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Re: [GRAPHS] g-box - abstraction or concrete?

From: Nathan <nathan@webr3.org>
Date: Sun, 27 Feb 2011 21:15:07 +0000
Message-ID: <4D6ABEDB.5070002@webr3.org>
To: Ivan Herman <ivan@w3.org>
CC: public-rdf-wg WG <public-rdf-wg@w3.org>, Sandro Hawke <sandro@w3.org>, Pat Hayes <phayes@ihmc.us>, Manu Sporny <msporny@digitalbazaar.com>
Ivan Herman wrote:
> On Feb 25, 2011, at 21:48 , Nathan wrote:
> [snip]
>> a g-box is a container of statements which form a particular view of a subset of the universe of discourse, the container is stateful such that it (potentially) contains different statements over time, at any one time the statements in the container form a set which can be considered the current state of that container (g-snap) and they form a current view of a the particular subset of the universe of discourse which they describe.
>> A g-box is a stateful abstraction whose state is managed by an abstract protocol, the abstract protocol is realized via various machine protocols which manage the state of the g-box via messages and pass full or partial representations of the current state (g-snap) in various lexical forms (g-texts).
>> A g-box can be given a name, and when a g-box is given a name the name becomes a namespace since the g-box is a container, and this namespace serves as the scope for all things within the g-box (statements/names/nodes). Thus a named-g-box becomes an Aristotelian abstraction where the current state of that named-g-box forms a particular scoped view of subset of the universe of discourse.
>> Since a g-box is an abstraction, it cannot be duplicated or replicated (I'm tempted to say a g-box is a Platonic abstraction and a named g-box is an Aristotelian abstraction), however two g-box's can share the same name(s) and machine protocols can be used to try and synchronize the current state of the g-box's sharing the same name such that they all offer the same view of the subset of the universe of discourse which they describe. This process can be seen as forking a g-box at it's current state to create a new g-box with the same current-state (g-snap), then pulling/pushing changes to the state in order to keep them aligned and sharing the same view / saying the same thing.
>> make sense?
> Hm. That is not exactly the way I understood things although it may not be so far off after all... I just try to extrapolate.
> Going back a bit to the root of the discussions, ie, Pat's mail:-): he talks about abstract graphs which, in my mind, is the same as Sandro's g-snaps. These are mathematical abstractions, ie, sets, which never exist in the real world. If you want to go back to the Greek world (and I am not ashamed to say that I may be wrong in my understanding of the greek philosophers), in my mind a g-snap is an ideal, a Platonic abstraction. And a g-text is a textual description of a g-snap.


> A g-box is a shadow of a g-snap in Plato's cave allegory; a concrete, tangible representation of a g-snap. Well, it is a little bit smarter because when poked, it can give you some sort of a representation of a g-snap (eg, in the form of a g-text). But no, for me a g-box is not an abstraction, it is a real thing somewhere. Because it is a real thing, it can have a name, and two g-boxes are different things even if they represent the same g-snaps.

agree to some level, although just as a g-snap is a platonic abstraction 
which has a realization (g-text), so I think a g-box is an abstraction 
which has a realization (usually in some form of computer memory) - the 
distinction I make between a g-box and a g-snap is that a g-snap is 
snapshot of the contents of a g-box, it's state at a particular time, 
whereas a g-box is a container which spans time and has different 
contents/states at different times. The key in your text above is "a 
representation of a g-snap" (not "the representation of the g-snap"), so 
the relation between g-snap and g-text is 1:N (many representations, 
g-texts, of the same g-snap) and the relation between a g-box and g-snap 
is also 1:N over time (one g-snap at a single time, many g-snaps over 
time). As for poking, well it's the realization of the abstract g-box 
which can be poked to get a g-text.

So, I conclude that previously we had one term "RDF Graph" and used it 
to in specs to refer to both an abstract graph (g-snap) and a 
realization of it (g-snap), Pat's original issue. Then I brought up that 
it was also being used to refer to other things which we later 
established to be g-box's. And now I'm saying that we're doing the same 
thing with g-box's as we did with RDF Graphs, using the one term to 
refer to both the abstraction and the realization of it.

> Trying to use the terminology... if I have something like
> { <a> <b> <c> }
> in SPARQL (or n3), what is it? Is it a particular g-text representing a g-snap? Probably...
> if I describe a rule (I use N3 syntax here because it is simpler than RIF would be, but that is just syntax):
> { ?a <b> <c> } => { <e> <f> ?a }
> what does it mean in our terminology? Both sides describe a pattern for a family of g-snaps. Would one say that
> "If a g-box's g-snap matches the rule's left hand side, then extend the g-box's g-snap to include the right hand side"? 

