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RE: The Airplane Example (was Re: Streaming XML)

• To: 'Elliotte Rusty Harold' <elharo@m...>, Benjamin Franz <snowhare@n...>
• Subject: RE: The Airplane Example (was Re: Streaming XML)
• From: "Bullard, Claude L (Len)" <len.bullard@i...>
• Date: Tue, 4 Jan 2005 11:15:52 -0600
• Cc: XML Developers List <xml-dev@l...>

Not exactly true.   Until you work with real time systems, 
it is easy to think software always behaves the same way 
every time.

Here is a thought experiment.   In a separate thread, 
the concept of information entropy was discussed again. 

Consider entropy a cost value:

o  In a state network, a decision can have multiple 
options.   The value of any information is the number 
of these outcomes it eliminates.

o  A state network may be multi-dimensional.  While 
we think of it as a single choice path, it can be 
multiple paths occurring virtually simultaneously 
in real time.

o  Where there are dependencies among these paths, 
the cost of controlling these and the value of that 
control information must be known to determine the 
energy budget for the system.   If one can optimize 
that decision cost, the information value is inversely 
proportional to the proximity to its application in 
real time.

The network effect is related to the service cost in 
real time.   The reliability of software is a function 
of that cost.

len


From: Elliotte Rusty Harold [mailto:elharo@m...]

At 7:03 AM -0800 12/31/04, Benjamin Franz wrote:

>People treat software like it isn't real. Software machines are just 
>as real as hardware machines - and often control hardware machines. 
>If you are killed by a radiation therapy machine because of broken 
>software - you are just as dead as if the cause was broken hardware.

Yes, but there is a qualitative difference between software failures 
and hardware failures (though the effects of either can be equally 
damaging). Software mostly fails due to outright bugs and failure to 
anticipate certain conditions it encounters. However, if it works in 
a certain condition, it always works. Hardware can fail for these 
reasons, but it also has an additional way to fail most software 
doesn't: it decays over time as parts wear. It is completely 
plausible for a piece of hardware to work 10,000 times in a row and 
then fail the 10,001st time, even though nothing has changed. This 
style of failure is very rare for software. Software needs to be 
upgraded and maintained to handle changes in the environment where 
the software runs, not because the software wears out.

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