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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

> 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


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Received on Thursday, 19 March 2009 22:09:10 UTC

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