[SystemSafety] What do we know about software reliability?

[Peter Bernard Ladkin]

On 2020-09-16 11:46 , yorklist at philwilliams.f2s.com wrote:
> If A is dependent on some temporal event, and the testing is conducted prior to that event – what
> does the testing tell you about the outcome after that event?

Such things are issues, but I am not sure of the value of posing it so abstractly.

The abstract answer is that the occurrence of the temporal event TE is an environmental predicate:
the characteristic pre-TE or post-TE is part of the environment. So the answer to your question
logically is: it tells you nothing at all because the environment has changed.

But that is hardly helpful. Here is a more concrete example. What does statistical testing prior to
Y2K tell you about how your system works post-Y2K? Or, what does statistical testing prior to 2038
tell you about the operation of your 32-bit Unix system in 2038?

The answer is: design your statistical tests so that both environmental states are represented. Then
you know.

Those are known possible-dependencies. You can - and did, and would - shield your system from such
effects.

Then there are unknown ones. An easter egg triggered by the clock. A GPS-dependency tracing its way
through a library you used.

I don't know of any general answer/prophylaxis in abstract terms. The known dependencies you just
handle individually in whatever way is appropriate. I think you may be able to detect easter eggs by
modified dead-code analysis. I think you handle internal temporal dependencies by performing an
impact analysis on clock values. GPS dependencies can be detected through jamming in the environment
E. And so on.

PBL

Prof. Peter Bernard Ladkin, Bielefeld, Germany
Styelfy Bleibgsnd
Tel+msg +49 (0)521 880 7319  www.rvs-bi.de





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