Weaving together requirements and architectures

Software development organizations often choose between alternative starting points-requirements or architectures. This invariably results in a waterfall development process that produces artificially frozen requirements documents for use in the next step in the development life cycle. Alternatively, this process creates systems with constrained architectures that restrict users and handicap developers by resisting inevitable and desirable changes in requirements. The spiral life-cycle model addresses many drawbacks of a waterfall model by providing an incremental development process, in which developers repeatedly evaluate changing project risks to manage unstable requirements and funding. An even finer-grain spiral life cycle reflects both the realities and necessities of modern software development. Such a life cycle acknowledges the need to develop software architectures that are stable, yet adaptable, in the presence of changing requirements. The cornerstone of this process is that developers craft a system's requirements and its architecture concurrently, and interleave their development.