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Re: Importing RDF documents from RIF - part 1

From: Axel Polleres <axel.polleres@deri.org>
Date: Tue, 19 Jan 2010 14:17:55 +0000
Cc: public-rif-wg@w3.org
Message-Id: <DA2F118E-33D7-477F-87C3-D62E19B389B7@deri.org>
To: Axel Polleres <axel.polleres@deri.org>

On 19 Jan 2010, at 00:09, Axel Polleres wrote:

> Hi all,
> 
> I had some discussions about this very issue with sandro and chris at ISWC.
> This issue is also problematic for SPARQL, if we want to give a semantics to SPARQL over RDF+RIF
> since there is no way to define or link to a ruleset from a "normal" SPARQL datased.
> 
>   I suggest I will summarise the options in an email shortly and get involved when it is discussed in a telecon, apologies for my otherwise silence recently, being swamped with other issues.
> 
> best,
> Axel


Dear all, 

this will come in two parts to make it easier to digest. 
This first part is just a dump of options I discussed with Sandro and Chris over 
lunch at ISWC (quite a while ago, I admit),
The second part will be a brief summary with one concrete proposal, following in a separate mail.

So at ISWC we were talking about the problematic issue
that it is well possible to import RDF from RIF documents, but there is no way to - in
reverse - import RIF from RDF, which is something I'd really NEED,
e.g. in SPARQL, see my comment above.

Ideally, as it is possibly too late to get this in the official spec, we could at least publish 
a WG note, maybe jointly with the SPARQL WG.

I think there is a very simple "abstract" solution for this, using something similar to
owl:imports. For that, there seem to be two obvious Options, which I will
outline in the following:

* Option 1: the following addition to the semantic conditions of models for
RIF-X-combinations:

"The models of a RIF-X-model of a RIF-X-combination (G,R)
RIF-X-entailing the RDF graph
   [ rif:imports R1] .
where R1 is an IRI  referring to a RIF document are restricted to those models which are also
models of (G, R') where R' is identical to the RIF document R, with the
only addition that R' has additional imports clauses
 Imports( R1 )
 Imports( G <http://www.w3.org/2007/rif-import-profile#Generic> )
"

I am a bit worried a bit here about the somewhat recursive nature of this
definition using "RIF-X-entailing the RDF graph
   [ rif:imports R1] ."
... so, here's a simpler Option:

* Option 2: ... an alternative may be to view rif:imports
 purely syntactical, i.e. require simply
that a triple " [ rif:imports R1] ." is explicitly mentioned in G)... it seems we'd
be safe with that in the sense that we'd follow the same "path" as OWL did with
their imports property: Note that particularly the RDF-based semantics of OWL
leaves out to take a clear definition of the semantics of owl:imports [1], and seems
to define the semantics of owl:imports on a purely syntactical level [2], I am not 100% sure here, though.
Particularly, it seems to me that [2] does NOT define e.g. what should be the behavior in case
someone writes a "weird" graph like.

 my:imports rdfs:subPropertyOf owl:imports.
 <o1> my:imports <o2> .

In that sense, it might be sufficient to similarly just say:

"The models of a RIF-X-model of a RIF-X-combination (G,R)
where G contains a triple
  [ rif:imports R1] .
where R1 is an IRI referring to a RIF document are restricted to
those models which are also models of (G, R') where R' is
identical to the RIF document R, with the only addition that R'
has an additional imports clauses
 Imports( R1 )
 Imports (G <http://www.w3.org/2007/rif-import-profile#Generic>).
"

The second Imports statement in both options,
 Imports (G <http://www.w3.org/2007/rif-import-profile#Generic>).
intuitively re-importing the RDF graph G into R may seem to be redundant, but
but may be useful when a RIF processor capable of handling RIF-X-combinations
is only given an RDF graph with rif:imports clauses as input, to determine the single
highest profile to be used. Say, a RIF processor loads
the RDF graph G which could contain the statement
 <G> rif:imports <R1>.
Then, single highest profile would simply be determined by the highest
profiles of the imported RDF graphs and ontologies in R1.


An alternative in both Option 1 an Option 2 to using
 Imports (G <http://www.w3.org/2007/rif-import-profile#Generic>).
would be that we'd  require rif:imports to specify the intended profile, e.g.
to require G to have some triples
 [ rif:imports [ rdf:value <R1>; rif:imports-profile <importsprofile-URI> ]]
or - simpler just
 R1 rif:usedWithProfile P .
and that would determine
 Imports (G P).
instead.


Note that, we'd NOT cover recursive imports with that notion that go via owl:imports, in
neither Option 1 nor Option 2. I mean, e.g. that the behavior of

G:
G owl:imports O1.
O1:
O1 rif:imports R1.

would not really be covered, since that would need to alter the meaning of owl:imports, as
rif:imports properties do not form part of the importsClosure of an OWL document.
I think that's ok. We should not necessarily get to the idea to remedy this by
reusing owl:imports, as that seems to violate the domain and range restrictions
of owl:imports (unless we'd find agreement that it's ok that the "class of RIF
documents is a then implicitly a subclass of owl:Ontology).

So, summarizing everything discussed in this mail, orthogonal to
Option 1 or 2 there would be several overall syntactic options
to write an import in RDF within a graph G, let's number them alphabetically, not to
confuse them with the orthogonal Option 1 an Option 2 mentioned so far:

Option A:
[ rif:imports R1 ] .
... generic profile, blank node.
Option B:
G rif:imports R1 .
... generic profile, graph URI (might be inconvenient when the graph is merged)
Option C:
[ owl:imports R1 ] .
... generic profile, blank node, somewhat "hijacking" owl:imports, with the consequences outlined above.
Option D:
G owl:imports R1 .
... generic profile, graph URI, somewhat "hijacking" owl:imports, with the consequences outlined above.
Option E:
[ rif:imports [rdf:value R1; rif:profile P] ].
... specific profile using rdf:value, blank node, that admittedly looks awkward.
Option F:
G rif:imports [rdf:value R1; rif:profile P] .
... specific profile using rdf:value, graph URI
Option G:
[ owl:imports [rdf:value R1; rif:profile P] ].
... specific profile using rdf:value, blank node, "hijacking" owl:imports
Option H:
G owl:imports [rdf:value R1; rif:profile P] .
... specific profile using rdf:value, graph URI, "hijacking" owl:imports
Option I:
 R1 rif:usedWithProfile P .
... neat...

My favorite is I, my worst choice is G, I guess...

I think, if we can agree on a solution (there are several coin-flips in the proposals
summarized in this mail...) then it would be possible to publish this
- as an additional WG Note,
- as an informative section of http://www.w3.org/TR/rif-rdf-owl
- as a normative section of http://www.w3.org/TR/rif-rdf-owl
I guess it might be too late for the latter, the first one might sound simplest, but I am not sure whether a WG Note
could be reused by other Rec track specs, as I mentioned, this imports mechanism could be useful for SPARQL.

That's all (probably too much at once...),
Axel

1. http://www.w3.org/TR/owl2-rdf-based-semantics/#Content_of_Ontologies_.28Informative.29
2. http://www.w3.org/TR/2009/REC-owl2-syntax-20091027/#Imports
Received on Tuesday, 19 January 2010 14:18:29 GMT

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