Re: [SystemSafety] Software reliability (or whatever you would prefer to call it)

From: David Crocker < >
Date: Sat, 14 Mar 2015 11:37:26 +0000


Bertrand, would you care to give an example of the pressure to build a concept of composability of equipment properties (as opposed to composability of components), and explain why you believe Gödel's theorem is relevant here?

David Crocker, Escher Technologies Ltd.
http://www.eschertech.com
Tel. +44 (0)20 8144 3265 or +44 (0)7977 211486

On 11/03/2015 14:37, RICQUE Bertrand (SAGEM DEFENSE SECURITE) wrote:
>
> Hi All,
>
>
>
> I am also somewhat puzzled by the two aspects of the situation in IEC
> 61508 in particular, and on the market in general, not knowing which
> one is the egg and the hen :
>
> · First, the segregated approach of IEC 61508 between HW and
> SW misses (for the moment as this is discussed for edition 3) the
> complexity of the interaction between HW and SW and the potentially
> unwanted emerging properties at system level. A simple example is the
> fact that isomorphism issues between HW architecture and SW
> architecture are even not foreseen. This a significant weakness. Any
> tentative to try to keep the two worlds so separated seems clearly not
> going to help to improve system safety.
>
> · Second, there is a heavy pressure on the market from the
> manufacturer’s side, to build a concept of “composability” of the
> equipment properties to automatically obtain the requested properties
> at system level, from both software and hardware components. This
> seems to be absurd and dangerous because of Gödel theorem.
>
>
>
> I thus support this approach to talk about “complex design”. Knowing
> that complexity emerges very soon with apparently simple functionalities…
>
>
>
> Bertrand Ricque
>
> Program Manager
>
> Optronics and Defence Division
>
> Sights Program
>
> Mob : +33 6 87 47 84 64
>
> Tel : +33 1 58 11 96 82
>
> Bertrand.ricque_at_xxxxxx >
>
>
> *From:*systemsafety-bounces_at_xxxxxx > [mailto:systemsafety-bounces_at_xxxxxx > Behalf Of *GRAZEBROOK, Alvery N
> *Sent:* Tuesday, March 10, 2015 4:33 PM
> *To:* Littlewood, Bev
> *Cc:* systemsafety_at_xxxxxx > *Subject:* Re: [SystemSafety] Software reliability (or whatever you
> would prefer to call it)
>
>
>
> Hi Bev.
>
>
>
> Thanks for addressing the issue of language / terminology.
>
>
>
> In the world of embedded control systems, I have seen various attempts
> to dodge standards for design, by playing with the semantics around
> the word “Software”. There are two specific classes of dodging I can
> think of,
>
> 1. – using programmable electronics or high-state digital
> circuitry and claiming that software design practices don’t apply. In
> civil aero world they introduced DO-254 in addition to DO-178 to cover
> this.
>
> 2. – using data tables to describe behaviour, and claiming that
> only the table interpreter not the contents are software.
>
> I’m sure list members will think of other examples. If the language of
> the standards talked of “system behaviour” or “design behaviour”
> including Software, I think this would remove such issues.
>
>
>
> My feeling is that it would be helpful to talk of “complex design”
> including the software, attached electronics, and if applicable
> complexities in the controlled equipment and “plant”, and consider the
> (systematic) design reliability of all of this. Separating the part
> that is labelled as “software” from its electronic and physical world
> context isn’t helpful.
>
>
>
> This sits alongside the “traditional” component reliability approaches
> that deal with the (non-systematic) failure of equipment due to
> limited life, damage, random failure etc.
>
>
>
> **Note: these are my personal opinions, not necessarily those of my
> employer**
>
>
>
> Cheers,
>
> Alvery.
>
>
>
> *From:*systemsafety-bounces_at_xxxxxx > <mailto:systemsafety-bounces_at_xxxxxx > [mailto:systemsafety-bounces_at_xxxxxx > Behalf Of *Littlewood, Bev
> *Sent:* 10 March 2015 11:45 AM
> *To:* C. Michael Holloway
