piclist 2009\01\30\084243a >
Thread: Agile programming (was Re: [P|C] Banksel)
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face BY : Jake Anderson email (remove spam text)



Alan B. Pearce wrote:
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I have been trying to work out why it is we can't make reliable
software, but we can (generally) build a bridge that is going to last
100 years.
It comes down to margins. When you are building something you say, well
this beam has a load of X so I'll put a beam in that will take 2X as a
load without breaking. Then down the track when it turns out the builder
is using particularly heavy bricks that beam sees 1.3X the load it was
meant to see everything is still handled.

What I am yet to see is how you can apply "margin" to software design,
its too "perfect" a system in many respects, If you miss one check to
see if a number is zero before doing a divide your hosed. Sure you can
try and "handle" those kinds of errors, but that seems more like
propping up the building after the balcony has fallen off.

You can apply margin to things like speed and the like but the only
program/system I'm aware of that feels like it has that "2x" safety
factor was the lunar module guidance computer.
http://klabs.org/history/apollo_11_alarms/eyles_2004/eyles_2004.htm


Anybody have any thoughts on "margin" as it applies to software?

<498303B8.8050004@vapourforge.com> 7bit

In reply to: <FCB48E2BA4A44D9FA56F161C9E9FEA45@space.rl.ac.uk>
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Subject (change) Agile programming (was Re: [P|C] Banksel)

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