On the Criteria To Be Used in Decomposing Systems into Modules D.L. Parnas Carnegie-Mellon University

MODULARIZATION What is modularization ? Decomposing a system into smaller independent segments that have their own well defined inputs and outputs. These segments are generally called as Modules and the process of this segmentation is called Modularization. Why is it required ? Managerial Product flexibility Comprehension

CRITERIA Based on every major step in the program Flowchart Suitable for small systems Based on Information hiding Design decisions Hides knowledge from others Independent from each other

SPECIFIC CRITERIAS Few examples of specific modules that are advisable Data structures, its access and modifying procedures The call to a routine and the routine itself Formats of control blocks Character codes, alphabetic orderings and similar data The sequence in which certain items will be processed Note: Decomposition is not the same as hierarchical structure.

CONCLUSION The criteria for decomposing a program must be based on information hiding and design decisions rather than flow of procedure and major actions. This enables the programmers to change the functionality of the program by changing the respective module and not rewriting parts of modules that rely on that functionality. The processes of integration, testing and maintainability are simplified

Discussion Do you agree with the author ? Any other criteria for decomposition ? Importance of hierarchy while decomposing Examples of bad decomposition