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RE: rambling about safety Was: Protocol/query interaction - comp utational cost of a query

From: Seaborne, Andy <andy.seaborne@hp.com>
Date: Tue, 4 May 2004 13:35:35 +0100
Message-ID: <E864E95CB35C1C46B72FEA0626A2E808028A3329@0-mail-br1.hpl.hp.com>
To: RDF Data Access Working Group <public-rdf-dawg@w3.org>

I have nothing aginst these crtieria used as examples per se but I do think
that too many parameters does not make for high levels of interoperability.
Even levels of facilties often fails in practice as either every system
assumes the lowest level (max interoperability) or requires some higher
level (hence the WG would have missed the target in some sense), neglecting
the lowest, most interoperable level.  What happens in deployed practice is
hard to determine beforehand.

Having some mechanisms for communicating specialised parameters is fine but
just let's not put too much on this point.  As to actual negotiating them,
rather than the simple case of just the client specifiying them and gettign
the server to reject a request, would, for me, need a stronger justification
than just an extension mechanism.

(well - the subject does say "rambling")

	Andy

-------- Original Message --------
> From: Thompson, Bryan B. <mailto:BRYAN.B.THOMPSON@saic.com>
> Date: 3 May 2004 17:44
> 
> Eric,
> 
> I also like the notion that the "extensibility" mechanism
> would provide
> a means for a client and a server to negotiate a variety of different
> kinds of things. 
> 
> Some examples:
> 
>  - cost limits (space & time or other quantities) for the query;
> 
>  - whether the query MUST provide a response based solely on
> ground terms
>    (no inference);
> 
>  - whether or not the server MUST trust the ground terms (and any
>    rules) used in generating the query response;
> 
>  - whether or not a cached response is acceptable;
> 
> Etc.
> 
> The working group might (or might not) want to standardize
> some of these
> "negotiable" criteria, but we could certainly provide the
> framework for
> such negotiations.
> 
> -bryan
> 
> -----Original Message-----
> From: public-rdf-dawg-request@w3.org
> [mailto:public-rdf-dawg-request@w3.org]
> On Behalf Of Eric Prud'hommeaux
> Sent: Sunday, May 02, 2004 8:36 PM
> To: Seaborne, Andy
> Cc: RDF Data Access Working Group
> Subject: rambling about safety Was: Protocol/query interaction -
> computational cost of a query 
> 
> 
> 
> On Thu, Apr 29, 2004 at 01:15:16PM +0100, Seaborne, Andy wrote:
> > 
> > 
> > One of the things the comparison of query languages talks about is
> > "safety". There is an interaction with remote access here - does the
> > query language allow or encourage queries which do not terminate or
> > incur excessive computation?
> 
> [[
> - Safety A query language is considered safe, if every query that is
>   syntactically correct returns a finite set of results (in a finite
>   data set). Typical concepts that cause query languages to be unsafe
>   are recursion, negation, and built-in functions.
> ]] - http://www.aifb.uni-karlsruhe.de/WBS/pha/rdf-query/rdfquery.pdf
> -- hand-transcribed from the original document because I can't   
> cut/paste PDFs. 
> 
> It seems like the designation as "safe" really indicates the
> lack of the
> contraindications such as recursion, which seems to mean "support for
> transitive properties". I believe we are chartered to produce
> a language
> that can be used for ground facts or inferences without any syntactic
> distinction. 
> 
> [[
> 1.8 Derived Graphs
> 
> The working group must recognize that RDF graphs are often constructed
> by aggregation from multiple sources and through logical
> inference, and that
> sometimes the graphs are never materialized. Such graphs may
> be arbitrarily
> large or infinite. ]] -
> http://www.w3.org/2003/12/swa/dawg-charter#derivedGraphs
> 
> > In sending a query to a remote server, we should be aware we are
> > sending around a computation.  A query language that can cause
> > infinite, or excessive, computation at the server is not a good idea.
> > A query could be analysable as to be being safe or the language could
> > be designed so that it is not possible, or probably not natural, to
> > express unsafe constructs.
> 
> I'm skeptical that the ability to query the product of large
> or infinite
> inferences is worse than the ability to underconstrain graph queries.
> 
> > I don't like the idea that "DAWG-QL" needs to be analysable; I would
> > prefer that the language does not have features that allow excessive
> > or infinite computation unless that feature is really important.
> > 
> > This isn't a clear cut matter - if you ask a query that is not very
> > selective of a large dataset, you will get back a lot of results and
> > it may consume a lot of system resources.  But we can design things to
> > avoid traps for the application writer.
> 
> I added an underconstraints checker to algae which warns you when you
> underconstrain up a query. (It also keeps you from doing cross
> products.) I did not bother to add any syntax to indicate
> whether the client
> *wanted* this check done. Consequently, there was no
> syntactic affect from
> this feature.
> 
> Perhaps the mythic *extensibility* feature could be uesd to communicate
> underconstraints and inference-limiting flags.
> 
> > 	Andy
> > 
> > PS I like the phrase "RDQL is safe" which reminds me of the
> > Hitch-Hikers Guide to the Galaxy: "mostly harmless".  Not sure that
> > translates to other parts of the globe.
Received on Tuesday, 4 May 2004 08:36:12 GMT

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