DESIGN PROCESS AND CONCEPTS

Design process s/w design is an iterative process through which requirements are translated into a “blueprint” for constructing the s/w. There are 3 charecteristics for evaluating a good design 1. The design must implement all the explicit requirements and it must accommodate all the implicit requirements. 2. The design must be readable, understandable for those who generate code and test the s/w. 3. The design must provide a complete picture of the software,addressing the data,functional and behavioral domains.

Cont.. Design Guidelines A design should exhibit an architectural structure A design should be modular A design should contain distinct representations of data, architecture, interfaces, & components A design should lead to data structures that are appropriate for the objects to be implemented A design should lead to components that exhibit independent functional characteristics A design should lead to interfaces that reduce the complexity of connections between modules and external environment A design should be derived using a repeatable method that is driven by information obtained during SW requirement analysis. A design should be represented using a notation

Design Concepts Abstraction Refinement Modularity Architecture Information Hiding Functional Independence

1.Abstraction concentrate on the essential features and ignore details that are not relevant(ie.,low level details) Procedural abstraction is a sequence of instructions that has a specific & limited function Data abstraction is a collection of data that describes a data object Control abstraction implies a program control mechanisms without specify internal details

2.Refinement stepwise refinement is a top down strategy refinement is actually a process of elaboration. develop hierarchy by decompose a procedural abstraction in a stepwise fashion until programming languages are reached abstraction & refinement are complementary concepts Abstraction Refinement

3.Modularity system is decomposed into a number of modules software architecture and design patterns represent modularity 5 criteria to evaluate a design method with respect to its ability to define effective modular system i. Modular decomposability - provides a systematic approach for decomposing the problem into sub problems ii. Modular composability - enables existing design components to be assembled into a new system iii. modular understandability - module can be understood as a standalone unit (no need to refer other modules) iv. modular continuity - small changes to the system requirements result in changes to individual modules v. modular protection - unexpected condition occurs within a module & its effects are constrained within that module.

4.Architecture is the structure or organization of program components (modules) 5 different types of models are used to represent the architectural design : i. structural models : represent architecture as an organized collection of program components ii. framework models : identify repeatable architectural design framework that similar to the types of applications iii. dynamic models : address the behavioral aspects of the program architecture iv. process models : focus on the design of the business or technical process of a system v. functional models : represent functional hierarchy of a system

5. Information Hiding Modules should be specified & designed so that information (procedure & data) contained within a module is inaccessible to other modules that have no need for such information. The use of information hiding for modular systems provides the greatest benefits when modifications are required during testing and later, during s/w maintenance.

6. Functional Independence It is a direct outgrowth of modularity and the concepts of abstraction and information hiding. Cohesion-is a measure of the relative functional strength of a module. Coupling-is a measure of the relative interdependence among modules.