Re: [xml-dev] The Airplane Example (was Re: [xml-dev] Streaming XML)

[Rick Marshall <rjm@zenucom.com>]

Elliotte Rusty Harold wrote:

> 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.

i basically agree (i use the argument to beat up users all the time - 
they changed the procedure, not me changing the software that caused the 
failure ;) ). however my experience to date is that 1) software failure 
cost can be analysed the same way as hardware failure cost - probability 
* expected cost = expected loss (and we use this to prioritise 
maintenance work); 2) software systems do degrade, as mentioned by 
changing external systems. but with modern hardware reliability i think 
hardware now has more in common that software in this respect. eg most 
of the hardware upgrades we complete are now due to obsolesence rather 
than failure - or if you prefer performance below expectation is a 
failure mode that increases with time 3) in spite of 40+ years of 
research there is still much to do in the understanding of software 
failure modes. the discussion on exception handling demonstrates just 
how complex failure detection/management can be and 4) the isolation of 
software from the hardware it runs on, seems to me to be a conceptual 
error. to some extent the reliability of software is a function of the 
reliability of the particular hardware involved (much as we'd like to 
ignore it). some of the dicsussions on memeory / disk limits point to 
the subtleties of this - what sort of failure is it when you can't 
process an xml document in memory because you ran out of memory? 
hardware or software?

rick
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