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From: gerety@hpfcbig.SDE.HP.COM (Colin Gerety)
Newsgroups: comp.software-eng
Subject: Re: Re: Tolerance (was Re: Reusability considered harmful??(!!))
Message-ID: <440005@hpfcbig.SDE.HP.COM>
Date: 8 Feb 91 20:07:16 GMT
References: <1401@ucl-cs.uucp>
Organization: HP SESD, Fort Collins, CO
Lines: 30


> engineering is to be considered an *engineering* discipline, rather
> than a science, the notion of tolerance should be developed.
...

> modern production line, and is not original. If you buy a new spark
> plug or a tyre, then it has to fit in place, within a certain margin
> of error - tolerance. Cylinders fit into an engine block with a
> certain gap, within a certain range of error. And so on.

> Can the tolerance idea get off the starting blocks?

> Gordon Joly 

  While I think that specifed, guaranteed behaviour is extremely
important to software re-use, the notion or tolerance probably won't
get off the starting block.

   In most physical systems there is some continuity between structure
and behaviour.  If you have a beam, making it bigger usually makes it 
stronger.  This is not the case in software.  Changing a single character 
in a program may lead from a correct program to catastrophic failure.  It
is not clear that adding more code will either detect or be able to
correct for these errors.

  For a good discussion of this, see "Evaluation of Safety-Critical
Software" by Parnas, Schouwen, and Kwan in CACM June 1990.

Colin Gerety
gerety@hpfclp.sde.hp.com