Chapter 6 - General Systems Design

A) Introduction

• consider alternative conceptual designs that fulfil users' requirements
• get it down on paper before trying to refine the design too early
• interact with users, check with team members & technicians
• redesign, check, check, recheck

B) What are the three categories of Systems Design?

1. Global-based systems
   

   involves a complete overhaul or replacement of all the systems design components

   examples:

   • monthly tabular reports changed to online screen displays
   • new processes are developed
   • input is captured via scanning devices
   • hierarchical database replaced by relational database
   • controls like UPS, disaster recovery plans, etc. are installed
   • new network topology

   need a General Systems Design Worksheet (Fig. 6.3)

2. Group-based systems
   services a branch office or department
3. Local-based systems
   designed for a few people
   ex. EIS for senior executives

C) Using the Four Key Elements of Rapid Application Development to Design Systems

1. RAD combines elements that work together so that the total effect is greater than the sum of the individual effects
   Dillinson's Department Store - pg. 201-3
2. Joint application development (JAD)

   best with global-based systems

   need to bring together

   • the "designer's mental design model",
   • the "user's mental design model" and
   • the conceptual model using DFDs and ERDs
3. Specialists with advanced tools (SWAT) teams

   3-4 highly skilled and motivated systems professionals using CASE

   • Computer-aided systems and software engineering (CASE) tools
   • CASE adds discipline to systems development;
   • reduces design errors and omissions and reduces systems rework
4. Prototyping

   users are shown what they're going to get

D) Using the Structure-Oriented Design Approach to Design Systems

process-oriented approach

• examines current input, output and processing to determine data needs
• use DFD to examine data requirements (Fig. 6.5)
• (Fig.6.6-8)
• this approach effective for transaction-based applications

data-oriented approach

used if processes, inputs and outputs are less defined

• 1) need to discuss potential decisions that will be made from the system
• 2) model the systems using an ERD (Fig. 6.9)
• 3) divide each criterion into its attributes (Fig. 6.10)

Create the Data Dictionary for the Systems

• define size, type, description, limits & exceptions, ranges, security level, and access privileges for each attribute
• DD is dynamic; must describe data fully, must be arranged in standard order; expect more refinement during detailed systems design

E) Using the Object-Oriented Design Approach to Design Systems

Designing & Building Systems Using Reusable Software Objects

What are the Key Elements of Object-Oriented Design?

1. Objects - "entity" with behaviours (methods)
2. objects behave in response to messages
3. classes - set of objects that share a common structure and behavior
4. inheritance - sharing of properties with subclasses
5. polymorphism - ability of objects to respond to the same message and implement it differently (ex. +)

What Steps are Required to Perform Object-oriented Design at the General Systems Design Level?

• 1) Identify object classes and select relevant ones (Fig. 6.14)
• 2) Identify Relationships (Fig. 6.15)
• 3) Identify Attributes (focus on most important ones)
• 4) Identify Inheritance and Build a Class Hierarchy (Fig. 6.16)

F) What is the Difference between Structure-oriented and Object-oriented design?

• Modularity
• Structured - unit of software code that performs a well-defined function
• Object - object that encapsulates attributes and methods