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Re: federated query review, reprised

From: Carlos Buil Aranda <cbuil@fi.upm.es>
Date: Tue, 03 May 2011 17:03:57 +0200
Message-ID: <4DC0195D.6060007@fi.upm.es>
To: Lee Feigenbaum <lee@thefigtrees.net>
CC: SPARQL Working Group <public-rdf-dawg@w3.org>, Eric Prud'hommeaux <eric@w3.org>, Axel Polleres <axel.polleres@deri.org>
  dear all,

I addressed all comments but the one about the conformance section, 
which I do not really know what to write there.

Carlos

On 28/04/2011 5:09, Lee Feigenbaum wrote:
> I've revisited my review of the federated query document based on 
> Carlos's changes documented on 
> http://www.w3.org/2009/sparql/wiki/To_Last_Call/Federated_Query_Review. This 
> also discharges ACTION-433, as I took a look at the protocol service / 
> endpoint terminology.
>
> == Substantive Comments ==
>
> These comments must be resolved before the document is ready for Last 
> Call. Editorial comments follow after, which can be addressed either 
> before or after Last Call, though many of them would help the 
> readability of the document.
>
> 2.3 -- we can't link to a definition on a wiki. If we need that 
> definition, it needs to be incorporated into this document. It also 
> should be incorporated into the semantics section, rather than in the 
> context of an example.
>
> 3.1
>
> * The syntax transformation needs to handle "SILENT"
>
> Definition: Evaluation of a Service Pattern
>
> * Invocation(IRI, S, P, B) needs to be extended to take a 5th 
> parameter, the boolean for silent invocation or not.
>
> * Why are only variables in the intersection of vars and bound 
> selected? This means that I can't do this:
>
> SELECT ?x {
>   SERVICE <iri> { ?x a foaf:Person }
> }
>
> ...which seems strange to me. Is this intentional?
>
> * The second call to "Invocation" needs to include silentOp
>
> * " return unbounded variables." needs to be more formal. It needs to 
> return a single solution with no bindings.
>
> 4 SPARQL 1.1 Federated Query Grammar -- this document only defines 
> SERVICE, not BINDINGS. This section should be changed to reflect that.
>
> 5 Conformance
>
> I'm still not sure that this conformance section makes sense here. I 
> need to think more about what would make sense. Perhaps Eric or Axel 
> has a suggestion here.
>
> Remove section 7 completely.
>
> == Editorial Comments ==
>
> 1. Introduction
>
> Suggest rewording as follows:
>
> """
> The SERVICE extension allows one to direct a portion of a query to a 
> particular SPARQL query service, similar a GRAPH graph pattern, which 
> "directs" queries to particular named graphs in the (local) dataset . 
> This specification defines the syntax and semantics of this extension.
> """
> =>
> """
> This specification defines the syntax and semantics of the SERVICE 
> extension to the SPARQL 1.1 Query Language. This extension allows a 
> query author to direct a portion of a query to a particular SPARQL 
> endpoint to be executed against local graphs. Results are returned to 
> the federated query processor and are joined with results from the 
> rest of the query.
> """
>
> (and add a link from "SPARQL endpoint" to 
> http://www.w3.org/2009/sparql/docs/protocol-1.1/Overview2.xml#terminology)
>
>
> 1.1.2 Result Descriptions
>
> Suggest adding this and removing the similar sentence from 1.1.3:
>
> """
> This example illustrates a solution sequence for a query that projects 
> three variables, ?x, ?y, and ?z. The solution sequence has a single 
> solution mapping in which ?x is bound to the plain literal "Alice", ?y 
> is bound to the IRI http://example/a, and ?z is not bound.
> """
>
> 1.1.3 Terminology
>
> Remove " (corresponds to the Concepts and Abstract Syntax term "RDF 
> URI reference")" after "Solution Mapping".
>
> Remove "In this result set..."
>
>
> 2 SPARQL 1.1 Basic Federation Extension
>
> Suggest renaming section to "SPARQL 1.1 Federated Query Extension"
>
> Suggest rewording the first paragraph based on the new introduction 
> text, as follows:
>
> """
> Queries over distributed SPARQL endpoints often involves querying one 
> source and using the acquired information to constrain queries of the 
> next source. This section illiustrates how this can be achieved using 
> SPAQL1.1's SERVICE Graph patterns by examples.
> """
>
> =>
>
> """
> The SERVICE keyword instructs a federated query processor to invoke a 
> portion of a SPARQL query against a remote SPARQL protocol service. 
> This section presents examples of how to use the SERVICE keyword. The 
> following sections define the syntax and semantics of this extension.
> """
>
> 2.1 Simple query to a remote SPARQL endpoint
>
> Suggest rewording:
>
> """
> This example shows how to query a remote SPARQL endpoint and join the 
> returned data with the data at local RDF data store. Imagine we want 
> to know who do I know. Data about people is in 
> <http://people.example/sparql> endpoint:
> """
> =>
> """
> This example shows how to query a remote SPARQL endpoint and join the 
> returned data with the data from the local RDF data store. Consider a 
> query to find the names of the people I know. Data about the names of 
> various people is available at the <http://people.example/sparql> 
> SPARQL endpoint:
> """
>
> 2.1 and following -- the examples are much improved. There is still 
> some inconsistency between the data in the remote endpoints and the 
> URI of the endpoints. 2.1 uses "people.example" and 2.2 uses 
> "people.example.org", for instance. It would be good to make these all 
> match up to help the reader follow the examples without being confused.
>
>
> 2.2
>
> I think this example needs a couple of changes:
>
> * The URIs of the services in the description does not match the URIs 
> in the query
>
> * The query nests one SERVICE inside another. This requires the first 
> service to be a federated query processor. This should be mentioned in 
> the example, or the example changed so that the OPTIONAL is outside of 
> the SERVICE clause (in which case the foaf:interest triples would need 
> to be local).
>
>
> 2.3 Variable Services
>
> "in the default graph" => "in the local default graph"
>
> """
> When having variables for specifying the address of a SPARQL endpoint 
> in a SERVICE operation this variable must be bounded.
> """
> =>
> """
> A variable used with the SERVICE keyword must be bound.
> """
>
> (See comment above.)
>
> 2.4
>
> """
> the query will stop returning the corresponding SPARQL error.
> """
> =>
> """
> the query will stop and return the error.
> """
>
> 2.4 - Can we add &nbsp; or something in the table cell in the results 
> to make it render as a full-height empty cell to show that there is 
> one row in which ?name is not bound?
>
>
> 2.5 As per 
> http://lists.w3.org/Archives/Public/public-rdf-dawg/2011AprJun/0119.html, 
> I'd remove this example completely and substitute the text in that 
> email message. While SERVICE and BINDINGS _can_ be used in the same 
> query, that is not the "natural" intent of things -- instead, the 
> natural intent is that a federated query implementation would 
> _generate_ queries with BINDINGS to help constrain the results from 
> the remote endpoint. An example showing this would be helpful, but is 
> probably too much before Last Call, so I'd remove this example entirely.
>
>
> thanks,
> Lee
>
Received on Tuesday, 3 May 2011 15:04:20 GMT

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