The need for levels of testing
Execution-based
software testing, especially for large systems, is usually carried out at
different levels. In most cases there will be 3-4 levels, or major phases of
testing: unit test, integration test, system test, and some type of acceptance
test as shown in Figure 6.1. Each of these may consist of one or more sublevels
or phases. At each level there are specific testing goals. For example, at unit
test a single component is tested. A principal goal is to detect functional and
structural defects in the unit. At the integration level several components are
tested as a group, and the tester investigates component interactions. At the
system level the system as a whole is tested and a principle goal is to
evaluate attributes such as usability, reliability, and performance. System
test begins when all of the components have been integrated successfully. It
usually requires the bulk of testing resources. Laboratory equipment, special
software, or special hardware may be necessary, especially for real-time,
embedded, or distributed systems. At the system level the tester looks for
defects, but the focus is on evaluating performance, usability, reliability,
and other quality-related requirements.
The
approach used to design and develop a software system has an impact on how
testers plan and design suitable tests. There are two major approaches to
system development—bottom-up, and top-down. These approaches are supported by
two major types of programming languages— procedure-oriented and
object-oriented.
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