Software Engineering-A Software Testing Strategy

The software engineering process may be viewed as the spiral illustrated in the figure. Initially, system engineering defines the role of software and leads to software requirements analysis, where the information domain, function, behavior, performance, constraints, and validation criteria for software are established. Moving inward along the spiral, we come to design and finally to coding. To develop computer software, we spiral inward along streamlines that decrease the level of abstraction on each turn.

A strategy for software testing may also be viewed in the context of the spiral . Unit testing begins at the vortex of the spiral and concentrates on each unit (i.e., component) of the software as implemented in source code. Testing progresses by moving outward along the spiral to integration testing, where the focus is on designand the construction of the software architecture. Taking another turn outward on the spiral, we encounter validation testing, where requirements established as part of software requirements analysis are validated against the software that has been constructed. Finally, we arrive at system testing, where the software and other system elements are tested as a whole. To test computer software, we spiral out along streamlines that broaden the scope of testing with each turn.

Considering the process from a procedural point of view, testing within the context of software engineering is actually a series of four steps that are implemented sequentially. The steps are shown in figure. Initially, tests focus on each component individually, ensuring that it functions properly as a unit. Hence, the name unit testing. Unit testing makes heavy use of white-box testing techniques, exercising specific paths in a module's control structure to ensure complete coverage and maximum error detection. Next, components must be assembled or integrated to form the complete software package. Integration testing addresses the issues associated with the dual problems of verification and program construction. Black-box test case design techniques are the most prevalent during integration, although a limited amount of white-box testing may be used to ensure coverage of major control paths. After the software has been integrated (constructed), a set of high-order tests are conducted. Validation criteria (established during requirements analysis) must be tested. Validation testing provides final assurance that software meets all functional, behavioral, and performance requirements. Black-box testing techniques are used exclusively during validation.

The last high-order testing step falls outside the boundary of software engineering and into the broader context of computer system engineering. Software, once validated, must be combined with other system elements (e.g., hardware, people, databases). System testing verifies that all elements mesh properly and that overall system function/performance is achieved.

Criteria for Completion of Testing

A classic question arises every time software testing is discussed: "When are we done testing—how do we know that we've tested enough?" Sadly, there is no definitive answer to this question, but there are a few pragmatic responses and early attempts at empirical guidance.

One response to the question is: "You're never done testing, the burden simply shifts from you (the software engineer) to your customer." Every time the customer/ user executes a computer program, the program is being tested. This sobering fact underlines the importance of other software quality assurance activities. Another response (somewhat cynical but nonetheless accurate) is: "You're done testing when you run out of time or you run out of money."

Although few practitioners would argue with these responses, a software engineer needs more rigorous criteria for determining when sufficient testing has been conducted. Musa and Ackerman suggest a response that is based on statistical criteria: "No, we cannot be absolutely certain that the software will never fail, but relative to a theoretically sound and experimentally validated statistical model, we have done sufficient testing to say with 95 percent confidence that the probability of 1000 CPU hours of failure free operation in a probabilistically defined environment is at least 0.995."

Using statistical modeling and software reliability theory, models of software failures (uncovered during testing) as a function of execution time can be developed . A version of the failure model, called a logarithmic Poisson execution-time model, takes the form

               f(t) = (1/p) ln [l0 pt + 1]         (1)

where f(t) = cumulative number of failures that are expected to occur once the software has been tested for a certain amount of execution time, t,l0 = the initial software failure intensity (failures per time unit) at the beginning of testing, p = the exponential reduction in failure intensity as errors are uncovered and repairs are made.
The instantaneous failure intensity, l(t) can be derived by taking the derivative of  f(t)

               l(t) = l0 / (l0 pt + 1)      (2 )

Using the relationship noted in Equation (2), testers can predict the drop-off of errors as testing progresses. The actual error intensity can be plotted against the predicted curve . If the actual data gathered during testing and the logarithmic Poisson execution time model are reasonably close to one another over a number of data points, the model can be used to predict total testing time required to achieve an acceptably low failure intensity.

By collecting metrics during software testing and making use of existing software reliability models, it is possible to develop meaningful guidelines for answering the question: "When are we done testing?" There is little debate that further work remains to be done before quantitative rules for testing can be established, but the empirical approaches that currently exist are considerably better than raw intuition.
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