W3C home > Mailing lists > Public > semantic-web@w3.org > March 2011

Re: a blank node issue

From: Reto Bachmann-Gmuer <reto.bachmann@trialox.org>
Date: Thu, 3 Mar 2011 08:55:33 +0100
Message-ID: <AANLkTin_j=6F8AAHTY-J-HXBSyE8+a3e8yuA-VfJhNK8@mail.gmail.com>
To: Richard Cyganiak <richard@cyganiak.de>
Cc: Jiří Procházka <ojirio@gmail.com>, Pat Hayes <phayes@ihmc.us>, William Waites <ww@styx.org>, nathan@webr3.org, Ivan Shmakov <oneingray@gmail.com>, semantic-web@w3.org
On Wed, Mar 2, 2011 at 8:35 PM, Richard Cyganiak <richard@cyganiak.de>wrote:

> Reto,
>
> On 2 Mar 2011, at 18:50, Reto Bachmann-Gmür wrote:
> > > Is there any practical difference between bnodes and normal nodes,
> > > except the scope (and necessity) of their name?
> >
> > Yes, a graph with bnodes can potentially be simplified: the same meaning
> may be expressed with a more lean graph, i.e. with less nodes and triples.
> If all your nodes are uris you cannot do simplifications with rdf
> entaillment.
>
> Reality check please!
>
I thought the discussion was about how to best use the technologies at hand,
not about which technologies are widely deployed.

>
> When was the last time you saw such a non-lean RDF graph in the wild,
> outside of examples and test cases?

Your foaf file could be simplified by 6 triples, but ok in this case only
using OWL entailment.

When was the last time you aggregated data and the resulting union was lean
right away?


> Can you name a production system that routinely performs the simplification
> you talk about, with user benefit?
>
No, general purpose leaninification can be computationally expensive. But,
I've rarely dealt with RDF applications that do not have mechanisms to
prevent or remove redundancies. While this code typically uses application
and domain specific knowledge, it is important to know by means of RDF
entailment that the simplifications do not change the meaning expressed by
the graph.


> The question was about practice. You describe a thought experiment. I think
> it's a good example of a complication in RDF that was added for sound
> theoretical reasons, but has failed to deliver any value whatsoever in
> practice.
>

not sure where I used thought experiments....

Even if automatic leanification cannot be used on a large scale, the
concepts are still important to decide how to best express things. you may
not be able to convince people to leanify graphs, but you have strong
arguments that their non-lean graphs express the same as their lean
counterpart.

Besides creating redundant graphs starts on the first slides of an rdf
introduction, a simple aggregator:

g = new Model
while(true) {
  g load <http://.../some.rdf>
  sleep(...)
}

If some.rdf contains bnodes g will be non-lean after two iterations.

Cheers,
Reto
Received on Thursday, 3 March 2011 07:56:10 UTC

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