Re: bNodes as graph identifiers

Hi Steve,

(This is not a formal response.)

As a database practitioner and implementer I can appreciate your 
position, but of late I've not seen a conflict between an RDF data model 
that supports bnodes as graph labels, and a database technology that 
does not and cannot use them internally.  The RDF data model in this 
case applies to possible serialization formats and parsing, but it seems 
to me that the internal representation of bnodes inside of a database 
will always be tied to some kind of more universal identifier.

We know that bnode identifiers are meaningless outside of the context of 
a given document.  Consequently we can't require that bnode identifiers 
be preserved when copying/merging datasets; so the requirement that your 
system be able to ingest (and skolemize) bnodes doesn't seem to require 
that RDF itself not use bnodes as graph labels.

In other words, I don't see why this change would necessarily affect 
your implementation, except as far as ingestion goes.

Does that at all start to mitigate your objection?

Charles


On 05/29/2013 10:47 AM, Steve Harris wrote:
> [ as a side note I find it bizarre that I'm having to advocate NOT changing a 14
> year old, industrially deployed spec, at the 11th hour of the standardisation
> process, to add a feature that's used by a tiny minority of deployed systems -
> if anything was to strike an outsider as peculiar about this WGs process, it
> would surely be this feature ]
>
> TL;DR: don't mess.
>
> We know that bNode graph identifiers are possible (I've designed a system myself
> that had them) and that there are usecases that are addressed by it, but I've
> not heard anything yet that can't already be addressed using RDF/SPARQL as it
> stands. It is the opinion of some people that bNodes as graph identifiers
> address it better, in some way, but that's another matter.
>
> There are however some costs to extending RDF (datasets) to require that bNodes
> be usable as graph identifiers:
>
> * We (Experian) have invested millions of dollars in our RDF engine - it's very
> tightly optimised to the current specs, and opening up the space of graph
> identifiers from a single class (URIs) to two classes (URIs and bNodes) would
> have a significant engineering, and storage cost. Put simply, we wouldn't do it,
> and would just step away from later RDF specs, becoming an RDF/SPARQL flavoured
> graph database.
>
> * RDF is already too complex for people coming into it to learn easily. Every
> time we add a new feature to the language we increase the barrier to entry.
>
> * There's no practical way to refer to long lived bNodes in SPARQL (without
> enforced skolemisation), people will import datasets with bNode graphs, and then
> realise they can't isolate their data (presumably after posting on stack
> overflow or similar :) ).
> The following will not retrieve your original data* and this will just promote
> more confusion:
>
> 	Data:
> 	_:abc { :s :p :o }
>
> 	Query:
> 	SELECT * WHERE { GRAPH _:abc { :s ?p ?o } }
>
> 	you could possibly do something like:
>
> 	SELECT * WHERE { GRAPH ?g { :s ?p ?o } } FILTER(STR(?g) =abc")
>
> 	That's pretty inconvenient, in many ways, and isn't required to work by SPARQL 1.1.
> 	It is only possible at all in systems that preserve bNode labels, which is not
> required.
>
> * Confusion with bNodes-in-graphs, and bNodes-as-graph-identifiers - the
> discussion seems to assume that they're separate kinds of thing, maybe with
> identifier bNodes not being existential variables? Which ever way it goes the
> relationship between bNodes-in-graphs, and bNodes-identifying-graphs is going to
> be complex.
>
> * Of all the extensions that are implemented by a small number of systems as an
> extension, this seems like an odd one to pick. IMHO there are far more serious
> problems with RDF. There is a cost (to this group, and the wider community) of
> any changes, so lets pick our battles wisely.
>
> * There's very little implementation experience - compared to the other things
> we're standardising: URI quads, bNode skolemisation, Turtle, NQuads. It's not
> clear how far the existential variable-ness should extend - do we sanction graph
> leaning? Do URI-identified graphs infer identical graphs identified by bNodes?
> If not, why not? What do bNodes with a given label, in graphs identified by a
> bNode with a different label refer to, etc.
>
> 	_:abc {
> } _:def (
>
> } One graph, or two, or undefined? I don't think we know the right answer yet.
> So, in summary, I think the cost is high, and the benefit is vanishingly small.
> Nothing stops people that feel they really need it adding them to RDF systems,
> as they have in the past. One counter argument is that JSON-LD will do it
> anyway, but that's fine - if it is widely used, it can be adopted into RDF 1.2,
> with plenty of implementation experience. In the meantime JSON-LD serialisers
> can skolemise when transforming JSON-LD into RDF - there's other places where
> the transform is lossy anyway, as far as I understand it. - Steve * this was
> possibly an error in the SPARQL 1.0 spec, but sadly the bNodes as variables
> feature is quite widely used, and many people argued in favour of the feature.
>
> -- Steve Harris Experian +44 20 3042 4132 Registered in England and Wales 653331
> VAT # 887 1335 93 80 Victoria Street, London, SW1E 5JL

-- 
Charles Greer
Senior Engineer
MarkLogic Corporation
charles.greer@marklogic.com
Phone: +1 707 408 3277
www.marklogic.com

Received on Thursday, 30 May 2013 20:21:57 UTC