Internal and External Interface Design

The design of internal program interfaces sometimes called interlobular interface dewing is driven through the data which must flow among modules and the characteristics of the programming language in that the software is to be implemented. In common the analysis model contains much of the information needed for interlobular interface design. The data flow diagram defines how data objects are transformed as they move by a system. The transforms of the Data Flow Display are mapped into modules within the program structure.   Thus the arrows flowing into and out of each Data Flow Display transform must be mapped into a design for the interface of the module which corresponds to that Data Flow Display transform.

The External interface design starts with an evaluation of each external entity represented in the DFDs of the analysis model. The control and data requirements of the external entity are appropriate and determined external interfaces are designed. Example for the Safe Home software discussed previously in this chapter needs interfacing with a variety or external security sensors. Design of the external interface for each and every sensor is predicated on the exact control and data items needs for the sensor.

Both techniques external and internal interface designs must be coupled with error handling algorithms and data validation within a module because side effects propagate across program interfaces it is essential to check all data flowing from module to module to ensure in which the data conform to bounds recognized during requirement analysis.