Data Flow Diagram

1. System Analysis & DesignSystem Analysis & Design

2. 
Where do they fit in?
 Analysis (What do we do?)
 Fact finding
 investigate business process
and the current system
 modelling the current and
required systems
 deliverables -
 requirements specification
 logical models of the
required system
 Life Cycle Phases
 Planning
 Feasibility Study
 Analysis
 Design
 Code and Unit test

3. 
3
 DFDs describe the flow of data or information into
and out of a system
 what does the system do to the data?
 A DFD is a graphic representation of the flow of data
or information through a system
Data Flow Diagrams
(DFD)

4. 
 external entity - people or organisations that
send data into the system or receive data from
the system
 process - models what happens to the data i.e.
transforms incoming data into outgoing data
 data store - represents permanent data that is
used by the system
 data flow - models the actual flow of the data
between the other elements
4 Main Elements

5. 
Notation
Process box
D Data Store
External
Entity
Data Flow
• Data Flow
• Process
• External Entity
• Data Store

6. 
DFD Shapes from Version
F r o m F lo w C h a r t /
D a t a F lo w D ia g r a m
P r o c e s s
D a t a S t o r e
E x t e r n a l E n t it y
F r o m S o f t w a r e D ia g r a m /
G a n e - S a r s o n D F D
P r o c e s s
ID #
ID
#
E x t e r n a l
E n t it y
D a t a S t o r e1
External
Entity
Data Store
Process
From Flow Chart /
Data Flow Diagram
Version 5.x Version 2000

7. 
4
 Even a small system could have many processes and
data flows and DFD could be large and messy
 use levelled DFDs - view system at different levels of
detail
 one overview and many progressively greater detailed
views
Levelled DFDs

8. 
 models system as one process box which represents
scope of the system
 identifies external entities and related inputs and
outputs
 Additional notation - system box
Level 0 - Context Diagram
System boxExternal
entity
Data flow out
Data flow in

9. 
 gives overview of full system
 identifies major processes and data flows between
them
 identifies data stores that are used by the major
processes
 boundary of level 1 is the context diagram
Level 1 - overview
diagram

10. 
 level 1 process is expanded into more detail
 each process in level 1 is decomposed to show its
constituent processes
 boundary of level 2 is the level 1 process
Level 2 - detailed
diagram

11.  Duplicates marked by diagonal line in corner
 System Boundary
 Elementary Processes - star in corner
 Process that is levelled - dots on top
Other Notation

12. 
5
 Numbering
 Labelling
 Balancing
Rules for DFDs

13. 
 On level 1 processes are numbered 1,2,3…
 On level 2 processes are numbered x.1, x.2, x.3…
where x is the number of the parent level 1 process
 Number is used to uniquely identify process not to
represent any order of processing
 Data store numbers usually D1, D2, D3...
Numbering

14. 
 Process label - short description of what the process
does, e.G. Price order
 Data flow label - noun representing the data flowing
through it e.G. Customer payment
 Data store label - describes the type of data stored
 Make labels as meaningful as possible
Labelling

15. 
 Balancing
 any data flows entering or leaving a parent level must
by equivalent to those on the child level
 Data stores
 data stores that are local to a process need not be
included until the process is expanded
Balancing and data
stores

16. 
 Allowed to combine several data flows from lower
level diagrams at a higher level under one data flow
to reduce clutter
 Flows should be labelled except when data to or
from a data store consists of all items in the data
store
Data Flows

17. 
 Find the people who send data into the system
 Often data is part of a PHYSICAL transaction
 When handing a bar of chocolate to a shopkeeper, you
are handing him/her a barcode.
 Find the people who get data out of the system.
 The only data you need is data that is transformed or
sent completely out of the system – not data that is
handled by an operator within the system.
Context Diagram

18. Design Strategy
Context Diagram

19. Design Strategy
DFD

20. Design Strategy
DFD

21. Design Strategy
DFD

22. Design Strategy
DFD

23. 
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