Systems Analysis and Design

Systems Analysis and Design
Alan Dennis, Barbara Haley Wixom, and Roberta Roth John Wiley & Sons, Inc. Slides by Candace S. Garrod Red Rocks Community College PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

Program Design Chapter 10
PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

Program design - creating instructions for the programmers
Key Definitions Program design - creating instructions for the programmers The top-down, modular approach - begin with the “big picture” and gradually add detail Program design document – all structure charts and specifications needed by programmers to implement the system PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

MOVING FROM LOGICAL TO PHYSICAL PROCESS MODELS
Analysis phase – focus on logical processes and data flows Design phase – create physical process models showing “how” the final system will work Physical process models convey the “system view” of the new system PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

The Physical Data Flow Diagram
The physical DFD contains the same components as the logical DFD, and the same rules apply (e.g. balancing, decomposition, etc.) There are five steps to perform to make the transition to the physical DFD PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

Steps to Create the Physical Data Flow Diagram

The Physical Data Flow Diagram (The How)

Resist temptation to write code too quickly
Designing Programs Resist temptation to write code too quickly System quality is enhanced with top-down, modular design Program design document is the final deliverable for this task Used by programmers to write code PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

Using a Top-Down Modular Approach

The Structure Chart Important program design technique
Shows all components of code in a hierarchical format Sequence (in what order components are invoked) Selection (under what condition a module is invoked) Iteration (how often a component is repeated) PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

Structure Chart Example

Structure Chart Elements

Building the Structure Chart
Processes in the DFD tend to represent one module on the structure chart Afferent processes – provide inputs to system Central processes – perform critical system operations Efferent processes – handle system outputs The DFD leveling can correspond to the structure chart hierarchy (e.g., the process on the context-level DFD would correspond to the top module on the structure chart) PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

Types of Structure Charts
Transaction structure – control module calls subordinate modules, each of which handles a particular transaction Few afferent processes Many efferent processes Higher up levels of structure chart Using inputs to create a new output PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

Transaction Structure

Transform Structure This structure has a control module that calls several subordinate modules in sequence after which something “happens.” These modules are related because together they form a process that transforms some input into an output. PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

Transform versus Transaction Structures

Steps in Building the Structure Chart
1. Identify top level modules and decompose them into lower levels 2. Add control connections (loops, conditional lines, etc.) 3. Add couples (identify the information that has to pass among the modules i.e. data couple and control couple) 4. Review and revise again and again until complete PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

Design Guidelines High quality structure charts result in programs that are modular, reusable and easy to implement. Measures include: Cohesion Coupling Appropriate levels of fan-in and fan-out PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

Cohesion Cohesion refers to how well the lines of code within each structure chart module relate to each other Ideally, a module should perform only one task, making it highly cohesive Cohesive modules are easy to understand and build because their code performs one function PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

Types of Cohesion (GPA = grade point average)

Cohesion Decision Tree (Adopted From Page-Jones, 1980)

Process of dealing with “low” cohesion
Factoring Process of dealing with “low” cohesion Separates tasks into different modules Reduces use of control flags For example, a more cohesive design for the print grade listing and dean’s list example would be to factor out print dean’s list and print grade listing into two separate modules. PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

Coupling Coupling involves how closely modules are interrelated.
The second guideline for good structure chart design states that modules should be loosely coupled In this way, modules are independent from each other, which keeps code changes from rippling throughout the program PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

Your Turn What, if anything, happens to coupling when you create modules that are more cohesive? What, if anything happens to the cohesiveness of modules when you lower the coupling among them? PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

Create High Fan-in Fan-in describes the number of control modules that communicate with a subordinate; a module with high fan-in has many different control modules that call it. This is a very good situation because high fan-in indicates that a module is reused in many places on the structure chart, which suggests that the module contains well-written generic code PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

Avoid High Fan-out Although we desire a subordinate to have multiple control modules, we want to avoid a large number of subordinates associated with a single control There is a limit to the number of employees that a boss can effectively manage The general rule of thumb is to limit a control module’s subordinates to approximately seven PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

Examples of Fan-in and Fan-out

Quality Checklist 1. Library modules have been created where ever possible 2. The diagram has a high fan-in structure 3. Control modules have no more than 7 subordinates 4. Each module performs only one function (high cohesion) 5. Modules sparingly share information (loose coupling) 6. Data couples that are passed are actually used by the accepting module 7. Control couples are passed from “low to high” 8. Each module has a reasonable amount of code associated with it PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

PROGRAM SPECIFICATION

Program Specifications Content
Written documents that include explicit instructions on how to program pieces of code No standard approach Include program information Note events that trigger actions List inputs and outputs (identified by the data couples and control couples) Include pseudocode Present additional notes and comments PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

Program Specification Form

Process Description – Analysis and Design

Summary The structure chart shows all of the functional components needed in the program at a high level. Components of the structure chart show sequence, selection, and iteration Follow design guidelines regarding cohesion, coupling, and fan-in/fan-out. Prepare detailed specifications to programmers including pseudocode to show important programming structures. PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

Copyright © 2009 John Wiley & Sons, Inc.
All rights reserved. Reproduction or translation of this work beyond that permitted in Section 117 of the 1976 United States Copyright Act without the express written permission of the copyright owner is unlawful. Request for further information should be addressed to the Permissions Department, John Wiley & Sons, Inc. The purchaser may make back-up copies for his/her own use only and not for redistribution or resale. The Publisher assumes no responsibility for errors, omissions, or damages, caused by the use of these programs or from the use of the information contained herein. PowerPoint Presentation for Dennis, Wixom, & Roth Systems Analysis and Design, 4th Edition Copyright 2009 © John Wiley & Sons, Inc. All rights reserved.

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