3.3.1
• What is a Data Flow Diagram?
• Why are they used?
• How to draw them.
“A data flow diagram is used for the visualization of data
processing. It illustrates the processes, data stores, external
entities, data flows in a business or other system and the
relationships between these things.”
- Wikipedia
• In order to design a new system you need to be able to
work out how data flows through a system.
• During the analysis stage of the system life cycle the
analyst would have worked this out by using Data Flow
Diagrams (DFDs).
• The DFD will show:
–
–
–
–
External entities
Processes
Data stores
Flow of data and information
Anything that interacts with the system.
Could be people, organisations or other systems.
Generally, external entities input or receive data.
This is an activity that takes place inside the system.
It uses inputs to produces outputs.
This is where data is stored (database, spreadsheet,
paper form, filing cabinet etc).
Show the direction that data flows through a system.
2.0
Check Stock
Check stock levels aren’t
below value n.
The process has three boxes.
Top Left:
This indicates the process number.
Top Right:
This is the process name.
Bottom:
This explains what the process does.
D2
Stock
The process has two boxes
M2
Invoices
Left:
This indicates the type of data store.
D: Computerised data store (database etc)
M: Manual data store (paper, filing cabinet etc)
Right:
This is the name of the data store.
Stock
If a data store is repeated then this is indicated by
removing the identifier from the repeated symbol and
placing an additional vertical line into the symbol.
Customer
Customer
This box contains the name of the entity.
If the entity repeats
for example, a customer inputs data into the
system and the same customer retrieves an output
from the system.
then a diagonal line is drawn to represent this.
• A DFD can been drawn to different levels of
complexity.
• At a basic level, (aka Context Level or Level 0),
the diagram only gives a summary of the system.
• It will only show the external entities, the inputs
and the outputs.
• It DOES NOT show the processes involved.
Customer details
Reject rental
DVD Rental System
Customer
Customer
DVD details
Accept rental
Customer details
Reject rental
DVD Rental System
Customer
Customer
DVD details
Accept rental
As you can see, this context diagram only shows:
- External Entities
- Basic Inputs
- Basic Outputs
Customer details
Reject rental
DVD Rental System
Customer
Customer
DVD details
Accept rental
As you can see, this context diagram only shows:
- External Entities
- Basic Inputs
- Basic Outputs
Customer details
Reject rental
DVD Rental System
Customer
Customer
DVD details
Accept rental
As you can see, this context diagram only shows:
- External Entities
- Basic Inputs
- Basic Outputs
Customer details
1.0
Receive
Receive details into
the system.
Customer
DVD details
Customer details
1.0
Receive
Receive details into
the system.
Customer
DVD details
Detail
2.0
Valid
Check membership.
< 3 DVDs rented?
Banned?
D1
Membership
Customer details
1.0
Receive
Receive details into
the system.
Customer
DVD details
Detail
2.0
Valid
Check membership.
< 3 DVDs rented?
Banned?
D1
Membership
D2
Rentals
Detail
3.0
Update
Add loan details to
rental database.
Customer details
1.0
Receive
Receive details into
the system.
Customer
DVD details
Detail
2.0
This diagram shows more
detail compared to a
context diagram.
Valid
Check membership.
< 3 DVDs rented?
Banned?
D1
Membership
D2
Rentals
D3
DVD
Detail
You can see the
processes and the data
stores.
3.0
Update
Add loan details to
rental database.
Detail
4.0
Customer
Update DVD
Update status of
borrowed DVD.
• Look closely at the previous DFD.
• Notice how where there is a data store you could relate this to a
table in a database.
• Notice how where there is a process you need to have a query.
• Notice how where there is data flow you need to have an input into
a system.
• Notice how where there is data flow out of a system there is need
for a report.
• A DFD can help design a database if done correctly.
• If you look closely, you can notice that some
sections can be broken down further.
• Section 2 can be broken down to show the
processes of checking if the person is a
member, and doesn’t already have 3 DVDs on
loan and isn’t blocked on the system.
• This would be known as a Level 2 DFD.
• Draw and label the four symbols you’d expect
to find in a DFD diagram.
• Why is it sometimes more suitable to draw a
diagram than explain a system in words?
• Explain how a DFD could help a database
designer.
• Create a Level 0 and Level 1 DFD which looks at what
happens when you buy a pair of trainers from a shoe
shop.
• The scenario...
– You have gone into a shoe shop and found a pair of really
wicked trainers that you like. You have asked the shop
assistant if they have your size. The shop assistant checks
on her PDA to see if your size is available. Success! They
have your size and you purchase the trainers. The system
automatically updates the stock database. You walk away
safe in the knowledge that your new trainers will allow you
to run and jump faster than everyone else now!