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Re: What we mean by "graph" / Named Graphs in SD

From: Steve Harris <steve.harris@garlik.com>
Date: Wed, 21 Jul 2010 09:50:54 +0100
Cc: SPARQL Working Group <public-rdf-dawg@w3.org>
Message-Id: <7459A513-1683-4F81-AFCD-C6E24270D345@garlik.com>
To: Sandro Hawke <sandro@w3.org>
Sandro,

I think you're right, but it's probably already too late. Although SPARQL 1 didn't sanction mutable graphs - in reality most implementations have supported this since before SPARQL 1, and I'm pretty sure that users I interact with think of "graphs" as mutable objects.

Probably us implementors are guilty of spreading confusion here, I don't think I ever put any real effort into using correct terminology in this area.

- Steve

On 2010-07-20, at 19:22, Sandro Hawke wrote:

> One of the downsides of trying to do a lot is that we make mistakes.
> Or, at least, I do...
> 
> On the Service Description issue, I made the mistake of thinking a
> NamedGraph was a Graph.   
> 
> Andy made a comment that led me to actually look in Carroll et al 2005
> and see that, no, formally speaking "Named Graphs" would be better
> called "graph namings".  They are disjoint from RDF graphs; they consist
> of at least a "name" and an "rdfgraph".   So when I say it's crazy that
> named graphs are named with themselves -- the craziness really rests in
> Carroll et al calling something a named graph when it is not a graph.
> The NamedGraph may well have a name, but that's distinct from its
> "name", which is what it associates with its "rdfgraph".   (Arg!)
> 
> I wonder if there's a way out of this mess....    I guess we can at
> least clarify it in the SD document, with some warnings.   Anyone
> interesting in us getting away from the misleading term "NamedGraph"?
> It may seem like it's entrenched in SPARQL 1.0, but this is just
> editorial, as long as it keeps the GRAPH and FROM NAMED keywords, which
> actually seem fine to me.   (I expect we'll have a new RDF Core WG
> working on this issue in a few months; that's when this will really get
> entrenched.  I expect to strongly oppose this misleading use of the term
> "named graph" during that work, if necessary.)
> 
> Aside from that, it would help a little to change sd:name to
> sd:graphName, but it should still be a string if it's like that.
> (Maybe we should add RIF's pref:iri-string as a builtin, to address
> Andy's use case.  Remind me why RIF and SPARQL have their own builtins
> and you can't just borrow from the other?)   Alternatively just use the
> name as the URI label on the graph node; that seems fine...
> 
> Hmmm.
> 
> Meanwhile, I've been meaning to send a question about our use of the
> term "Graph", which is connected here.
> 
> It seems to me there are two different common meanings for the term
> "RDF Graph".  To use the AI terms for each of them:
> 
>        1. A Knowledge Base (KB); a specific repository or store of RDF
>        triples.  As in, "Please update your graph to remove the triple
>        <a> <b> <c>."
> 
>        2. A Formula; a mathematical set of RDF triples.   As in, "Graph
>        G1 entails infinite other graphs".
> 
> The most crisp distinction may be around identity.   Two formulas are
> identical if and only if they contain the same triples.  Meanwhile, KBs
> can have the same triples while remaining distinct.   It also makes
> sense to talk about the state of a KB, and a KB changing over time.  It
> makes no sense to say such things about a formula; it's just a pure
> mathematical set.
> 
> I think we can agree that formally, technically, only definition 2
> (formulas) is correct.  But I think meaning two is in common use; I
> expect most of us use it often.    When I say "graph" in the sense of
> definition 1, I mean it as shorthand for "graph storage location",
> "graph data structure", or "graph store".   In spoken language, the
> context usually makes it clear whether people mean KB or formula.
> 
> The distinction also didn't matter much in SPARQL 1, I think, because
> it was agnostic on mutability, identity, etc.  I guess this will come
> up in the update semantics for 1.1.  And it may come up in the SD
> issue, above.
> 
> I wonder if we could agree on a standard name for sense 1, and try to
> use it.   (Or maybe we already did, and I missed it.)   As long as it's
> a term like "graph storage location", then using the keyword "GRAPH" as
> we do in the query language seems fine.
> 
>    -- Sandro
> 
> 
> 
> 

-- 
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Received on Wednesday, 21 July 2010 08:51:31 GMT

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