Data Flow Diagrams

Data Flow Diagrams
Introduction
SSADM
What is a Data Flow Diagram?
Why do we use DFDs?
Levelling
Conventions
Decomposition and Abstraction
The Elements
Process and Data Stores
Outside Entity
Data Flow
The Levels
Rules
The Procedure for Constructing DFD’s
The Document Flow Diagram
The Context Diagram
Draw the external entities and data stores
Level 1 Physical DFD - Complete
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
What is a Data Flow
Diagram?
• Known as DFDs
• A way to model a real
world situation
• They are the interface
between the real world
activities and an
understanding of how this
can be converted into a
computer system.
Why do we use DFDs?
• It is a way of taking the
physical view and
converting it into a logical
view.
• The physical view - all
documents involved
• The logical view - the data
they contain
• Their main purpose is to
communicate with the
user, the analyst’s
understanding of the
scope of the required
system
Levelling
• DFDs are expanded or
decomposed into levels.
• Separating each process
into sub processes
• Uncovers more and more
detail
Conventions
Balancing
Process at lower level should
have identical data flows if
they flow out of a process
Modelling Data Stores
Only use DATA STORES used
within this process on the
diagram
Numbering
1 - 1.1 - 1.1.1
1.2 - 1.2.1
Labels
Should carry as much meaning
as possible
Decomposition and
Abstraction
• Decomposition - Divide
and subdivide into
manageable size
problems
• Abstraction Concentrate on the
important issues and
ignore the irrelevant
The Elements
The four main elements of
DFDs notation
 Data Flows, with a
label to indicate what
data is flowing
 Processes, that handle
the data
 Data stores, within the
system (diary, filing
cabinet or computer
file)
 Outside entities,
outside sources of
data
Process and Data
Stores
A process is made up of
Destination
(Place or Name)
Process Number
Process
description Should
be descriptive,
starting with a
verb.
Data Stores
Can be M for manual
or D for computer
base data stores.
M1
Name of
Store
Outside Entity
Is anything outside the
system that is of interest to
the system. Can be a
person, a company or
another system.
Customer
a
Outside entity shows the
Name and a lowercase
alpha character is used to
uniquely identify it.
Customer
a
If an outside entity is
repeated for the
purpose of neat layout
a line is added across
the top.
Data Flow
Is shown by a line with an
arrowhead, indicating the
direction of the flow of
data. Each data flow
should be named to
indicate what data is being
passed. Nouns or
adjectives only no verbs
are permitted.
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 that
can be used.
Rules
Sequence 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
Understand
Hard to know where to stop
Rule of Thumb
If there are more than 8 data
flows break it
Process of Identifying major
Processes
The Procedure for
Constructing DFD’s
 Draw a document flow diagram
of the current situation
 Draw a systems boundary
around the agencies that are
part of the system
 Draw a Context Diagram
 Identify processes in the system
 Complete the level 1 Current
Physical DFD
The Document Flow Diagram
The task of modelling a business
situation can be daunting at first. It is
best to start with something simple
such as a document flow diagram.
Production
Planning
Production
Plan
Stock Control
Stock
Withdrawal
Note
Factory
Supplier
Delivery
Note
Purchase
Order
Supplier Details Update Form
Delivery Note
Material Requirements List
Purchasing
Bill of
Materials
Design
The Context Diagram

You decide which agencies are to be
part of the system that you are
examining.

These agencies fall inside the system
boundary and are reduced to one box in
the centre.

This is a Context Diagram
a
Production
Planning
b
Supplier
Production
Plan
Stock Control
e
Design
Deliv ery
Note
Bill of Materials
Maintain
Stock System
Stock
W ithdrawal
Note
d
Factory
(Lejk & Deeks)
Supplier Details Update Form
Deliv ery Note
Material Requirements List
c
Purchasing



All data flows going into the
system must be received by a
process.
All data flows going out of the
system must be generated by
process.
The first task is therefore to
identify these processes:
1
Stoc k c lerk
Maintain
planned c all-of f
2
Stoc k c lerk
Maintain
s toc k c ards
3
Stoc k c lerk
Prepare
m aterial reqm nts
lis t
Draw the external entities and
data stores.
1
a
Stock clerk
M1 Bill of materials
Production
Planning
Maintain
planned call-of f
M2 Stock cards
b
Supplier
2 Stock clerk
c
Factory
Maintain
stock cards
3
Stock clerk
d
Purchasing
Prepare
material reqmnts
list
Level 1 Physical DFD - Complete
Finally draw in the data flows to give a
completed diagram. Note that a data flow
must have a process at the end .
1
a
Production
Plan
Production
Planning
Stock clerk
B O M details
M1 Bill of materials
Maintain
planned call-of f
Planned call-of f
details
b
M2
Supplier
Deliv ery note
Stock details
2 Stock clerk
c
Factory
Maintain
stock cards
Stock
withdrawal note
Stock details
Updated
supply details
d
Purchasing
Deliv ery
note
Material requirements
list
(Lejk & Deeks)
3
Stock clerk
Prepare
material reqmnts
list
Stock cards
Hairdressing Salon Level 1
Physical DFD
1
Receptionist
New client
details
Register
Appointm ent
Appointm ent
details
Existing client
details
M1 Client card index
Confirmation
Request
Appointm ent
details
Confirmation
Details
Confirmation of
arrival
a
Client
M2 Appointm ent diary
Appointm ent
details
2
Receptionist
Confirm
arrival
Change of
hairstyle etc.
3
Hairdresser/Rcptnst
(Lejk & Deeks)
Conduct
appointment
(Lejk & Deeks)
Appointm ent
details
Change of
hairstyle etc.
Process 3 Level 2
3
Client
a Clie
a
Hair/Reception
3.1
Hairdresser
Appointment Details
Conduct Appointment
Hair Details
M2
3.2
Diary
Hairdresser
Inform Reception
Change of Hair Details
3.3
Receptionist
Complete Appointment
M3
Client Card
Naming of DFD
processes
Level 0
Level 1
Level 2
2
Level 3
2.2
1
Process
Man
Overall
Process
2
Process
Process
3
2.1
2.1.1
Sub
Process
Sub - Sub
Process
2.2
Sub
Process
Process
Man
Level 4
2.1.1
Elementary
process
descriptions.
Decision trees
Decision table
Structured
English
2.1.2
Sub - Sub
Process
2.3
Sub
Process
There must be consistency between levels, with
all the data appearing on the higher level DFD.
If a data store is used only for one process it is
placed with that process. Outside entities are
always shown outside the boundary of a lower
level DFD process, even if they only
communicate with that one process.
Summary
SSADM
What a DFD is & Why
we use them
The different
conventions
What the elements are
Example
Next Week:- Entity
Relational Model