> *Cc:* systemsafety_at_xxxxxx > <mailto:systemsafety_at_xxxxxx > *Subject:* Re: [SystemSafety] Software reliability (or whatever you
> would prefer to call it)
>
>
>
> Hi Michael
>
>
>
> Seems you /are/ speaking for Nick! (see his most recent posting) Of
> course the distinction you make here is an important one - I think we
> can all agree on that. Not least because our actions in response to
> seeing failures from them will be different (in the case of design
> faults - inc. software faults - we might wish to remove the offending
> fault).
>
>
>
> But excluding design faults as a source of (un)reliability results in
> a very restrictive terminology. I realise that appealing to “common
> sense” in a technical discussion is often the last refuge of the
> scoundrel… But I don’t think that the man in the street, contemplating
> his broken-down car (in the rain - let’s pile on the pathos!), would
> be comforted to be told it was not unreliable, it just had
> /design/ faults.
>
>
>
> And, of course, your interpretation seems to rule out the contribution
> of human fallibility (e.g. pilots) to the reliability and/or safety of
> systems. This seems socially unacceptable, at least to me.
>
>
>
> Cheers
>
>
>
> Bev
>
>
>
>
>
> On 10 Mar 2015, at 10:34, C. Michael Holloway
> <c.m.holloway_at_xxxxxx >
>
>
> I can't speak for Nick, but I object to the use of the term
> "reliability" being applied to anything other than failures (using
> the term loosely) resulting from physical degradation over time.
> I believe it is important to maintain a clear distinction between
> undesired behavior designed into a system, and undesired behavior
> that arises because something ceases to function according to its
> design. (Here "designed / design" is used broadly. It includes
> all intellectual activities from requirements to implementation.)
>
> --
>
> /*c*//*M*//*h*/
>
> *C. Michael Holloway*
> The words in this message are mine alone; neither blame nor credit
> NASA for them.
>
>
>
> On 3/10/15 5:50 AM, Peter Bishop wrote:
>
> Now I think I understand your point.
> You just object to the term *software* reliability
>
> If the term was *system* reliability in an specified
> operational environment, and the system contained software
> and the failure was always caused by software
> - I take it that would be OK?
>
> A alternative term like *software integrity* or some such
> would be needed to describe the property of being correct or
> wrong on a given input.
> (In a lot of mathematical models this is represented as a
> "score function" that is either true or false for each
> possible input)
>
> Peter Bishop
>
> Nick Tudor wrote:
>
> Now back in the office...for a short while.
>
> Good point David - well put.
> I would have responded: There exists a person N who knows a
> bit about mathematics. Person N applies some mathematics and
> asserts Truth. Unfortunately, because of the incorrect
> application of the mathematics, the claims N now makes cannot
> be relied upon. The maths might well be correct, but the
> application is wrong because - and I have to say it yet again
> - the application misses fails to acknowledge that it is the
> environment that is random rather than the software. Software
> essentially boils down to a string of one's and nought's.
> Given the same inputs (and that always comes from the chaotic
> environment) then the output will always be the same. It
> therefore makes no sense to talk about 'software reliability'.
>
> Nick Tudor
> Tudor Associates Ltd
> Mobile: +44(0)7412 074654
> www.tudorassoc.com <http://www.tudorassoc.com/>
> <http://www.tudorassoc.com> <http://www.tudorassoc.com/>
> *
> *
> *77 Barnards Green Road*
> *Malvern*
> *Worcestershire*
> *WR14 3LR**
> Company No. 07642673*
> *VAT No:116495996*
> *
> *
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> <http://www.aeronautique-associates.com>
> <http://www.aeronautique-associates.com>
> <http://www.aeronautique-associates.com/>*
>
> On 9 March 2015 at 12:26, David Haworth
> <david.haworth_at_xxxxxx > <mailto:david.haworth_at_xxxxxx > <mailto:david.haworth_at_xxxxxx > <mailto:david.haworth_at_xxxxxx >
