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

From: Thompson, Bryan B. <BRYAN.B.THOMPSON@saic.com>
Date: Mon, 3 May 2004 12:44:13 -0400
Message-Id: <D24D16A6707B0A4B9EF084299CE99B390E2C9456@mcl-its-exs02.mail.saic.com>
To: Eric Prud'hommeaux <eric@w3.org>, "Seaborne, Andy" <andy.seaborne@hp.com>
Cc: RDF Data Access Working Group <public-rdf-dawg@w3.org>

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.

-- 
-eric

office: +1.617.258.5741 NE43-344, MIT, Cambridge, MA 02144 USA
cell:   +1.857.222.5741

(eric@w3.org)
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Received on Monday, 3 May 2004 12:44:50 GMT

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