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6.4 Lifecycle models: showing how the activities relate 187
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6.4.2 Lifecycle models in software engineering
Software engineering has spawned many lifecycle models, including the water-
fall, the spiral, and rapid applications development (RAD). Before the waterfall
was first proposed in 1970, there was no generally agreed approach to software
development, but over the years since then, many models have been devised, re-
flecting in part the wide variety of approaches that can be taken to developing
software. We choose to include these specific lifecycle models for two reasons:
First, because they are representative of the models used in industry and they
have all proved to be successful, and second, because they show how the empha-
sis in software development has gradually changed to include a more iterative, 1
user-centered view.

The waterfall lifecycle model

The waterfall lifecycle was the first model generally known in software engineer-
ing and forms the basis of many lifecycles in use today. This is basically a linear
model in which each step must be completed before the next step can be started
(see Figure 6.8). For example, requirements analysis has to be completed before

Figure 6.8 The waterfall lifecycle model of software development.