Data Flow Diagrams
• Often a good way of summarising sources
and destinations of data and the
processing that takes place
• Shows how data is transformed into
information as it moves through a system
and what data needs to be stored
• Can be used at both the Analysis (existing
system) and Design (proposed system)
stages of a project
Context diagram
• Analyst should start with this before
drawing a full DFD
• Focuses on the inputs and outputs
• Only uses the source/destination and data
flow symbols
• A blank circle is drawn where the rest of
the DFD should appear
• Identifies the system’s users and their
interactions with the system
Diagram Levels
• Diagrams are in layers or levels with each
providing a greater degree of detail
• Context diagrams show the whole context
and are also called Level 0
• Level 1 shows a process to handle each
incoming data flow and a process to
generate each output data flow
• Level 2 is part of the system with more
detail of processing
Example Context Diagram
Club
secretary
MembershipNo
DatePaid
amount
System
Printed
members
letters
ErrorMessage1
Club
secretary
Club
secretary
Data Flow Diagrams
Data source or data
destination e.g. user
documentation or OMR
or
ID
Description
Data Store - where data is
held within the system e.g.
stock file
ID
Bottom
Process e.g. validate code
Location
Data Flow e.g. customer ID
DFD for an electricity meter
Meter
reading
Bill
Validate
entry
Calculate
bill
Customer
details
Customer file
Updated
details
Customer file
A library loans system identifies each book in its stock by a unique
BookID. This is encoded in a bar code and attached to the book. When a
borrower returns a book it is scanned and any fine that is due is
calculated by extracting from the library database the date that the book
was due back.
Copy and complete the given DFD that describes this part of the library
system.
Book
Bar
code
Borrower
2
1
1
Example
Mr Jollifant owns a children’s party entertainment business and
employs a number of clowns, conjurers etc on a part-time basis.
All bookings are kept in a large ledger type book. Mrs Jollifant
produces a word processed list of bookings for the next month
for each entertainer. At the end of the year she goes through the
books, and produces a table similar to the one shown below.
Jollifant Jolly Parties
Summary of Events 1998
Entertainer
No. of Events
Total Hours
Total Pay
Cleo the Clown
Magic Marcus
Jumping Jimmy
5
2
24
13
6
65
319.50
117.83
1926.21
Inputs/Process/Storage Output
Input/output
• booking details from client (input)
• monthly schedules produced (output)
• annual statistical summary (output)
Storage
• bookings ledger
•Processes
• calculate available bookings and store in ledger
• produce monthly schedules
• calculate statistics
DFD
Client
tai
ls
of
mo
nth
ly
Make
Booking
Details of
new
booking
Clown
L1 Bookings' Ledger
s
ing
ok
bo
of
ils
ta
De
Location
Location
De
ID
Booking
details
Produce
monthly
schedule
le
du
he
Sc
bo
ok
ing
s
ID
ID
Produce
end of year
statistics
Location
s
tic
s
i
at
St
Mr
Jollifant