dependent a la OPTIONAL

      - { ... ?X ... SERIVCE ?X {...} } is different from { SERVICE ?X {...} ... ?X ...  } 
      - { ... SERVICE <a> {... ?X ...}  ... SERVICE ?X {...} } would mean that the bindings obtained from service <a> serve as "input" 
        for the SERVICE ?X call...

     however, then It is fairly arguable then why 

     - { ... SERVICE <a> {... ?X ...}  ... SERIVCE <b> {... ?X ... } }

     shouldn't behave in the same, order-dependent way, or, respectively, why we shouldn't allow variables in BINDINGS, following the same 
     rationale of order dependence.

  c) a third alternative seems to be - analogous to our notion of Dataset - to define the notion of "service set" meaning that variables 
     in a SERIVCE position can only bind to values from that service set ... that would be analogous to the treatment of GRAPH patterns, 
     which syntactically already resemble SERVICE a lot.

     With c) it seems we can drop any restrictions on "potentially boundness", 
     however - small side note - it seems that we'd need analogous to FROM NAMED, new keywords FROM SERVICE or alike 
     to define the service set.


At this moment, c) seems to me the most straightforward to define and in line with the treatment of GRAPH, whereas a) seems to be very restrictive and for b) I am afraid it raises questions ... Opinions?

Axel

1. http://www.w3.org/2009/sparql/docs/fed/service.xml
2. http://lists.w3.org/Archives/Public/public-rdf-dawg/2010JulSep/0433.html
3. http://www.w3.org/2009/sparql/wiki/Potentially_bound


On 22 Sep 2010, at 10:31, Andy Seaborne wrote:

> >> > > 13 Basic Federated Query
> >> > > See basic federated query doc
> Much of the Federation document is written in a very casual and
> narrative fashion (very different than the Query doc; I suspect this
> will be very obvious if the federation text is just merged with the
> query document).
> 
> The document never discusses the "UNDEF" token that is introduced in the
> grammar.
> 
> "Solution Mapping (corresponds to the Concepts and Abstract Syntax term
> "RDF URI reference")" -- seems like a copy-paste typo.
> 
> "For instance, an edpoint" -- sp. "endpoint".
> 
> The examples are hard to follow because they are so domain-specific.
> 
> "The mechanics of executing a query over a graph" -- is this meant to be
> referring to "executing a query over a *named* graph"?
> 
> "Typically, a GRAPH constraint is matched against an RDF graph which is
> in the querying system, perhaps as the result of parsing the response to
> an HTTP GET on the named graph." -- This is needless detail. A GRAPH
> pattern is matched against named RDF graphs contained within the dataset
> being used for the query.
> 
> "GRAPH-constrained pattern" -- I don't know what this means.
> 
> "Note that WSDL defines the behavior with respect to constructing HTTP
> URLs from an endpoint and a set of query parameters, in particular
> appending '?' or '&' to an endpoint URL which may already have them." --
> I'm not totally sure what this means, but I'd like to suggest that there
> should be a way to query over a custom dataset at the remote endpoint
> using the standard SPARQL Protocol conventions (SERVICE
> <http://example/endpoint?default-graph-uri=foo> {...}).
> 
> "application/sparql-results" -- should be "application/sparql-results+xml"
> 
> "For any other response, the query fails." -- Should this fail or just
> return an empty result set? I can think of arguments for both, but
> SERVICE blocks within OPTIONALS and UNIONS would be more useful if they
> didn't cause the entire query to fail.
> 
> "queryier" ??
> 
> In the example for section 3 BINDINGS, the ?id variable is bound to
> plain literals, but the example data from earlier in the document uses
> xsd:integer typed literals.
> 
> >> > > [FED]4.2 Definition of BINDINGS
> "If a WhereClause has a BindingsClause" -- WhereClause doesn't 'have' a
> BindingsClause. The grammar associates these two through SelectQuery
> (with an intervening SolutionModifier).
> 
> Section 4.2 doesn't seem to follow the same conventions as the query doc
> . For example, "eval(BindingsSolutionSequence(P,V,St)) = Join(Rbc, P)"
> -- isn't P (a GGP) an AST, not an algebra, concept?.
> 
> >> > > [FED]5 SPARQL Federation Extensions Grammar
> """
> It is a syntax error if to use a variable as the first argument to a
> ServiceGraphPattern if that variable is not bound (at least optionally)
> in the left hand side of a join with the ServiceGraphPattern on the right.
> """
> 
> "if to use" -- should be "to use"
> This text should align with Axel's(?) proposed "potentially bound"
> concept, but in general it seems like it's trying to talk about a syntax
> error defined in terms of the algebra which is going to be confusing for
> people who otherwise don't need to ever think about the algebra. Also,
> join ordering doesn't have to use the lexical ordering, so "left hand
> side" here isn't particularly useful.
> 
> 
Received on Tuesday, 5 October 2010 17:37:32 GMT

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