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Re: Bandwidth efficiency

From: Eric Prud'hommeaux <eric@w3.org>
Date: Fri, 4 Jun 2004 08:38:54 +0900
To: DAWG public list <public-rdf-dawg@w3.org>
Message-ID: <20040603233853.GC8052@w3.org>

On Wed, Jun 02, 2004 at 02:33:13PM +0100, Steve Harris wrote:
> 
> On Wed, Jun 02, 2004 at 05:58:28 -0700, Howard Katz wrote:
> > > The situation where the namespaces are declared after the results is not
> > > much better as the client cant interpret them untill its parsed all the
> > > namespace decls anyway.
> > 
> > Right. So what you're saying is that the tradeoff is basically between :
> > 
> > (1) taking processing time to compute the namespace of every uri in the
> > result set before they depart the scene, or
> > (2) leaving it to the client to do the inverse operation following arrival,
> 
> I dont think I follow. If the server just streams the results with fully
> qualified URIs then the server and client doesnt have to do anything, but
> it takes more bandwidth.
>  
> > I wouldn't have thought that (1) was all that expensive offhand -- I need to
> > have some hands-on play with this one.
> 
> It means that the sever has to hold onto the reuslt set, which can be
> quite large.

Serializing in a language with nested namespace scope ala RDF/XML, the
serializer can make a tradeoff between holding onto some amount of the
result set and risking repeated namespace decls. Pushing the slider
towards holding more of the document gives the serializer a chance to
serialize all the namespaces for nested elements once in the parent
element:

  <rdf:RDF xmlns:rdf="ob0">
    <ns1:Thing rdf:about="ob1" xmlns:ns1="..." xmlns:ns2="...">
      <ns2:p1>...</ns2:p1>
      <ns2:p2>...</ns2:p2>
    </ns1:Thing>
  </rdf:RDF>

Pushing the slider the other way introduces some inefficiency, but
still results in valid XML/RDF (note xmlns:ns2):

  <rdf:RDF xmlns:rdf="ob0">
    <ns1:Thing rdf:about="ob1" xmlns:ns1="...">
      <ns2:p1 xmlns:ns2="...">...</ns2:p1>
      <ns2:p2 xmlns:ns2="...">...</ns2:p2>
    </ns1:Thing>
  </rdf:RDF>

Taking a non-nested namespace scope, we can eliminate storing more
than one statement if the language allows namespace declarations to be
interspersed with the statements (and those declarations are valid for
the remainder of the document or until overriden by another decl).

I'm not sure what the authoritative n3 grammar is, but this one [1]
doesn't quite meet this goal:

  document ::= void 
           | statementlist;

  statement ::= subject space+ property_list
            | directive;

  statementlist ::= (statement space* ("." space*)?)* ;

  directive ::= "bind" space+ nprefix ":" uri_ref2
            | "@prefix" space+ nprefix ":" space+ uri_ref2;

For instance,
N3variantA:
  @prefix rdf=<...>
  @prefix ns1=<...>
  <ob0> rdf:type ns1:Thing .
  @prefix ns2=<...>
  <ob0> ns1:p1 "..." ;
        ns1:p2 "..." .

is legal and demonstrates minimal result set storage. However, 
N3variantB:
  @prefix rdf=<...>
  @prefix ns1=<...>
  <ob0> rdf:type ns1:Thing ;
  @prefix ns2=<...>
        ns1:p1 "..." ;
        ns1:p2 "..." .

is illegal (according that the above grammar), and, 
N3variantC:
  @prefix rdf=<...>
  @prefix ns1=<...>
  @prefix ns2=<...>
  <ob0> rdf:type ns1:Thing ;
        ns1:p1 "..." ;
        ns1:p2 "..." .

requires some lookahead (or holding onto the result set).

Note, graphs with bNodes as the subject of one arc and the object of
others can't be serialized in N3variantA.

  @prefix rdf=<...>
  @prefix ns1=<...>
  @prefix ns2=<...>
  <ob0> ns1:p0 [ns1:p1 "..." ;
                ns1:p2 "..." ] .

I bet that allowing clients to request RDF/XML is "good enough". An
ASCII (i18n alarms ring!) protocol (well, non-XML) that allows ns
decls anywhere would be more efficient, but I'd rather not wait for
the working group to invent one.

I *thought* that the XML specification said that a non-well-formed XML
document [2] was not XML and thusly, the document consumer (query
client, in this case) really ought to read an entire RDF/XML document
before extracting any triples from it. However, I can't find that
text; perhaps it was relaxed to account for looong XML streams.

> > There's one other alternative of course, but only if the store was created
> > to your own specification (which makes me realize I only have a vague
> > understanding of the various possible scenarios) :
> > 
> > (3) store the namespace (or at least some sort of integer key representation
> > of it) along with the uri for each node in the repository as it's added.
> > Then there's no latency or bandwith issues on the way out. However there
> > might be other internal processing issues that would arise in the area of
> > self-consistency and the like, and of course increased data size.
> 
> Thats what 3store v1 did, for prety much that reason, actually to
> reconstruct RDF/XML cheaply. It was too much overhead though. Slows down
> assertion and query.
>  
> > Our painful motto: No free lunch in computerland,
> 
> Too true

[1] http://dev.w3.org/cvsweb/2001/blindfold/sample/n3.bnf?rev=1.4&content-type=text/x-cvsweb-markup
[2] http://www.w3.org/TR/2004/REC-xml11-20040204/#sec-well-formed
-- 
-eric

office: +1.617.258.5741 NE43-344, MIT, Cambridge, MA 02144 USA
cell:   +1.857.222.5741

(eric@w3.org)
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Received on Thursday, 3 June 2004 19:38:53 GMT

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