[Mulgara-general] Sommer

[Paul Gearon]

Hi Henry,

On 9/7/06, Henry Story <henry.story at bblfish.net> wrote:
> Hello, I see things have  been moving on with Mulgara...

It's moving, but not always in anticipated directions.  I know that
each of us has features we're trying to develop, which means that some
essential features are not being worked on yet.

Just saying this so people know that we're doing stuff, even if
doesn't show up as the bug fixes or features they are waiting for.

> So I was wondering if the following had been implemented yet:
>    - Is there a way to query the store in a way that one can keep
> track of anonymous nodes? In sesame one can query a graph and
> anonymous resources will be identical between nodes

Yes and no.

If someone is deleting and inserting blank nodes, then the IDs will be
re-used.  This is why we make no guarantees about the IDs behind a
blank node from one query to the next.  This is completely consistent
with the RDF specification.

That said, there are ways around it.

- If you can guarantee that no one else is writing to the database,
then the nodes will be the same.  Not an easy guarantee to make
though.
- If you keep the same session object, you will always get a
consistent view of the data.  This is because you read a fixed phase,
and no writes will be visible until you update to the latest phase.
Make sure you keep the same session to guarantee that the data stays
consistent.  This is only a viable option if you are doing a series of
reads at one time, rather than coming back some time later.  I rely on
this behaviour in the rules engine.
- Follow RDF Semantics, and select blank nodes according to a unique
labelled node (URI reference or a Literal), or unique set of nodes.
This will always work, but can lead to unwieldy queries in some cases.


>    - can one query such a graph and have inferencing.

Inferencing requires writes from another session.  If you kept the
current session alive then you wouldn't see the inferred statements.
If you DON'T keep the current session alive, then you don't know if
anyone deleted and then inserted statements.

That said, inferencing does not delete any statements (this is a
consequence of the open world assumption), so if you know that no one
else will be doing any writing, then you're OK.  You're skating on
thin ice though.  :-)

In reality, the best way to handle it in this situation is to use
labelled nodes to find your blank ones.  This is always possible,
otherwise you'd have no way of accessing those nodes in the first
place!


>    - does one still need to connect via a client server?

I don't understand this question.  The client is not, and has never
been, a server.  The "client" is just a library found in the
itql-1.0.0.jar file.  You access it with the ItqlInterpreterBean
class, which is really just a thin wrapper around the ItqlInterpreter
class.  The library also includes the ItqlSession class, which is the
GUI console for interactively making queries.  (this "interactive"
console is the "i" in "iTQL").

I understand that ItqlInterpreterBean has been made to work with SOAP,
so non-Java applications can use it, but I heard that this may not be
working all that well at the moment.

Hmmmm, I should also mention that servers have a client built in, and
that this client is accessible from a web page.  However, no
real-world system should ever use this, as it was created solely for
demonstration purposes, and it is HTML only.  It also has a history of
bugs which are enitrely due to it's integration into the server JVM.
If you are asking about this, then my answer is to learn to use the
proper interfaces (ItqlInterpreterBean).

Did this answer your question, or are you referring to something else?

Regards,
Paul Gearon