Hi Erik,
OWLIM computes inferences at load time (forward chaining rule-based
inference) and considers triples from any named graph (context) as input
to the reasoner. The decision to put inferred triples into the default
graph is really down to the fact that there is no other sensible place
to put them.
For example, if a triple in context A and a triple in context B leads to
the inference of a new triple, should it go in context A, context B or
somewhere else? It is purely because inferred statements are computed as
a result of triples in many contexts that it is just simpler to put them
in the default context.
I hope this makes sense,
barry
On 08/01/13 15:21, Erik Boye wrote:
Hi Ken, Barry,
Thank you so much for your answers to my question. Including the
default context by supplying null (in a addition to the URI of the
specific named graph) to setReadContexts-method solves my immediate
issue. But as Barry is pointing out, "the default graph can include
inferred statements produced from explicit statements not present in
the selects contexts", but this is currently not an issue for my
current usage.
Just out of curiosity: What would be wrong if OWLIM produced inferred
statements in the same context as the explicit statement? How/why
would that be wrong?
Regards,
Erik
*From:*Barry Bishop [mailto:barry.bis...@ontotext.com]
*Sent:* 7. januar 2013 14:18
*To:* Wenzel, Ken
*Cc:* Erik Boye; owlim-discussion@ontotext.com
*Subject:* Re: [Owlim-discussion] Inference with named graph (context)
Hi Ken, Erik,
To reiterate what Ken mentioned, there is no standard place to put
inferred statements and OWLIM simply adds these to the default graph.
If restricting the execution of certain methods to specific named
graphs (contexts) only is required, then it will be necessary to
include the default graph. Note that at present OWLIM does not compute
inferences at query time, so the default graph can include inferred
statements produced from explicit statements not present in the
selects contexts.
For selecting the named graph, please see the Sesame javadoc:
http://www.openrdf.org/doc/sesame2/api/org/openrdf/repository/RepositoryConnection.html
This explains how to select the default graph. I believe it is
possible to pass something like the following to the vararg parameter
for contexts: new URI[] {context1, null, context2} - where 'null'
represents the default graph. However, if this fails, then with
OWLIM-SE it might be possible to use the special URI for inferred
statements: <http://www.ontotext.com/implicit>
<http://www.ontotext.com/implicit>
I hope this helps,
barry
--
Barry Bishop
OWLIM Product Manager
Ontotext AD
Tel: +43 650 2000 237
email:barry.bis...@ontotext.com <mailto:barry.bis...@ontotext.com>
skype: bazbishop
www.ontotext.com <http://www.ontotext.com>
On 04/01/13 10:47, Wenzel, Ken wrote:
Hi Erik,
in addition to your specific named graphs, you also need to
include the default context (with a null URI) when
executing your query. This ensures that the inferred triples can
also be read.
Unfortunately, at the moment it is not possible to retrieve only
those triples which are inferred from a
certain combination of named graphs. I also think that it is
difficult, if not impossible, to implement this
on top of a forward chaining reasoner since it would require to
store tons of provenance information.
A backward chaining reasoner may be better suited for this usage
scenario.
Virtuoso provides a mechanism called rule sets. These rule sets
are associated with a URI (e.g., <urn:ruleset1>) and
are based on RDFS/OWL descriptions retrieved from a combination of
named graphs (e.g. <urn:graph1>, <urn:graph2>, ...).
When executing a SPARQL query a rule set can be selected with a
Virtuoso specific SPARQL extension:
define input:inference '<urn:ruleset1>'
select ?s where { ?s ?p ?o }
Afaik, Virtuoso's rule sets are implemented by using query rewriting.
Maybe, OWLIM can implement something comparable for its forward
chaining approach?
For example, the user may specify multiple rule sets and
associated named graphs for the inferred triples:
<urn:graph1>, <urn:graph2>, ... => <urn:graph-inferred-with-ruleset1>
Now, OWLIM's reasoner is executed for each rule set (and
combination of named graphs) instead of using
only one global reasoner.
I don't know if this approach is scalable since it would produce
many duplicated inferred triples and
also may lead to many reasoner runs if a named graph is changed
that takes part in multiple rule sets.
Another approach can be that OWLIM uses some sort of filtering to
exclude those inferred triples
where not at least one asserted triple within the queried named
graphs exists that mentions its subject,
predicate or object.
I've simulated this approach by using SPARQL filters but it tends
to be rather slow with OWLIM SE:
select ?o where {
?s ?p ?o
filter (!bound(?o) || isLiteral(?o) || exists {
graph ?g { { ?o ?p1 ?o1 } union { ?s2 ?o ?o2 } union { ?s3
?o3 ?o } }
}
}
Note: "!bound(?o)" is not required in this query - I've just
included it to demonstrate the generic case.
Best regards,
Ken
*From:*owlim-discussion-boun...@ontotext.com
<mailto:owlim-discussion-boun...@ontotext.com>
[mailto:owlim-discussion-boun...@ontotext.com] *On Behalf O**f
*Erik Boye
*Sent:* Thursday, January 03, 2013 5:16 PM
*To:* owlim-discussion@ontotext.com
<mailto:owlim-discussion@ontotext.com>
*Subject:* [Owlim-discussion] Inference with named graph (context)
Hi,
We are in the process of establishing an infrastructure for
storing and querying power system network models based on OWLIM
and Sesame. We need to track different versions of the power
system network model (at the same time), and thought could be
achieved using named graphs: One named graph per version of the
power system network model.
The queries that are performed on the power system network model
heavily depends on OWLIM inference. With only one named graph in
the repository, all works fine. But when there are more than one
named graph in the repository, and queries need to be restricted
to a specific named graph, our queries fail:
To restrict the queries to a specific named graph the following
Sesame API method is used:
ContextAwareConnection#setReadContexts(URI[])
The problem seems to be related to the fact that statements
inferred by OWLIM are stored in the unnamed graph (default
context), and therefore not included when restricting the queries
to a specific named graph. Is this the expected behavior? If yes,
why? And does anyone have a suggestion for a workaround, except
from the obvious one: separation on the repository level?
Best regards,
Erik
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