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Re: RDF Concepts - What is a Node?

From: Richard Cyganiak <richard@cyganiak.de>
Date: Mon, 28 May 2012 18:41:55 +0100
Cc: RDF WG <public-rdf-wg@w3.org>, David Wood <david@3roundstones.com>
Message-Id: <64BD4E4D-C96E-43C7-8BBB-CC56A1317995@cyganiak.de>
To: nathan@webr3.org
Hi Nathan,

On 28 May 2012, at 17:02, Nathan wrote:
> Generally we see an RDF Graph as a Set of Statements, and the meaning of that is the conjunction of all the Statements in the Graph; and where each Graph entails its powerset.

Yes.

> However, it seems like there is an unwritten assumption in the community, that the meaning of a Node within a Graph is the conjunction of all the statements made where that node is in the subject or object position.

Nodes in an RDF graph don't have meaning. Only triples and graphs have. Nodes are simply the subjects and objects of triples in a graph. So they are IRIs, literals or blank nodes. They refer to things, but they don't say anything about the things, and thus have no meaning.
http://dvcs.w3.org/hg/rdf/raw-file/default/rdf-concepts/index.html#dfn-node

When you talk about the “meaning of a node within a graph”, then you're probably thinking of something like the CBD around that node:
http://www.w3.org/Submission/CBD/

> This leads me to several questions:
> 
> Given a graph:
> 
> {
> [] a :Person ;
>    :mailbox <mailto:foo@bar.com> .
> 
> [] a :Person ;
>    :mailbox <mailto:bar@foo.com> .
> }
> 
> then how many nodes does this contain? 4?

No. Five. Two blank nodes, the IRI :Person, and two mailto IRIs.

> regarding the second { [] a :Person } statement:
> - is it redundant?

No. The graph says: “There's a person with email address A. And there's a person with email address B.” If you drop the second rdf:type statement, you get: “There's a person with email address A. And there's something with email address B.” This is obviously different, because you no longer have the assertion that the thing with email address B is a person.

> - does removing it change the meaning of the graph? (logically I read no, practically I read yes)

Yes, it changes the meaning.

> Another example:
> 
>  </bar> a :Person .
>  </bar> :mailbox <mailto:bar@foo.com> .
> 
> In the above is </bar> a Node?

Yes, because it appears as the subject of a triple in the graph.

> or is the node an abstract concept who's values are the conjunction of the statements made about it?

No.

> is the IRI value of </bar> a name for the node

IRIs have to be an absolute IRI to be valid in an RDF graph. Let's assume </bar> was an absolute IRI.

</bar> *is* an IRI. Speaking of the “IRI value of </bar>” doesn't really make sense, it is redundant. And </bar> *is* a node, because it appears in the subject of a triple in the graph.

> , or is each mention of bar an instance of some label which is used to refer to the node, or?

I don't even know what that is supposed to mean.

The IRI </bar> *is* a node in the RDF graph. It's as easy as that.

> Skipping a fair few subsidiary questions, ultimately what I'm getting at is that the definition of Node isn't clear, or well defined, and that it appears there are a lot of unwritten assumptions in the community about rdf nodes.

http://dvcs.w3.org/hg/rdf/raw-file/default/rdf-concepts/index.html#dfn-node
http://www.w3.org/TR/rdf-concepts/#dfn-node

I think that's a very clear definition.

Best,
Richard




> 
> Best & hope that makes sense!
> 
> Nathan
Received on Monday, 28 May 2012 17:42:28 GMT

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