Structured design

The mapping technique to be presented enables a designer to derive reasonably complex call and return architectures from data flow diagrams within the requirements model. Structured design is often characterized as a data flow-oriented design method because it provides a convenient transition from a data flow diagram to software architecture.

The transition from information flow (represented as a DFD) to program
structure is accomplished as part of a six-step process:

(1) the type of information flow is established;
(2) flow boundaries are indicated;
(3) the DFD is mapped into program structure;
(4) control hierarchy is defined;
(5) resultant structure is refined using design measures and heuristics; and
(6) the architectural description is refined and elaborated.

The type of information flow is the driver for the mapping approach required in step 3. In the following sections we examine two flow type

14.5.1 Transform Flow
Information enters the system along paths that transform external data into an internal form. These paths are identified as incoming flow. At the kernel of the software, a transition occurs. Incoming data are passed through a transform center and begin to move along paths that now lead "out" of the software. Data moving along these paths are called outgoing flow. The overall flow of data occurs in a sequential manner and follows one, or only a few, "straight line" paths. When a segment of a data flow diagram exhibits these characteristics, transform flow is present.

Transform Mapping
Transform mapping is a set of design steps that allows a DFD with transform flow characteristics to be mapped into a specific architectural style. In this section transform mapping is described by applying design steps to an example system—a portion of the SafeHome security software