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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
Preparematerial reqmnts
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
Preparematerial reqmnts
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