A strategy for software testing may also be viewed in the context of the spiral
(Figure 17 .1). Unit testing begins at the vortex of the spiral and concentrates on each
unit (e.g., component, class, or WebApp content object) of the software as implemented
in source code. Testing progresses by moving outward along the spiral to
integration testing, where the focus is on design and the construction of the software
architecture. Taking another turn outward on the spiral, you encounter validation
testing, where requirements established as part of requirements modeling are validated
against the software that has been constructed. Finally, you arrive at system
testing, where the software and other system elements are tested as a whole. To test
computer software, you spiral out in a clockwise direction along streamlines that
broaden the scope of testing with each turn.