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Re: Protocol extensions for federated querying

From: Andreas Langegger <al@jku.at>
Date: Wed, 21 Oct 2009 15:02:42 +0200
Cc: public-rdf-dawg@w3.org
Message-Id: <FBD3462F-33CD-4387-BA3E-C60F76014791@jku.at>
To: Paul Gearon <gearon@ieee.org>
Hi Paul,

+1 - would like to see that in SPARQL/Query1.1 also!

However, I think it would be more convenient, compact and also require  
less markup if initial bindings can be submitted as part of the query  
and not in the post attachment. Small queries could still be issued  
via GET and if there are many bindings, the client just can use POST  
anyway.

I have implemented a BINDINGS extension in ARQ, demo running at
http://ramses.faw.uni-linz.ac.at:8900/snorql/?query=SELECT+*+WHERE+%7B%0D%0A++%3Fs+a+%3Ftype%0D%0A%7D+BINDINGS+%3Ftype+%7B%0D%0A++bsbm%3AProduct+.%0D%0A++foaf%3APerson+.%0D%0A%7D

Example with multiple variables (empty bindings may be specified with  
"null"):
SELECT * WHERE {
   ?s :p ?a ; :p ?b ...
} BINDINGS ?a ?b {
   bsbm:Product "34"^^xsd:int .
   null "23"^^xsd:int .
   foaf:Person . // remaining slots are interpreted as empty (null)
}

The evaluation is simply a Join in ARQ against an OpTable which is the  
materialized solutions supplied. Very simple to implement actually and  
worth having it in future SPARQL.

For scalable federation over public SPARQL endpoints I'm however more  
than sceptical since I've done much research and experiments towards  
this direction. My SemWIQ [1] mediator is working with patched  
endpoints only that support SPARQL BINDINGS and RDFStats [2]. I think  
issuing COUNT queries before may not scale well. Initial bindings  
mainly reduce the latency times for HTTP connections, but it does only  
linearly speed up federation. If there are many distributed joins,  
even bind joins (dynamic optimization by substitution) becomes  
troublesome...

Regards,
Andy

[1] http://semwiq.sourceforge.net
[2] http://rdfstats.sourceforge.net


On Oct 20, 2009, at 9:51 PM, Paul Gearon wrote:

> Hi everyone,
>
> This meets the commitment I made for ACTION-124.
>
> So far, all the comments I've seen on federated queries have been
> about the suggested query syntax. To date I'm in agreement with what
> I've seen proposed.
>
> I am also interested in extending the protocol to support federation a
> little better. At the moment, all queries are done as a simple request
> via a GET or a POST. In the case of POST, the endpoint alone is
> provided in the URL, and the query appears in the body.
>
> I'd like to see a form of POST that includes a SPARQL variable binding
> result in the body (a la http://www.w3.org/TR/rdf-sparql-XMLres/). In
> this way the receiving query engine can work with prebindings that are
> provided to it, allowing it to reduce the result that is to be
> streamed back to the calling engine.
>
> To give an example, I'll reference the two datasets found in 8.3 of
> the SPARQL Query Language document:
> http://www.w3.org/TR/rdf-sparql-query/#queryDataset
>
> If we make the presumption that the named graph
> http://example.org/foaf/aliceFoaf can be found at
> http://sparql.org/sparql/, then I might want to issue the following
> query to get the names of people whose nicknames are in the bobFoaf
> graph:
>
> SELECT ?nick ?name
> FROM <http://example.org/foaf/bobFoaf>
> WHERE {
> ?p1 foaf:nick ?nick .
> ?p1 foaf:mbox ?mbox
> SERVICE <http://sparql.org/sparql/> {
>   SELECT ?mbox ?name
>   FROM <http://example.org/foaf/aliceFoaf>
>   WHERE { ?p2 foaf:mbox ?mbox . ?p2 foaf:name ?name }
> }
> }
>
>
> The part of the query in the SERVICE block would usually return the  
> following:
> <?xml version="1.0"?>
> <sparql xmlns="http://www.w3.org/2005/sparql-results#">
> <head>
>   <variable name="mbox"/>
>   <variable name="name"/>
> </head>
> <results>
>   <result>
>     <binding name="mbox"><uri>mailto:alice@work.example</uri></ 
> binding>
>     <binding name="name"><literal>Alice</literal></binding>
>   </result>
>   <result>
>     <binding name="mbox"><uri>mailto:bob@work.example</uri></binding>
>     <binding name="name"><literal>Bob</literal></binding>
>   </result>
> </results>
> </sparql>
>
> Note that this is information for both Bob and Alice. This can then be
> joined to the remainder of the query, which reduces the results to
> just Bob.
>
> However, a query engine may instead want to evaluate Bob first. This
> may be desirable if some COUNT queries have already been issued, and
> the query engine knows that the results of the SERVICE block will
> return a large number of results, while the local data would bind
> ?mbox to only a few values. In that case, the local binding of ?mbox
> could be sent along with the query (?p1 and ?nick are not necessary
> for the remote service). This could be accomplished using a POST that
> has the query in the URL, and the bindings in the body.
>
> POST /sparql/?query=SELECT+%3Fmbox+%3Fname+FROM+%3Chttp%3A%2F 
> %2Fexample.org%2Ffoaf%2FaliceFoaf%3E+WHERE+%7B+%3Fp2+foaf%3Ambox+ 
> %3Fmbox+.+%3Fp2+foaf%3Aname+%3Fname+%7D
> HTTP/1.1
> Content-Length: xxxxxx
> Content-Type: multipart/form-data;  
> boundary=ZpwZZc62ZXXjf0InvlrBjTWNrJSp--FL
> Host: sparql.org
> Connection: Keep-Alive
> User-Agent: example
>
> --ZpwZZc62ZXXjf0InvlrBjTWNrJSp--FL
> Content-Disposition: form-data; name="query-prebinding"
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
>
> <?xml version="1.0"?>
> <sparql xmlns="http://www.w3.org/2005/sparql-results#">
> <head>
>   <variable name="mbox"/>
> </head>
> <results>
>   <result>
>     <binding name="mbox"><uri>mailto:bob@work.example</uri></binding>
>   </result>
> </results>
> </sparql>
>
> --ZpwZZc62ZXXjf0InvlrBjTWNrJSp--FL--
>
> With this pre-binding, the remote query engine is able to reduce it's
> results to just the one for Bob, thereby cutting the returned size
> down by nearly half.
>
> One potential issue is for very long queries that also want to be
> placed into the body of a POST. In that case we could simply define
> the names of each section (in the example above I've used a name of
> "query-prebinding").
>
> What do others think? Does this proposal have merit?
>
> Regards,
> Paul Gearon
>


http://www.langegger.at
----------------------------------------------------------------------
Dipl.-Ing.(FH) Andreas Langegger
FAW - Institute for Application-oriented Knowledge Processing
Johannes Kepler University Linz
A-4040 Linz, Altenberger Straße 69
Received on Wednesday, 21 October 2009 13:29:16 GMT

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