Re: The Airplane Example (was Re: Streaming XML)
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
begin:vcard fn:Rick Marshall n:Marshall;Rick email;internet:rjm@z... tel;cell:+61 411 287 530 x-mozilla-html:TRUE version:2.1 end:vcard
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