Lec2data Flow Diagram Power Point Slides

7/31/2019 Lec2data Flow Diagram Power Point Slides

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Data Flow Diagrams


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IntroductionSSADMWhat is a Data Flow Diagram?Why do we use DFDs?LevellingConventionsDecomposition and AbstractionThe ElementsProcess and Data StoresOutside EntityData Flow

The LevelsRulesThe Procedure for Constructing DFDsThe Document Flow DiagramThe Context DiagramDraw the external entities and data storesLevel 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 Diagrams Entity Relational Model

(Logical Data Stores) Normalisation


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What is a Data FlowDiagram?

Known as DFDs A way to model a real

world situation They are the interface

between the real worldactivities and anunderstanding of how thiscan be converted into acomputer system.


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Why do we use DFDs? It is a way of taking the

physical view andconverting it into a logical

view. The physical view - all

documents involved

The logical view - the datathey contain Their main purpose is to

communicate with theuser, the analystsunderstanding of thescope of the required

system


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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 thediagram

Numbering1 - 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 sizeproblems

Abstraction -Concentrate on theimportant issues and

ignore the irrelevant


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The Elements

The four main elements of DFDs notation Data Flows, with a

label to indicate whatdata is flowing

Processes, that handlethe data

Data stores, within thesystem (diary, filingcabinet 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)

Processdescription Shouldbe descriptive,starting with averb.

M1Can be M for manualor D for computerbase data stores.

Name of Store


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Outside EntityIs anything outside the

system that is of interest tothe system. Can be a

person, a company or another system .

Outside entity shows the Name and a lowercasealpha character is used touniquely identify it.

If an outside entity is

repeated for the purpose of neat layouta line is added acrossthe top .

Customer a

Customer a


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Data FlowIs shown by a line with an

arrowhead, indicating thedirection of the flow of

data. Each data flowshould be named toindicate what data is being

passed. Nouns or adjectives only no verbsare 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 a

level 2 process There is no rule as to how

many levels of DFD thatcan be used.


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RulesSequence not important - getting the Process

correct is Context or Level 0 - Identifies

the system/ boundary/External

Links Level 1 - Overview of function Level 2 - Breakdown to

UnderstandHard to know where to stopRule of ThumbIf 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 diagram

of the current situation Draw a systems boundary

around the agencies that arepart of the system

Draw a Context Diagram Identify processes in the system Complete the level 1 Current

Physical 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.

ProductionPlanning

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 system

boundary 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

Bill 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 aprocess.

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 anddata stores.

Produc t ion

S tock c le rk

Main ta in

a Bi ll o f m a te r i a l sM 11

2

3

Stock c le rk

S t o c k c a r d sM 2

Stock c le rk

p lanned ca l l-o f f

Main ta ins to c k c a r d s

P r e p a r em ate r ia l r eqm nts

list

P lann ing

Supp l ie r

b

Fac to ry

c

P u r c h a s in g

d


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Level 1 Physical DFD - Complete

F in a lly d raw in th e d a t a flo w s to g ivc o m p le te d d ia g ra m . N o te th a t a d a t am u s t h a v e a p ro c e s s a t th ee n d .

P r o d u c t i o n

S t o c k c l e rk

M a i n t a i n

aB ill o f m ate r iM 1

1

2

3

S t o c k c l e rk

S t o c k c a r d sM 2

D e l iv e r y n o t e

B O M d e t a ils

P l a n n e d c a l l- o f f

de t a i l s

S t o c k d e t a i ls

S t o c k c l e rk

p l a n n e d c a l l- o ff

M a i n t a i ns t o c k c a r d s

P r e p a r em a t e r i a l r e q m n t s

list

P l a n n i n gP r o d u c t i o n

P l a n

S u p p l i e r

b

F a c t o r y

c

S t o c kw it h d r a w a l n o t e

P u r c h a s i n g

d

M a t e r ia l re q u ir e m e n t slist

U p d a t e dsup p ly de t a i ls

D e l i v e r yn o t e

S t o c k d e t a i ls

( Lejk&


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H a ird re s s in g S a lo n L e v

P h y s ic a l D F D

Client

Receptionist

Register

a

Client card indexM1

1

Confirm

2

Hairdresser/Rcptnst

Conduct

3

arrival

Receptionist

Appointment

Appointment diaryM2

appointment

Appointmentdetails

Change of hairstyle etc.

Appointmentdetails Confirmation

Request

ConfirmationDetails

New clientdetails

Existing clientdetails

Appointmentdetails

Confirmation of arrival

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

Clienta

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

L ev el 0 L ev el 1 L ev el 2 L ev el 3 L ev el 4

M a n

O v era llP ro ce ss

M a n

P ro c es s

P ro c es s

P ro c es s

2

S u bP ro c es s

S u bP ro c es s

S u bP ro c es s

S u bP ro c es sS u bP ro c es s

P ro c es s

2 .2

S u b - S uP ro c es s

S u b - S uP ro c es s

E lem e nta r y p ro ce ssd e sc ri p tio n s.

D e cisi on tre eD e ci si o n ta bS tru ct u re dE n gli s h

2 .1 .1

T here must be consistency betw een levels, w iall the data appearing on the higher level D F

If a data store is used only for one process it ip laced w ith that process. O utsid e entities arealw ays show n ou tside the boundary of a lowlevel D FD process, even if they onlycommunicate w ith that one process.


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Summary

SSADMWhat a DFD is & Whywe use themThe differentconventions

What the elements areExample

Next Week:- EntityRelational Model

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