[Mulgara-general] Jena-Mulgara connector
andrae at netymon.com
Fri Jan 11 04:12:15 UTC 2008
On 04/01/2008, at 1:39 AM, Seaborne, Andy wrote:
> It's also "interesting" if you create a blank node by some small
> integer and see what node in the server graph you have really named.
> id=1 seems to rdf:type.
I wouldn't rely on that if I were you :). There are various points
in the database bootstrap process where common uris are preallocated
permanent ids - purely as an optimisation (believe it or not
localizing rdf:type floated to the top of a profile for a given class
of query at one point). It just happens that rdf:type is the first
common uri we preallocate - hence it currently gets '1'. If that
order should change for some reason, it would result in a different
small positive integer.
>> I always get around it by doing one of two things:
>> - Perform add/deletes as insert...select and delete...select
>> - Wrap all my read/writes in a transaction. That means I'm the only
>> writer, so I know the ID can't change. Unfortunately, once we get
>> multiple writers (some time in the future still) then this won't
>> Also, if I'm using TQL then I have to perform insert...select and
>> delete...select operations still, as this is the only way to refer to
>> the blank nodes.
Probably worth noting here that even with multiple writers this will
>> Of course, if I'm doing insert...select and delete...select then I
>> some guaranteed way of identifying the correct node. IMO this is
>> almost always doable if you have knowledge of the schema you are
>> but there is no general mechanism. The only case where you can't
>> get a
>> particular blank node is if it's indistinguishable from another... in
>> which case it doesn't matter when one you get. :-)
> It's the lack of general mechanism that's the issue here. In
> trying to write a general connector, as with writing any library
> code, the hope is that the application assumptions don't have to
> run all the way from top to bottom.
If the problem reusing a blanknode from a query for an insert, as
long as you perform the query within the same write-phase you will be
fine. Mulgara's understanding of insert/delete is as functions from
graphs to graphs, ie: insert :: Triple -> Graph -> Graph.
In other words our current semantic is that the result of an update
operation on a graph is a new graph, and as such blank-nodes from the
pre-update graph are not necessarily the same as the blank-nodes on
the post-update graph.
So a mulgara instance consists of a sequence of 4-uniform hypergraphs
defined as the graphs produced by a sequence of unification and
restriction operations starting with an empty graph, and we call the
resulting ordinal number of a given graph a phase.
This idea of graph immutability (operations don't change the graph,
but produce a new one) has formed the basis of our thinking regarding
blank-nodes and their interaction with transactions and queries. As
a result your blank nodes remain valid for the duration of a
transaction, for the moment that means holding the write-lock,
however over the next few weeks we will be introducing the ability to
obtain read-only transactions which will help alleviate some of your
> My canonical use case is an RDF (or OWL) editor. These tend to
> regard the graph as a syntactic entity, especially when it is
> between consistent states. And those RDF collections are always
> there to keep us on our toes. Viewing the graph syntactically is
> just the same as thinking of being inside the graph, not querying
> from the outside.
> This use case isn't the primary reason for wanting the connector
> so, by above all, documentation for correct use is needed.
For now if you want to perform update operations you have to hold the
write-lock. Shortly you will be able to round-trip blank-nodes while
browsing the graph within a single read-only transaction. For this
to be transparent requires us to eliminate the write-lock, ie.
providing support for multiple-writers. This is planned but isn't
feasible in the short-term.
>>> One issue I encountered: Session.modelExists() for
>>> RemoteSessionWrapperSession throws an exception if the model has
>>> existed but returns false if the model used to exist but has been
>> This is a bug. I thought I'd put the fix into the trunk, but I must
>> have missed it.
> I'll check the trunk - I have been using 1.1.1 distribution where
I should be fixed in trunk I know it's fixed in the various devel-
branches, so it should have been merged across by now.
> Please do not tell Pat Hayes about the skolemization of the blank
> nodes :-)
He says to a publicly archived mailing list ;)
andrae at netymon.com
Senior RDF/SemanticWeb Consultant
Netymon Pty Ltd
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