Information Technology Reference
In-Depth Information
chases one version of a software package, then the user is entitled to
free upgrades—perhaps for a year or more. In other cases, the user
may be required to purchase upgrades. In the latter situation, a de
veloper may have particular incentive to generate new versions fre
quently, because each new release represents another opportunity
for additional revenue and profit.
As we have just seen, some errors in software may be the result
of difficulties that were not discovered in testing. A mistake was
made in the development of the program, but that problem was not
revealed by comprehensive testing. Software errors, however, are
not composed solely of overlooked mistakes.
The correctness of programs requires a remarkable degree of
completeness and accuracy. Unfortunately, unforeseen problems
can arise well beyond the testing phase of software development.
Here are two illustrative examples:
WWMCCS Example
The World Wide Military Command and Control System is a computer network
designed to provide communications for the military both in peace and during
emergencies. In November 1978, a power failure disrupted communications be
tween Florida and Washington, D.C. When power was restored, the Washington
computer was unable to reestablish communications with the Florida machine.
In reviewing the specifications, it seems that mechanisms were available for
connecting new machines to the existing systems; additional computers could
“log on” to the network. No one, however, had anticipated the need for an ex
isting computer to log on to the system a second time, and this omission pre
vented the computers from reestablishing communications in a normal manner
following the blackout.*
* From
The Limits of Computing
, p. 97. For more information, see William Broad,
“Computers and the U.S. Military Don't Mix,”
Science
, Volume 207, March 14, 1980,
p. 1183, and quoted in
Software Engineering Notes
, Volume 11, Number 5, October
1986, p. 17.
