1.3. Underlying Principles
The specific techniques presented in this book are based upon general principles that we believe are fundamental to system development regardless of the particular techniques used. It is worth stating these principles right here, at the outset, to help you keep them firmly in mind as you progress through the book. We elaborate on them in later chapters, but here they are in summary:
- Every system below the level of the whole universe is a component of one or more larger systems. The larger systems are the context or environment in which the component system must work.
- Thus, systems comprise a layered set of subsystems below the layer with which we happen to be dealing, and a layered set of supersystems above that layer. This layered structure can be exploited both in representing systems and in defining the system development process.
- Most systems are members of multiple layered sets. The particular set or sets chosen to represent a system are determined by the view-point or viewpoints that are important for the particular system.
- Every system has a set of essential requirements, which meet the needs of the context or environment, without imposing any specific implementation, and a set of physical requirements, which reflect the architectural and design decisions made to satisfy the essential requirements.
- To carry out the essential requirements, and thereby to meet the needs of the environment, systems receive as inputs, produce as outputs, and process internally information and/or material and/or energy.
To achieve the dependability and flexibility needed in the development of complex systems, all of the system artifacts invoked by these principles must be represented separately, but with their relationships and interactions also represented. These artifacts include, at a minimum: the layered system structure and the relationships within it; the subsystems, the supersystems, and their relationships; the essential requirements, the physical requirements, and their relationships; the information, material, and energy that travel into and out of the sub- and supersystems; the processing of that information, material, and energy; and the links between the information, material, and energy, their processing, and the sub- and supersystems.
These, then, are the principles that we follow throughout this book.