W3C home > Mailing lists > Public > public-rdf-wg@w3.org > April 2012

Re: "layers" (was Re: the term "named graphs")

From: Sandro Hawke <sandro@w3.org>
Date: Sun, 29 Apr 2012 15:28:48 -0400
To: Pat Hayes <phayes@ihmc.us>
Cc: Dan Brickley <danbri@danbri.org>, Andy Seaborne <andy.seaborne@epimorphics.com>, public-rdf-wg@w3.org
Message-ID: <1335727728.9663.856.camel@waldron>
On Sat, 2012-04-28 at 22:44 -0500, Pat Hayes wrote:
> I am not following this AT ALL.  Can you expand slightly? Detailed questions inline below.

It turns out I can expand at great length, but not slightly.

I'm posting as a new, non-reply message about Layers, in just a minute.
I hope it addresses your questions.   

     -- Sandro

> On Apr 28, 2012, at 8:04 AM, Sandro Hawke wrote:
> 
> > On Sat, 2012-04-28 at 12:57 +0200, Dan Brickley wrote:
> >> On 28 April 2012 11:58, Andy Seaborne <andy.seaborne@epimorphics.com> wrote:
> >>> On 28/04/12 05:49, Sandro Hawke wrote:
> >>>> My concern is with how people
> >>>> use the term in practice, and whether that usage conflicts with the
> >>>> formal definition.
> >>> 
> >>> General usage is sloppy, imprecise and changes as convenient. Ambiguity in
> >>> spoken language is normal.  We all manage.  But we are not all managers.
> >> 
> >> Yes, we have same issue with 'property', 'triple', 'statement' and
> >> others; each might (if we're lucky) have a precise W3C RDF meaning,
> >> but they shade into other related uses that there can't be such strict
> >> standardised control over.
> >> 
> >> Property is probably the oddest. Sometimes in computing 'color',
> >> 'size' etc. are themselves called properties, sometimes the size of
> >> some particular thing is counted as one of its properties, and if it
> >> had two different colours, they're each a property. So RDF properties
> >> are close cousin to
> >> http://en.wikipedia.org/wiki/Property_(programming) but different too;
> >> I think we gain more by neighbourhood benefits than we suffer from
> >> sloppyness and confusion there.
> > 
> > Yeah, lots of situations distinguish between attributes and properties
> > and relations; we mush them together.   If we were starting with a blank
> > slate, I'd suggest that "aspect" might be the best match for what we
> > mean.
> 
> Blech. In logic they are all relations with different numbers of arguments. All other names are just flimflam. Aspect sounds totally wrong to me. Are our fathers aspects of us? If I own a house, is it one of my aspects? Doesnt make first cut. 
> 
> > 
> >> "Named graph" by contrast is pretty much our phrase to do with as we
> >> will (e.g. first page of google results are all "ours") . My guess is
> >> that usage will get murky if we don't have a sloppier not-so-nitpicky
> >> phrase also to throw around.
> > 
> > I think "named graph" is already being thrown around quite sloppily.
> > Trying to answer my own survey, I found even I was very comfortable with
> > the sloppy usage.
> > 
> > I'd rather come up with some precise new terms, and allow named graph
> > that sloppy usage (in addition to the precise (u.G) meaning it also has
> > in the SPARQL spec.
> > 
> >> I've pretty much convinced myself that "layer" is the best metaphor
> >> there, and that we could productively encourage talk of data 'layers'
> >> while leaving 'named graph' as the thing that has a much more rigid
> >> official meaning.
> > 
> > I like that term, "layer".  Excellent....
> 
> To me it suggests a vertical dimension being involved. One layer is 'above' another, and this presumably means something rather important. Deep layers are deep for a reason, I guess... What reason?  (if not, why this usage?) 
> 
> > 
> > For me, it works pretty well for what the fourth term in the quad
> > denotes.
> 
> ?? Not to me. What is the intuitive depth dimension for quad-4ths? Is it the date the IRI was coined, maybe?  When does one layer cover or hide another? 
> 
> >  It's similar to "graph container" but suggests much more
> > strongly that it functions best as part of a whole.
> 
> Wha?? What is it about "layer" suggesting that? The cretacious is a layer. What is it part of?  (Do we speak the same language? Come to think of it, maybe not.) 
> 
