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Systems Analysis

Lecture 9

Introduction to Design

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Lecture Objectives

• The transition to design

• Design Guidelines

• Design Trade-offs

• Prototypes

• Design Modelling Tools

• Process Flowcharts & Data Flow Models

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Transition to Design• If management decide to develop the

system in-house then the transition to the system Design phase begins

• This phase uses systems design guidelines

• It sometimes requires prototyping methods and tools

• System Requirements Specification / Document is key

• Logical and Physical Designs

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Design Guidelines

• The systems analyst must understand the logical design of a system before beginning the physical design

• First step is to revue the system requirements document

• The actual design process can then begin

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Steps in Design Phase

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Stages and Steps

• The systems design stage is not a series of clearly defined steps

• Many system components are interdependent

• Sometimes changes to one thing can cause a knock on effect to other areas of design

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System Design Objectives

• To satisfy the defined requirements and constraints

• Must be accepted by the user

• Must support the organisation’s business objectives

Must do what it was meant to do

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System Design Objectives

• The system must be reliable in order to adequately handle errors, hardware failures and human mistakes

• A good system will plan for errors, detect them as soon as possible and allow for their correction without damaging the system itself

Must be stable and resilient

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System Design Objectives

• The system must be maintainable

• Should be well designed and flexible

• Allow for future modifications

• Clearly and accurately documented

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System Design Considerations

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Activity• As the previous slide demonstrates

design considerations should include the following:

– users

– data

– Architecture

Consider what is meant by this…(Steps Slide)

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Logical Design

• Logical design defines the functions and features of the system (What)

– Inputs

– Outputs

– Processes

• This does not take into account how the tasks will be accomplished physically (How)

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Logical and Physical Design

• Physical design is the actual blueprint of the system describing the construction of the system

• It is built upon the logical design specification

• It is concerned with how the system will meet the requirements

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Physical Design

“In a typical system the physical design will describe the actual processes of entering, verifying and storing data.

The physical layout of data files

Sorting procedures, format of reports etc”.

p222 shelley cashman et al

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Data Design (NB)

Data design defines the

• Physical data structures

• Elements

• Relationships

NOTE: Data design and user interface will influence each other; could be data design driven

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Data Considerations

Delays cause Data Errors therefore:

• Data should be entered when it occurs

• It should be verified immediately to catch any errors

• Automated data entry should be used whenever possible

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More Data Error Guidelines

• Data should be entered into the system only once as data duplication should be avoided

• Every instance of entry and change to the data should be logged

• Changes to critical values should be reported to trace the changes (audit facility)

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User Considerations

Design of the system should include consideration of how the system will affect the user – points to remember

1. Input processes should be easy to follow and forgiving of errors

2. Output should be attractive and easy to understand – with an appropriate level of detail

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User Interface Guidelines

• Focus on business objectives

• Build an interface that is easy to learn and use

• Provide features that promote efficiency

• Make it easy for users to correct errors

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User Interface Guidelines

• Minimise input data problems

• Provide feedback to users

• Create an attractive layout and design

• Use familiar terms and images

(More in Lecture 10)

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Software Architecture Considerations

Consider the design of the architecture

• Modular design creates individual processing components (modules) which then connect to a higher level program

• Single function design modules are easy to understand, implement and maintain

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Component Architecture Considerations• Requirements and Constraints

regarding physical system architecture(e.g. server/workstation locations, networking, etc)

• Physical location of components, and their interaction

• Backup/Recovery

• Security considerations

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Design Trade-offs

“Design goals often conflict with each other. In the system design phase you must constantly analyse alternatives and weigh trade offs” (Shelley Cashman et al)

• Programming requirements might be more complex to make a system easier to use.

• Making the system more flexible may make maintenance more difficult

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Using Prototypes in Design

“Prototyping produces an early, rapidly constructed working version of the proposed information system.

It is a common technique which can be used to design anything from a new home to a computer network” – Shelly Cashman et al p228

Ref: Tutorial 4

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Other Modelling Tools

• Process Flowcharts

• Data flow models

• Object modelling tools

• CASE tools

• System flowcharts

• Activity Diagrams (more detailed)

• Sequence Diagrams etc

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System Flowcharts

Another method of describing the physical design of a system

Definition:

“A system flowchart is a tool that uses various symbols to identify input, output processes and files”

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American National Standards Institute (ANSI) Symbols

Display

Library / Pre-defined

ProgramDecision

Manual Input

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Flowchart using ANSI standardShelly et al Fig 5.32

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Data Flow Diagrams

Process

Data Flow

Real-time Link

External Agent

Data Store

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External agent, data flows, and the process come from

information about the event in the event table:

1

Lookup item

availability

Catalogue

Product Item

Inventory Item

Data stores come from

Entity-relationship

diagram

Customer

Item inquiry

Item availability details

Source

Trigger

Activity

Destination Respons

e

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In Conclusion

• The transition to design

• Design Guidelines

• Design Trade-offs

• Prototypes

• Design Modelling Tools

• Process Flowcharts & Data Flow Models