Re: Writing better software was: Design by Contract (was Re: Interesting thread in comp.lang.eiffel)

[Simon Brady <see@below.for.email.address>]

John Magness wrote:
> 
> In 1992 I took an Ada language course from an adjunct professor who was an
> IBM employee.
> He was a member of the shuttle flight software development team.  He told
> the class that NASA had asked IBM to determine the cost of the delivered
> code. The result of the study was $5000.00 per line of delivered code.
> That's late '80's dollars I think.

What could NASA do to reduce this cost? It seems to me they have three
options:

1. Accept (potentially) lower-quality software and fly with increased
risk.

2. Accept lower-quality software but make system-wide adjustments so
overall risk isn't reduced (e.g. put in more non-software backup)

3. Find a way to produce software of the same quality while using a
cheaper process

(1) is probably the easiest route to take, since risk is hard to
quantify (until it bites you) and "acceptable" risk is a function of all
sorts of malleable social and politcal factors. Of course we have to ask
"acceptable to whom?", and the tradeoffs don't seem at all obvious: how
does one determine the ultimate cost (to national morale, support for
the space program and related industries, etc.) of losing a Shuttle?

Speaking as a software geek, (2) feels like a step backward, yet it
still seems to be an embarressingly appropriate option for many systems
(remember Therac-25). But is it really practical for something as
complex as the Shuttle? Or have we reached a "damned if we do, damned if
we don't" point where we have no choice but to rely on software, creaky
as it may be?

(3) is, IMO, the holy grail of software engineering. If anyone has
realistic suggestions of how it could be done (for the Shuttle or any
similar CMM-5 shop), I for one would be interested to hear them.

Simon Brady                                            sjbrady
Research Assistant, Computer Science Dept.               at
University of Otago, Dunedin, New Zealand            acm dot org