W3C home > Mailing lists > Public > public-rdf-dawg@w3.org > January to March 2009

Re: [BlankNodeRefs] Questions on blank node refs

From: Chimezie Ogbuji <ogbujic@ccf.org>
Date: Thu, 19 Mar 2009 18:08:09 -0400
To: "Seaborne, Andy" <andy.seaborne@hp.com>, "Lee Feigenbaum" <lee@thefigtrees.net>, "SPARQL Working Group" <public-rdf-dawg@w3.org>
Message-ID: <C5E83A89.871C%ogbujic@ccf.org>
On 3/19/09 6:05 AM, "Seaborne, Andy" <andy.seaborne@hp.com> wrote:
>> Here are my questions & concerns:
>> 
>> 1/ implementation burden on systems that treat blank nodes as
>> existentials -- in particular here I'm thinking of any SPARQL engine
>> that has extended SPARQL BGP matching as per 12.6 in the spec [1] with
>> some level of entailment that looks at blank nodes as existentials.

Perhaps an example showing this burden would help?

>> 2/ interoperability - I question whether this is a key feature to
>> standardize to promote interoperability. Blank node labels used via this
>> feature will not, of course, be re-usable across implementations. I
>> realize that an argument can be made that this promotes interoperability
>> of applications that rely on re-using blank node refs, but I'm not
>> thoroughly convinced that that meets my (personal) bar for the need to
>> promote interoperability.
> 
> Agreed - they only make sense referring back to a data source that issued the
> blank node in the first place.

Yes, plus the motivating use case for this feature is where subsequent
queries are dispatched to the *same* data source.  So, I'm not sure if the
argument for interoperability applies here (as stated).
 
>> 3/ implementation cost for systems that do not maintain persistent
>> labels for blank nodes - I have in mind here things like systems that
>> download static RDF files on the fly to query against, or federated
>> query approaches that need to re-serialize blank nodes ids retrieved
>> from other SPARQL endpoints to avoid label clashes. It seems like a
>> potentially unnecessary burden to require these sorts of systems to
>> maintain new persistent state to handle blank node refs.
> 
> Agreed.  We can't require reading a file twice to preserve bnode labels
> because that's wrong.  It only applies to persistent data and even then some
> systems only guarantee the label for the duration of a session or some other
> system concept.

There should be some notion of 'compliance levels' (for a lack of a better
phrase) such that systems that do not have an identification mechanism for
bnodes (or even a system for persisting them) would not be able to provide
such a feature and the agent dispatching the query would be informed in some
way.

> One possibility here is to write a working group note that documents the usage
> but does not make it a fully-fledged feature of SPARQL (i.e. in a REC)

The number of implementations that provide non-standard ways to address
Bnodes is (for me) an indication of a legitimate need in the community.  It
would be nice to standardize this capability (as a REC track feature) rather
than to continue to have implementations do their own thing.

-- Chimezie


===================================

P Please consider the environment before printing this e-mail

Cleveland Clinic is ranked one of the top hospitals
in America by U.S. News & World Report (2008).  
Visit us online at http://www.clevelandclinic.org for
a complete listing of our services, staff and
locations.


Confidentiality Note:  This message is intended for use
only by the individual or entity to which it is addressed
and may contain information that is privileged,
confidential, and exempt from disclosure under applicable
law.  If the reader of this message is not the intended
recipient or the employee or agent responsible for
delivering the message to the intended recipient, you are
hereby notified that any dissemination, distribution or
copying of this communication is strictly prohibited.  If
you have received this communication in error,  please
contact the sender immediately and destroy the material in
its entirety, whether electronic or hard copy.  Thank you.
Received on Thursday, 19 March 2009 22:09:10 GMT

This archive was generated by hypermail 2.3.1 : Tuesday, 26 March 2013 16:15:38 GMT