Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!uccba!mead!gordon
From: gordon@mead.UUCP (Gordon Edwards)
Newsgroups: comp.databases
Subject: Re: Support for imprecise data: survey
Message-ID: <1055@meaddata.mead.UUCP>
Date: 2 Aug 90 18:11:25 GMT
References: <26231@usc.edu> <2060003@hpcuhc.HP.COM> <1990Aug1.152432.7861@sctc.com>
Sender: usenet@mead.UUCP
Reply-To: mead!gordon@uccba.uc.edu
Organization: Mead Data Central, Dayton OH
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In article <1990Aug1.152432.7861@sctc.com>, endrizzi@sctc.com (Michael
Endrizzi ) writes:
|> And the war starts.....
|> 
|> dhepner@hpcuhc.HP.COM (Dan Hepner) writes:
|> 
|> 
|> >This topic was discussed here a while back under the subject heading 
|> >"Fault Tolerant information recall", or something like that.
|> 
|> >Your title for this field of investigation is superior.
|> 
|> 
|> Fault Tolerance: A list of faults that will be tolerated and the 
|> post-fault level of service for each fault. The process of tolerating
|> a fault consists of 3 steps: fault detection, isolation, and recovery.
|> 
|> Fault Tolerant Information Recall:
|> 
|> 	1) Tolerates syntatic faults in databases
|> 	2) Post level of service guarantees data that is
|> 	   syntatically "closest" to query.
|> 	3) Fault-Detection: implicit and assumed
|> 	   Fault-Isolation: ???
|> 	   Fault-Recovery:  Query always returns with "best" answer 
|> 		            available.
|> 

I normally don't get involved in word games, but in this case I have to agree
with Dan.  Before comming to MDC, I was a member of an operating system group
supporting embedded airborne systems for the Navy.  On this particular project,
an ELINT bird, three AN/AYK-14(V) airborne computers were used to support
various mission and flight navigation devices.  We had to design the operating
system to detect failures among the three computers.  Example, AYK1 carried
the software of AYK2 and AYK3. In the event that AYK2 became incapacitated, 
AYK1 would assume AYK2's responsibility.  Okay, no problem with detection
and isolation, but recovery!  If you can't recover something to a known
state then you shut it down and notify the crew, not make a guess and continue
running! 

In summary, best guesses have no place in fault tolerance.  Call it value
approximation, imprecise information recall, etc...


--
                                Gordon Edwards
                         Mead Data Central, Dayton OH
                           mead!gordon@uccba.uc.edu
                              uccba!mead!gordon