Software Design Principles “Producing the software blueprint”

Objectives To understand the importance of design in developing quality software To describe the translation from the requirements analysis model to the design model To understand the principles that guide proper design of software

Designing A House If you are asked to design a house… D W W Kitchen Room 2 D WC D Living Room Room 1 D W W

What Is Design? Explaining the idea/concept of something Usually with graphical diagrams With the intention to build from the explanation The design is a representation of a product or a system with sufficient detail for implementation

The Second Task Problem Analysis Design Models Development Solution Testing

Designing Software From our understanding of the problem, we start building the software Translate the analysis model into the design model Map the information from the analysis model to the design representations - data design, architectural design, interface design, procedural design

Translation Model Data Object Description Entity- Relationship Diagram Data Object Description Process Specification (PSPEC) Procedural design Data Dictionary Data Flow Diagram Interface design State-Transition Diagram Architectural design Control Specification (CSPEC) Data design

Design Principles Design process should not suffer from “tunnel vision” The design should be traceable to the analysis model The design should not reinvent the wheel; Time is short The design should “minimize intellectual distance” between the software and the problem in the real world

Design Principles (Continued) The design should exhibit uniformity and integration The design should be structured to accommodate change The design should be structured to degrade gently.

Design Principles (Continued) Design is not coding, coding is not design The design should be assessed for quality as it is being created, not after the fact The design should be reviewed to minimize conceptual errors

Design Concepts Fundamental concepts which provide foundation to design correctly: Abstraction Refinement Modularity Software Architecture Control Hierarchy Structural Partitioning Data Structure Software Procedure Information Hiding

Abstraction Identifying important features for representation There are many levels of abstraction depending on how detailed the representation is required Data abstraction - representation of data objects Procedural abstraction - representation of instructions

Refinement Stepwise refinement - top-down design strategy by Niklaus Wirth Starting at the highest level of abstraction, every step of refinement ‘decompose’ instructions into more detailed instructions Complementary to abstraction

Modularity Software is divided into separately named and addressable modules “Divide and conquer” approach - problem is broken into manageable pieces Solutions for the separate pieces then integrated into the whole system

Divide And Conquer S1 S2 P1 P2 S5 P5 P4 P3 S3 S4

Software Architecture Modules can be integrated in many ways to produce the system Software architecture is the overall structure of the software The hierarchy of components and how they interact, and the structure of data used by the components Use of framework models, and possible reuse of architectural patterns

Software Architecture Patterns Recurring pattern help designers reuse successful designs by basing new designs on prior experience. A designer who is familiar with such patterns can apply them immediately to design problems without having to rediscover them.

Why use Design Patterns? Reuse successful practices. Not new – recognised that this is something that engineers have done for years. Improve communication Step towards a software engineer’s handbook

Examples of Software Architecture Program structure Batch sequential pattern S3 S2 S4 S5 S1

Control Hierarchy Hierarchy of modules representing the control relationships A super-ordinate module controls another module A subordinate module is controlled by another module Measures relevant to control hierarchy: depth, width, fan-in, fan-out

Structure Terminology Fan-out a b c Depth d e f g h Fan-in i Width

Structural Partitioning Program structure partitioned horizontally and vertically Horizontal partitioning defines separate branches for each major program function - input, process, output Vertical partitioning (aka factoring) defines control (decision-making) at the top and work at the bottom

Software Procedure Processing details of individual modules Precise specification of processing, including sequence of events, exact decision points, repetitive operations, and data organization/structure Procedure is layered - subordinate modules must be referenced in processing details

Information Hiding Information (procedure and data) contained within a module is inaccessible to other modules that have no need for such information Effective modularity is achieved by independent modules, that communicate only necessary information Ease of maintenance - testing, modification localized and less likely to propagate

References “Software Engineering: A Practitioner’s Approach” 5th Ed. by Roger S. Pressman, Mc-Graw-Hill, 2001 “Software Engineering” by Ian Sommerville, Addison-Wesley, 2001

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