[Mulgara-dev] Round-tripping Blank-nodes.

[Paul Gearon]

Just to comment for the sake of others who weren't there....

On Thu, Mar 27, 2008 at 11:38 PM, Andrae Muys <andrae at netymon.com> wrote:

<snip/>

As I mentioned, MUST is far too strong a term for this - SHOULD is
> the strongest you can legally use here, and MAY is probably more
> accurate. RDF Semantics are perfectly happy with a coordinated set of
> graphs sharing blank-nodes, and therefore we are perfectly entitled
> to provide a reference to them that satisfies blank-node semantics
> (note: *blank-node* semantics, *not* blank-node *id*s.).  On the
> other hand, I should note here that this only applies when we are
> talking about a blank-node reference returned from a query result -
> it does *NOT* apply as soon as the blank-node has been serialized as
> RDF/XML, N3, or similar.  At that point it's been inserted into a
> new, non-coordinated, graph and the id is then scoped strictly to
> that graph.
>
> > To imagine the difficulty, think of doing a query from the union of
> > graphs A and B, and insert the results into B.  Blank nodes from B
> > shouldn't change, but blank nodes from A are supposed to change
> > before being written.
>
> This isn't actually required by the spec.
>

As it  turned out, Andrae and I have different semantics in mind for our
definitions in Mulgara. He sees insert/select as being a graph union
operation, whereas I see it as a graph merge operation. For labeled nodes
there is no difference. However, for blank nodes, the difference is what
we've been discussing here. Whether insert/select is a union operation or a
merge operation is defined by the semantics of the database (and not by
RDF). In our case we never formally specified what an insert/select is
supposed to represent, but we have been effectively operating on the
presumption that it is a union operation.

Since I want to waste as little time as possible on this, I think the way to
resolve this is to take Andrae's "union" view.  Consequently most things
work already. I love fixing things by having a change in attitude.  :-)


Paul
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