Drexel University

CS 451Software Engineering

Winter 2009

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Yuanfang Cai

Room 104, University Crossings215.895.0298

yfcai@cs.drexel.edu

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Design within the Context of Software Engineering

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Software Design Between Requirement and Coding Including:

Data Design Architectural Design Interface Design Component Design Detailed Design

Need to be modeled, analyzed, and reviewed in industrial strength software.

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Translating the Analysis Model into the Design Model

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Data/Class Design

Architecture Design

Interface Design

Component Design

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Design Process and Desgin Quality

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Design Engineering Software design is an iterative process through

which requirements are translated into a “blueprint” for constructing software Abstraction Refinement

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Design Engineering A design must implement all of the explicit requirements

contained in the analysis model, and it must accommodate all of the implicit requirements desired by the customer.

A design must be a readable, understandable guide for those who generate code and those who test and subsequently support the software.

The design should provide a complete picture of the software, addressing, the data, functional, and behavioral domains from an implementation perspective.

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Design Quality FURPS – Functionality, Usability, Reliability,

Performance, and Supportability. Functionality – assessed by evaluating:

the feature set capabilities of the program.

Usability - assessed by considering: human factors, overall aesthetics, consistency, end-user documentation.

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Design Quality - Functionality, Usability, Reliability, Performance, and Supportability

Reliability – is evaluated by measuring: the frequency and severity of failure, the accuracy, of output results, the mean-time-to-failure, the ability to recover from failure, the predictability of the program.

Performance – is measured by: processing speed, response time, resource consumption, throughput, efficiency

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Design Quality - Functionality, Usability, Reliability, Performance, and Supportability

Supportability – combines: the ability to extend the program (extensibility), adaptability, serviceability testability, compatibility, configurability.

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Design Concepts

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Design Concepts Abstraction

Architecture Patterns Data

Modularity Information Hiding Functional Independence Refinement Refactoring

Design Classes12

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Design Concepts-Abstraction “Abstraction is one of the fundamental ways that

we as humans cope with complexity.” Grady Booch

“What kinds of things do we abstract? data objects procedures modules just about anything

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Design Concepts-Architecture Software architecture alludes to “the overall

structure of the software and the ways in which that structure provides conceptual integrity for a system.

Architecture is: the structure or organization of program components

(modules), the manner in which these components interact, the structure of data that are used by the

components.

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Design Concepts-Patterns “A pattern is a named nugget of insight which conveys

the essence of a proven solution to a recurring problem within a certain context amidst competing concerns.”

“Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice.” Christopher Alexander

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Design Concepts -Modularity MODULARITY

“Modularity is the single attribute of software that allows a program to be intellectually manageable”

Software is divided into separately named and addressable components, sometimes called modules, that are integrated to satisfy problem requirements.

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Design Concepts –Information Hiding Modules should be specified and designed so that

information (algorithms and data) contained within a module is inaccessible to other modules that have no need for such information.

This means that inadvertent errors introduced during modification are less likely to propagate to other locations within the software.

Changes to the internal representation of one module should have not have an effect on other modules.

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Design Concepts-Functional Independence

Functional independence is achieved by developing modules with “single-minded” function and an “aversion” to excessive interaction with other modules.

We want to design software so that each module addresses a specific subfunction of requirements and has a simple interface when viewed from other parts of the program structure.

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Design Concepts-Functional Independence Independence is assessed by using two

qualitative criteria: Cohesion – How related a module is to itself. It should

perform a single task and require little interaction with the rest of the program.

Coupling is an indication of the interconnectoin among modules in a software structure.

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Design Concepts-Refinement Stepwise refinement is when a program is

developed by successively refining levels of procedural detail.

Refinement is actually the process of elaboration.

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Design Concepts-Refactoring Refactoring is a reorganizational technique that

simplifies the design )of code) of a component without changing its function or behavior.

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Design Concepts-Design Classes Refine analysis classes by providing design details

Create a new set of design classes that implement a software infrastructure to support the business solution

Five types: User interface classes Business domain classes Process classes Persistent classes System classes

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