LIM, MARYLYN GRACE C.

OOA

            Book: Systems Analysis and Design: an Active Approach

            Author: George M. Marakas

            Reference No.: QA

                                    76.9

                                    S88

                                    M37

                                    2001

Chapter 12: Designing the System Internals

Quote: “Things should be made as simple as possible, but not any simpler.”

Review:

            This chapter talks about the things associated with internal design of the system. Modular design is an approach that is used in software design. Modular design is an approach that is used in software design. This approach helps analyst to decompose a large and complex software application a large and complex software application into a smaller interrelated components called modules. Modules are a group of executable instruction that has a single point of entry and exit.

            There are 4 important principles of good internal design that should be remembered. 1.) System Factoring. This principle said that the system should be decomposed into small modules that conforms both in size and cohesion guidelines of good design. In system factoring, there are 2 approached used. First is bottom-up approach. This identifies the process needed to be a part of the system then moves forward to code each process as a module that interfaces with all other processes. Second is top-down approach. This views the hardest possible sense then decomposed it into subsystem that works together to efficiently and effectively reach the objectives of the system. 2.) Module Span. This principle states that a parent should not exceed 5-7 children, subordinate and modules. 3.) Module cohesion. This is a measure of completeness. There are instructions contained within a module that pertains only to the function. 4.) Module Coupling. This states that the modules that are dependent to each other should be minimized and that the amount of communication between dependent modules must also be minimized.

            There are 7 types of cohesion. First is functional cohesion. In this type, the modules are designed so that the instructions would work collectively to accomplish a task. Second is sequential cohesion. This defines as the relationship between an instruction and the next in a given module. Third is communicational cohesion. This happens where a module is designed so that the instructions accomplish tasks that use the same data but the sequence of the instructions isn’t critical to a successful outcome. Fourth is procedural cohesion. This happens when modules that have instructions grouped together because of some common relationship based on time. Sixth is logical cohesion. This happens when instruction are related to each other only because they are in the same logical class of functions. Seventh is coincidental cohesion. This happens when modules that have instructions that have no relationship to each other.

            There are 4 types of coupling. First is data coupling. This states that the dependency among 2 modules is limited to the fact that they pass data between them. Second is stamp coupling. This states that data are passed among modules in the form of data structure. Third is common coupling. This states that to consider 2 modules as common coupled, they should both refer to the same global data area. Fourth is control coupling. This tells us that control information is passed among modules.

            Hierarchical structure diagram displays the relationships of the modules to each other and displays the flow and processing of data among the modules of the system. This is a top-down approach to system development using modules. The difference of hierarchical structure diagram and data flow diagram is that in DFD the intended audience is composed of business managers and end users while in HSD the intended audience is the application programmers.