Data Flow Diagrams.ppt

8/14/2019 Data Flow Diagrams.ppt

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Data Flow Diagrams


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IntroductionSSADM

What is a Data Flow Diagram?

Why do we use DFDs?

Levelling

Conventions

Decomposition and Abstraction

The Elements

Process and Data Stores

Outside Entity

DataFlowThe LevelsRules

The Procedure for Constructing DFDs

The Document Flow Diagram

The Context Diagram

Draw the external entities and data stores

Level 1 Physical DFD - Complete


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S.S.A.D.M.

S.S.A.D.M. - Structured

Systems Analysis and

Design Method Uses different techniques

to model a system

Data Flow DiagramsEntity Relational Model

(Logical Data Stores)

Normalisation


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What is a Data Flow

Diagram?

Known as DFDs

A way to model a realworld situation

They are the interface

between the real worldactivities and an

understanding of how this

can be converted into a

computer system.


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Why do we use DFDs?

It is a way of taking thephysical view and

converting it into a logical

view. The physical view - all

documents involved

The logical view - the datathey contain

Their main purpose is to

communicate with the

user, the analystsunderstanding of the

scope of the required

system


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Levelling

DFDs are expanded or

decomposed into levels.

Separating each process

into sub processes

Uncovers more and more

detail


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Conventions

BalancingProcess at lower level should

have identical data flows if

they flow out of a process

Modelling Data StoresOnly use DATA STORES used

within this process on the

diagram

Numbering

1 - 1.1 - 1.1.1

1.2 - 1.2.1

LabelsShould carry as much meaning

as possible


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Decomposition and

Abstraction

Decomposition - Divide

and subdivide intomanageable size

problems

Abstraction -Concentrate on the

important issues and

ignore the irrelevant


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The Elements

The four main elements ofDFDs notation

Data Flows, with a

label to indicate what

data is flowing

Processes, that handle

the data

Data stores, within thesystem (diary, filing

cabinet or computer

file)Outside entities,

outside sources of

data


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Process and Data

StoresA process is made up of

Data Stores

Process Number

Destination

(Place or Name)

Process

description Should

be descriptive,

starting with a

verb.

M1Can be M for manual

or D for computer

base data stores.

Name of

Store


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Outside Entity

Is anything outside the

system that is of interest to

the system. Can be a

person, a company oranother system.

Outside entity shows the

Name and a lowercase

alpha character is used to

uniquely identify it.

If an outside entity is

repeated for thepurpose of neat layout

a line is added across

the top.

Customer

a

Customer

a


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DataFlowIs shown by a line with an

arrowhead, indicating the

direction of the flow of

data. Each data flowshould be named to

indicate what data is being

passed. Nouns or

adjectives only no verbs

are permitted.


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The Levels

Context - Overview -

contains only one process

Level 1 - Utilises all four

elements

Level 2 - A breakdown of a

level 1 process

Level 3 - A breakdown of alevel 2 process

There is no rule as to how

many levels of DFD thatcan be used.


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Rules

Sequence not important - getting the Processcorrect is

Context or Level 0 - Identifies

the system/ boundary/External

Links Level 1 - Overview of function

Level 2 - Breakdown to

Understand

Hard to know where to stop

Rule of Thumb

If there are more than 8 data

flows break itProcess of Identifying major

Processes


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The Procedure for

Constructing DFDs

Draw a document flow diagramof the current situation

Draw a systems boundaryaround the agencies that arepart of the system

Draw a Context Diagram

Identify processes in the system

Complete the level 1 CurrentPhysical DFD


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The Document Flow Diagram

The task of modelling a businesssituation can be daunting at first. It isbest to start with something simplesuch as a document flow diagram.

Production

Planning

Stock Control

FactoryDesign

Purchasing

Supplier

Stock

NoteWithdrawal

ProductionPlan Purchase

Order

Delivery Note

Material Requirements List

Bill of

Materials

Supplier Details Update Form

DeliveryNote


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The Context Diagram

You decide which agencies are to be

part of the system that you areexamining.

These agencies fall inside the systemboundary and are reduced to one box inthe centre.

This is a Context Diagram

ProductionPlanning

Stock Control

Factory

Design Purchasing

Supplier

Stock

NoteWithdrawal

ProductionPlan

Delivery Note

Material Requirements List

Bil l of Materials

Supplier Details Update Form

DeliveryNote

MaintainStock System

a b

c

d

e

(Lejk & Deeks)


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All data flows going into thesystem must be received by a

process.

All data flows going out of thesystem must be generated byprocess.

The first task is therefore toidentify these processes:

Stock clerk

Maintain

1

2

3

Stock clerk

Stock clerk

planned call-off

Maintain

stock cards

Preparematerial reqmnts

list


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Draw the external entities and

data stores.

Production

Stock clerk

Maintain

aBill of materialsM1

1

2

3

Stock clerk

Stock c ardsM2

Stock clerk

planned call -off

Maintainstock cards


list

Planning

Supplier

b

Factory

c

Purchasing

d


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Level 1 Physical DFD - Complete

Finally draw in the data flows to give acompleted diagram. Note that a data flowmust have a process at the end .

Production

Stock clerk

Maintain

a

Bill of materialsM1

1

2

3

Stock clerk

Stock cardsM2

Delivery note

B O M details

Planned call-off

details

Stock details

Stock clerk

planned call-off

Maintainstock cards


list

Planning

Production

Plan

Supplier

b

Factory

c

Stockwithdrawal note

Purchasing

d

Material requirements

list

Updatedsupply details

Deliverynote

Stock details

(Lejk & Deeks)


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Hairdressing Salon Level 1

Physical DFD

Client

Receptionist

Register

a

Client card indeM1

1

Confirm

2

Hairdresser/Rcptnst

Conduct

3

arrival

Receptionist

Appointment

Appointment diaryM2

appointment

Appointmentdetails

Change ofhairstyle etc.

Appointmentdetails

ConfirmationRequest

ConfirmationDetails

New clientdetails

Existing clientdetails

Appointmentdetails

Confirmation ofarrival

Change of

hairstyle etc.

Appointment

details

(Lejk & Deeks)

(Lejk & Deeks)


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Process 3 Level 2

3 Hair/Reception

Clie

a

Hair Details

Client

a

3.1 Hairdresser

Conduct Appointment

3.2 Hairdresser

Inform Reception

3.3 Receptionist

Complete Appointment

M2 Diary

Appointment Details

M3 Client Card

Change of Hair Details


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Naming of DFDprocesses

Level 0 Level 1 Level 2 Level 3 Level 4

Man

Overall

Process

Man

Process

Process

Process

3

SubProcess

SubProcess

SubProcess

SubProcessSubProcess

Process

Sub - SubProcess

Sub - SubProcess

Elementary

process

descriptions.

Decision trees

Decision table

StructuredEnglish

There must be consistency between levels, withall the data appearing on the higher level DFD.

If a data store is used only for one process it isplaced with that process. Outside entities arealways shown outside the boundary of a lowerlevel DFD process, even if they only

communicate with that one process.


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Summary

SSADM

What a DFD is & Why

we use themThe different

conventions

What the elements areExample

Next Week:- Entity

Relational Model

Date post:	04-Jun-2018
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Data Flow Diagrams.ppt

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