> Peter,
>
> there's nothing wrong with the mathematics, but I've got
> one little nit-pick about its application in the real world.
>
> The mathematics you describe gives two functions f and g,
> one of which is the model, the other is the implementation.
>
> In practice, your implementation runs on a computer and so
> the
> domain and range are not "the continuum". If your model is
> mathematical
> (or even runs on a different computer), the output of one
> will
> necessarily be different from the output of the other. That
> may not be a problem in the discrete sense - you simply
> specify a
> tolerance t > 0 in the form of:
>
> Corr-f-g(i) = 0 if and only if |f(i)-g(i)| < t
>
> etc.
>
> The problem becomes much larger in the real world of control
> systems where the output influences the next input of the
> sequence. The implementation and the model will tend to drift
> apart. In the worst case what might be nice and stable in the
> model might exhibit unstable behaviour in the implementation.
>
> You're then in the subject of mathematical chaos, where a
> perfectly deterministic system exhibits unstable and
> unpredictable
> behaviour. However, this email is too small to describe
> it. :-)
>
> Cheers,
> Dave
>
> On 2015-03-09 11:48:57 +0100, Peter Bernard Ladkin wrote:
> > Nick,
> >
> > Consider a mathematical function, f with domain D and
> range R.
> Given input i \in D, the output is f(i).
> >
> > Consider another function, g, let us say for simplicity
> with the
> same input domain D and range R.
> >
> > Define a Boolean function on D, Corr-f-g(i):
> >
> > Corr-f-g(i) = 0 if and only if f(i)=g(i);
> > Corr-f-g(i) = 1 if and only if f(i) NOT-EQUAL g(i)
> >
> > If X is a random variable taking values in D, then
> f(X), g(X) are
> random variables taking values in
> > R, and Corr-f-g(X) is a random variable taking values
> in {0,1}.
> >
> > If S is a sequence of values of X, then let Corr-f-g(S)
> be the
> sequence of values of Corr-f-g
> > corresponding to the sequence S of X-values.
> >
> > Define Min-1(S) to be the least place in Corr-f-g(S)
> containing a
> 1; and to be 0 if there is no such
> > place.
> >
> > Suppose I construct a collection of sequences S.i, each
> of length
> 1,000,000,000, by repeated
> > sampling from Distr(X). Suppose there are 100,000,000
> sequences I
> construct.
> >
> > I can now construct the average of Min-1(S) over all the
> 1,000,000,000sequences S.i.
> >
> > All these things are mathematically well-defined.
> >
> > Now, suppose I have deterministic software, S. Let f(i)
> be the
> output of S on input i. Let g(i) be
> > what the specification of S says should be output by S
> on input
> i. Corr-f-g is the correctness
> > function of S, and Mean(Min-1(S)) will likely be very
> close to
> the mean time/number-of-demands to
> > failure of S if you believe the Laws of Large Numbers.
> >
> > I have no idea why you want to suggest that all this is
> nonsensical and/or wrong. It is obviously
> > quite legitimate well-defined mathematics.
> >
> > PBL
> >
> > Prof. Peter Bernard Ladkin, Faculty of Technology,
> University of
> Bielefeld, 33594 Bielefeld, Germany
> > Je suis Charlie
> > Tel+msg +49 (0)521 880 7319 www.rvs.uni-bielefeld.de
> <http://www.rvs.uni-bielefeld.de/>
> <http://www.rvs.uni-bielefeld.de>
> <http://www.rvs.uni-bielefeld.de/>
> >
> >
> >
> >
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> --
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> Disclaimer: my opinion, not necessarily that of my employer.
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> _______________________________________________
>
> Bev Littlewood
> Professor of Software Engineering
> Centre for Software Reliability
> City University London EC1V 0HB
>
> Phone: +44 (0)20 7040 8420 Fax: +44 (0)20 7040 8585
>
> Email: b.littlewood_at_xxxxxx >
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systemsafety_at_xxxxxx Received on Sat Mar 14 2015 - 12:37:39 CET

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