> >  Different
> > subgraphs are in different layers; you can look at just one layer, or
> > look at the union of several layers.   It's not entirely intuitive that
> > nodes and arcs (especially blank nodes) can be in multiple layers,
> 
> Indeed. FWIW, this is exactly what the 'surfaces' idea was supposed to *prevent*. Each bnode is on a single unique surface: that was the whole point. 
> 
> > but
> > it's not too counter-intuitive either, I think.   I picture any node
> > that occurs in the same location on two layers as being the same node.
> 
> Aaarghh. My head is exploding. The same LOCATION in two layers? what can that possibly even MEAN? That like saying the same street in two countries. 
> 
> > 
> > So, let's look at the example dataset I used on the survey, without its
> > default graph for now:
> > 
> >                @prefix :    <http://example.org/>
> >                :g1 { :a :b 10 }
> >                :g2 { :a :b 20 }
> >                :g3 { :a :b 10 }
> > 
> > If you make the unique names assumption, then we have three layers.
> > 
> > If you make the closed world assumption (as one has to do during some
> > database operations) -- that the triples we see here are all the triples
> > there are in these layers, then we have either two or three layers.  We
> > can't tell if g1 and g3 name the same layer.   Since they have  the same
> > set of triples on them, they might be the same layer.  We can tell that
> > g1 and g3 each do not name the same layer as g2, since clearly their
> > layers have different triples on them.
> 
> But with incomplete semantics, each could have the ones it has and some others, so they could all be the same or indeed all different (maybe :g1 also includes :c :d 15 and :g3 also has :f :g 26)
> 
> BUt in any case, isn this all about named graphs? What has this got to do with layers? 
> 
> > 
> > If you don't make either the UNA or the CWA, it would be possible that
> > even g1 and g2 would be names for the same layer.   For example, if we
> > later learned...
> > 
> >                :g2 { :a :b 10 }
> >                :g1 { :a :b 20 }
> > 
> > then, as far as we knew, the layers would have the same triples on them.
> > 
> > I continue to like that a lot. 
> > 
> > What about Web Architecture?  If the name of a layer is dereferenceable,
> > is it reasonable to expect/require the returned content to be a
> > serialization of all the triples on that layer?   I think it probably
> > is.    So we'd start to think about the different published and
> > maintained foaf files, doap files, environmental quality surveys (in
> > RDF), etc, each being a "layer".   The triples on the layer can change
> > over time, but the layer is still a thing.  
> 
> That was one of the purposes of the surfaces idea, yes. They are an abstract model for g-boxes. But I didnt pitch this to the WG as we already had g-boxes, and that seemed to work OK. 
> 
> > 
> > So, "layer" is the new "g-box".   (Maybe "RDF Layer" when we need to be
> > formal.)  I think it's a much better term.  One cool thing is how much
> > it raises the question, "layer in what?"    And that's a great question
> > to be asking; of course, a dataset is a collection of layers,
> > appropriately stacked, with their names attached, and one of them
> > flagged as the "default".
> > 
> > Preliminary +100 on "layer".
> 
> If we are going to start pitching names, I will re-open my "surfaces" idea, described in http://www.slideshare.net/PatHayes/rdf-redux. It does have the great merit of completely getting rid of the merge/union terminology. Those slides dont mention SPARQL datasets, but in that terminology they would be a set of named surfaces and a default surface, each with an RDF graph on it. OR, if we want bnodes to be shared, they would be a single surface with the named graphs and default graph on it. 
> 
> We dont have to stick to the name "surface". I do like words like 'sheet' (of paper), 'page', etc.., however. 
> 
> Pat
> 
> 
> > 
> >    -- Sandro
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> 
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Received on Sunday, 29 April 2012 19:28:58 UTC

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