Re: The Entailment bit (was Re: thoughts from Tuesday telecon)

On Sep 26, 2005, at 8:41 AM, Seaborne, Andy wrote:

> Bijan Parsia wrote:
>> On Sep 22, 2005, at 6:03 PM, Pat Hayes wrote:
> . . .
>>> 2a. The question arose: if SPARQL can be used (perhaps in the 
>>> future, but let us look ahead) with various notions of entailment, 
>>> should a query be able, or be required, to specify the kind of 
>>> entailment intended? Although this was discussed only briefly, there 
>>> was a consensus that it would be acceptable for the entailment in 
>>> use to be specified as part of the 'service', identified by the URI 
>>> currently used to name the target graph.
>> While I think that is a reasonable point in the design space, I talk 
>> with Jim and he flat out rejected it. So there's still play here. (I 
>> myself don't think it's the best design but do think it's workable.)
> [Bijan-
> I'm assuming that Jim's issue is wanting to query the same graph under 
> different semantics as discussed below.  If there other concerns, 
> could you say what they are?
> ]

As I understand it, the idea is that reasoning level is set on per 
endpoint basis. Endpoints provide access to 1 or more number of graphs 
(i.e., a dataset). Each endpoint has its own URI. If I understand the 
proposal, this would necessitate a distinct endpoint for each semantics 
offered by the service and the semantics must be(?) uniform over the 
dataset. I don't think that's the most useful only configuration (e.g., 
I might want the background graph to be with RDFS semantics containing 
info about the possible semantics offered by component graphs; a 
different issue would be being willing to drop down in the presence of 
a timeout, e.g., I accept OWL-DL results, then RDFS but prefer OWL-DL 
(similarly to connegging); if you timeout or are in danger of timing 
out with OWL-DL, just send the RDFS).

>>> We are indebted to Enrico for making this point vividly clear with 
>>> the 'little-house' (aka 'worker') example.
>>> (I would suggest, and this is purely my own personal view, that we 
>>> can adopt a compromise here, in which SPARQL in its current release 
>>> will refer to simple entailment; the issue is pointed out; the 
>>> actual spec. refers to virtual graphs identified by URIs, and refers 
>>> to the RDF and RDFS closure lemmas; and the possibility of using 
>>> URIs to identify services which offer other kinds of entailment is 
>>> pointed out as a future extension path.
>> Hmm. Doesn't this bias things against Jim's desire for one and the 
>> same URI identified graph to be queried under different semantics? In 
>> other words, does this close discussion on that protocol design 
>> decision before alternatives have been considered?
> This seems to be a different matter.
> The protocol paradigm is service-centric, not graph-centric (this was 
> after some debate so I think we have considered it, may be not exactly 
> as described).

Yes, sorry. I mean that how I, a server, offer different semantics for 
the graphs I query over.

> It is the combination of graph (by parameter) + service + query that 
> gives the results.  Querying the same graph under different semantics 
> is asking a different service unless we change the service-centric 
> emphasis of the protocol.

Yes, so the only question I see at the moment is whether one wants to 
force a distinct call (to a distinct endpoint) in the following 

A server knows it can reasonably handle with OWL-DL Ontologies A and B, 
but it can't deal with C and D except with RDFS (even though they 
species validate as OWL-DL). I cannot query over these services with 
the maximal semantics the server handles in one call. (I have to query 
C and D on the rdfs endpoint and A and B on the OWL-DL endpoint).

Actually, all the other situations are variants of this. I don't know 
if this is what jim has in mind.

> A hybrid would be to have a protocol argument (or query clause) which 
> is influencing the service.

Yes. So I might like on a per query basis the pattern of semantics for 
graphs offered by the service.

>  Some might say this is getting away from the service-centric protocol.

Doesn't seem more so than the various graph selection operations.

> I would not like to enumerate all the possible values of this argument 
> (mentioning some well-known cases is OK) in rq23 or the protocol doc 
> because of uses of subsets of OWL/RDFS entailment for tuned 
> performance.  [This would also for rules].

I would like distinguished designators for the current set of defined 
entailment relations with extensbility (for new variants). There are 
certainly a billion subsets and extensions of OWL alone which people 
might want to indicate. However, I think having the gross distinctions 
of: simple, rdf, rdfs, owl-lite, owl-dl, and owl-full is a useful base, 
matches the existing specs, and maps to behavior of e.g., webont (for 
their test cases) and software (species validators). (Swoop, for 
example distinguishes species validation (values supported by the w3c) 
and expressivity (wihich is more fine grained)).
>> There is a charter prohibition, but I would propose altering that as 
>> it all comes out so nice.
>> One question worth answering is whether there will be implementor 
>> support at this time. I believe I can pledge that Pellet will support 
>> SPARQL over OWL DL. Indeed, if Jena's SPARQL implementation separates 
>> the graph matching and the rest of the algebra, I believe it's a tiny 
>> hookup for us.
> Indeed it does.
> ARQ works over graphs so you can have your own graph implementation 
> but here it would be better to override the ARQ implementation of 
> basic pattern matching to add your own.  This has been done for 
> writing queries to legacy SQL databases so has been tried out.



Received on Monday, 26 September 2005 22:44:45 UTC