Software Engineering for Students A Programming Approach

254 Chapter 17 ■ Software robustness
SELF-TEST QUESTION
17.9 Write pre- and post-conditions for method pop.
Note that truth of assertions does not guarantee that the software is working correctly.
However, if the value of an assertion is false, then there certainly is a fault in the
software. Note also that violation of a precondition means that there is a fault in the
user of the method; a violation of a postcondition means a fault in the method itself.
There are two main ways to make use of assertions. One way is to write assertions as
comments in a program, to assist in manual verification. On the other hand, as indicated
by the notation used above, some programming languages (including Java) allow assertions
to be written as part of the language – and their correctness is checked at runtime.
If an assertion is found to be false, an exception is thrown.
There is something of an argument about whether assertions should be used only
during development, or whether they should also be enabled when the software is put
into productive use.
17.9 ● Discussion
Fault tolerance in hardware has long been recognized – and accommodated. Electronic
engineers have frequently incorporated redundancy, such as triple modular redundancy,
within the design of circuits to provide for hardware failure. Fault tolerance in software
has become more widely addressed in the design of computer systems as it has become
recognized that it is almost impossible to produce correct software. Exception handling
is now supported by all the mainstream software engineering languages – Ada, C++,
Visual Basic, C# and Java. This means that designers can provide for failure in an organized
manner, rather than in an ad hoc fashion. Particularly in safety-critical systems,
either recovery blocks or n-programming is used to cope with design faults and enhance
reliability.
Fault tolerance does, of course, cost money. It requires extra design and programming
effort, extra memory and extra processing time to check for and handle exceptions.
Some applications need greater attention to fault tolerance than others, and
safety-critical systems are more likely to merit the extra attention of fault tolerance.
However, even software packages that have no safety requirements often need fault
tolerance of some kind. For example, we now expect a word processor to perform
periodic and automatic saving of the current document, so that recovery can be performed
in the event of power failure or software crash. End users are increasingly
demanding that the software cleans up properly after failures, rather than leave them
with a mess that they cannot salvage. Thus it is likely that ever-increasing attention
will be paid to improving the fault tolerance of software.