The Strategy in Functional Modeling

The methodology of structured systems analysis & design provides a roadmap for the development of functional specifications for an accounting information system, shown in the Figure below.

 
Figure:   Structured Systems analysis & Design Methodology

The functional specifications are documented graphically in Dataflow Diagrams (DFDs) described in the next section below.

STEP 0: (Defining the scope of the system under study.) This accomplished by drawing the context diagram for the system.

STEP 1: (Documentation of how the existing system works.) This is accomplished by drawing the Physical DFDs of the existing system. These DFDs specify the current implementation of the existing system, and would answer questions such as:

• Who performs the tasks?
• How they are performed?

• When or how often they are performed?

• How the data is stored (media)?

• How the dataflows are implemented (media)?

These physical DFDs may be levelled, or, if the system is not very large, prepared all on a single DFD.

STEP 2: (Documentation of what the existing system does.)This is documented in Logical DFDs of the existing system. Deriving these logical DFDs of the existing system from the physical DFDs involve abstraction of all implementation details. Since the systems designer would not like to be tied down by the current implementation of the system, all such details are abstracted. These logical DFDs are usually levelled in order to reduce the perceived complexity of the system, and balanced in order to assure consistency in the design.

STEP 3: (Documentation of what the proposed system will do.) After step 2, the systems designer will examine why the existing system does not meet the user requirements, and how it can be modified in order to meet such needs. The result is a set of logical DFDs which describe what the modified (proposed) system will do. These functional specifications are devoid of implementation considerations, and therefore rather abstract specifications of the proposed system. These logical DFDs are also levelled and balanced.

STEP 4: (Documentation of how the proposed system will work.) The logical DFDs of the proposed system derived in step 3 above are then examined to determine which implementation of it meets the user requirements most efficiently. The result is a set of physical DFDs of the proposed system. They answer questions such as:

• Who will perform the various tasks?

• How they will be performed?

• When or how often they will be performed?

• How the data will be stored (media)?

• How the dataflows will be implemented (media)?

In this step, man-machine boundaries are drawn, and media selected for all dataflows & datastores.