From Building Evolutionary Architectures:
Software suffers from the unknown unknowns problem: developers cannot anticipate everything. During construction, some part of the architecture often shows troubling signs, and building fitness functions can prevent this dysfunction from growing. While some fitness functions will naturally come to light at the beginning of a project, many won’t reveal themselves until an architectural stress point appears. Architects must vigilantly watch for situations where nonfunctional requirements break and retrofit the architecture with fitness functions to prevent future problems.
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