Your "What" Is My "How": Iteration and Hierarchy in System Design

Systems are naturally constructed in hierarchies in which design choices made at higher levels of abstraction levy requirements on system components at lower levels of abstraction. Thus, whether an aspect of the system is a design choice or a requirement depends largely on one’s vantage point within the hierarchy of system components. Furthermore, systems are often constructed middle-out rather than top-down; compatibility with existing systems and architectures, or availability of specific components influences high-level requirements. We argue that requirements and architectural design should be more closely aligned: that requirements models must account for hierarchical system construction, and that architectural design notations must better support specification of requirements for system components. We briefly describe work to this end that was performed on the META II project and describe the gaps in this work that need to be addressed to meet practitioner needs.