The thing on the left and the thing on the right are what I'd refer to 
as quoted-graphs (or graph literals) in N3, each one being a g-text (a 
realization) of a g-snap (an abstract graph). The thing in the middle is 
of course a predicate/property using shorthand notation given by n3, it 
represents a logical constant / named node which itself is being used as 
a relation.

The full statement itself, is a g-text (a realization) of a g-snap (an 
abstract graph) which contains only one statement, use case for this 
particular statement is that it's going to be used as a rule.

To move forward with your use case in detail, I'm going to use four new 

box - an abstract box which can contain statements, and whose contents 
can vary over time

box-realization - a realization of a box, some process coupled to some 
memory which can manage realizations of the box's state/contents and 
change the state from one to another, change the contents of the box.

snapshot - an abstract snapshot of the state/contents of a box at time 
t, a mathematical set of statements, a g-snap

snapshot-realization - a realization of a snapshot, a distinct immutable 
collection of triples in memory, or some lexical representation of them, 
a g-text

   Issue 1:
   Snapshot-realizations are anonymous and there is no way to tell that
   two snapshot-realizations realize snapshots of the state of the same
   box, or to tell which state (Sn-1, Sn-5) they are snapshots of.

   Thus, in order to incorporate the concepts of box or box-realization
   in to RDF, some form of box identification, and some form of state
   identification would need to be added.

   If the two prior needs are not added, then the only notion of boxes
   that can exist is that of the a realization of the current state of
   some anonymous box; which is the definition of a snapshot-realization,
   thus pointless adding.

Okay, so you've given us a rule (R)

   { ?a <b> <c> } => { <e> <f> ?a }

Now, (thanks Ivan) you've given us a rule which has variable identifiers 
in it, so we better clear up what variables identifiers are too, and 
blank node identifiers whilst were here (so as not to confuse the two).

A Blank Node Identifier is temporary reference, bound to a blank node at 
a particular time - since it's an identifier it belongs in the 
realization space, and since it's a temporary identifier it belongs in 
the snapshot space, thus blank node identifiers are scoped to 
snapshot-realizations. Blank Nodes are therefore only existentially 
quantified within snapshots.

   Issue 2:
   Blank Nodes are only existentially quantified within snapshots, which
   means they aren't quantified at box level, which means they can't
   exist at box level.

   Thus, in order to incorporate the concepts of box or box-realizations
   in to RDF, the semantics of blank node identifiers and their scope
   of existential quantification would need to be changed. B.C. break.

Is it worth continuing this line of thought? boxes clearly do exist in 
semantic web land (sparql update of "named graphs" for example, and the 
need for "graph changes over time"), but they don't currently exist in 
RDF, and the two issues listed above are far from minor. Even if we 
incorporate boxes, both sparql and the web don't provide for any notion 
of time, and even if we did work out a way to have the concept of states 
over time in there, we'd need to change to a temporal logic.

My take away on this, is that if people want "named graphs" we can only 
accommodate "named snapshot realizations", which means that if you find 
at some point two different snapshot-realizations bearing the same name, 
well frankly you're up the creek without a paddle! We could provide for 
"quoted graphs" which would allow people to describe what a 
snapshot-realization is (retrieved from here at date x etc) but then 
we're moving more towards N3 (a good thing imho).

Another choice is to formalize what's required for the presence of 
boxes, such that boxes exist and can be given names, but you can only 
ever "get" the current state of the box (thus negating the need for 
mentioning state, state changes or moving to temporal logic), this would 
make room for other specs to piece together layers of the cake such as 
some dataset synchronization method, or say adding versioning meta data 
to http responses in order to cater for this need. The only thing that 
would need addressed for this would be the scope of blank node 

Personally, I'd say let's go for adding quoted graphs, variables, add 
the concept of box but only ever account for the current state, and 
scope blank node identifiers to being at box level. This would allow for 
the community to cover all use cases either in or out of RDF and layer 
on other bits to the sem web stack where needed. Practically this could 
be quoted graphs added to turtle, and some trig like format which could 
refer to a named-box and show the snapshot realization of the current 
state of that box.


Received on Sunday, 27 February 2011 21:15:53 GMT

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