Data Flow Diagrams
DT211 Stage 2
Software Engineering
Where do they fit in?
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Life Cycle Phases
Planning
Feasibility Study
Analysis
Design
Code and Unit test
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Analysis (What do we do?)
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Fact finding
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investigate business process and
the current system
modelling the current and
required systems
deliverables 
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requirements specification
logical models of the required
system
Data Flow Diagrams (DFD)
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DFDs describe the flow of data or information into and
out of a system
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what does the system do to the data?
A DFD is a graphic representation of the flow of data or
information through a system
3
4 Main Elements
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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
Notation
Data Flow
• Data Flow
Process box
• Process
External
Entity
• External Entity
• Data Store
D
Data Store
Levelled DFDs
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Even a small system could have many processes and data
flows and DFD could be large and messy
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use levelled DFDs - view system at different levels of detail
one overview and many progressively greater detailed views
4
Level 0 - Context Diagram
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models system as one process box which represents
scope of the system
identifies external entities and related inputs and outputs
Additional notation - system box
External
entity
Data f low out
Data f low in
Sy stem box
Level 1 - overview diagram
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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 2 - detailed diagram
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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
Other Notation
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Duplicates marked by diagonal line in corner
System Boundary
Elementary Processes - star in corner
Process that is levelled - dots on top
Rules for DFDs
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Numbering
Labelling
Balancing
5
Numbering
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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...
Labelling
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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
Balancing and data stores
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Balancing
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any data flows entering or leaving a parent level must by
equivalent to those on the child level
Data stores
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data stores that are local to a process need not be included
until the process is expanded
Data Flows
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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
Joe’s Yard
Joe’s builders’ suppliers has a shop and a yard. His system is entirely manual.
He has a stock list on the wall of his shop, complete with prices. When a
builder wants to buy supplies, he goes into the shop and picks the stock
items from the list. He writes his order on a duplicate docket and pays
Joe, who stamps the docket as paid. The builder takes the duplicate
docket and he goes to the yard and hands it to the yard foreman. The
yard foreman gets the ordered items from the yard and gives them to
the builder. The builder signs the duplicate docket and leaves one copy
with the foreman and takes one copy as a receipt. Every week, Joe looks
around the yard to see if any of his stock is running low. He rings up the
relevant suppliers and reorders stock. He records the order in his order
book, which is kept in the yard. The yard foreman takes delivery of the
new stock and checks it against what has been ordered. He pays for it
on delivery and staples the receipt into the order book. At the end of
every month, Joe goes through all the dockets and the order book and
produces a financial report for the shareholders.
Draw a context level DFD and a level-1 DFD for this system.
Context Diagram
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Find the people who send data into the system
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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.
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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
Shareholders
financial
report
Docket &
Payment
Joe's
Yard
Supply needs
Joe
Customer
Signed docket
Supply order
& payment
Supply invoice
Supplier
Level-1 DFD processes
Joe’s builders’ suppliers has a shop and a yard. His system is entirely
manual. He has a stock list on the wall of his shop, complete with
prices. When a builder wants to buy supplies, he goes into the shop
and picks the stock items from the list. He writes his order on a
duplicate docket and pays Joe, who stamps the docket as paid. The
builder takes the duplicate docket and he goes to the yard and
hands it to the yard foreman. The yard foreman gets the ordered
items from the yard and gives them to the builder. The builder signs
the duplicate docket and leaves one copy with the foreman and
takes one copy as a receipt. Every week, Joe looks around the yard
to see if any of his stock is running low. He rings up the relevant
suppliers and reorders stock. He records the order in his order
book, which is kept in the yard. The yard foreman takes delivery of
the new stock and checks it against what has been ordered. He
pays for it on delivery and staples the receipt into the order book.
At the end of every month, Joe goes through all the dockets and the
order book and produces a financial report for the shareholders.
Verbs from script
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Has (passive)
Buy supplies
Picks stock items
Writes order
Pays joe
Stamps docket
Takes docket to yard
Hands it to foreman
Gets items
Gives them to builder
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Builder signs docket
Takes copy as receipt
Looks around yard and
reorders
Records order in order
book
Foreman takes delivery –
checks
Foreman pays supplier
Staples receipt to order
book
Produces financial report
Remove passive verbs and
queries
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Joe then
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Passive: has stock list
Buy supplies
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Customer then
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Picks stock items (views list)
Writes orders
Pays joe
Stamps docket
Takes docket to yard
Hands it to foreman
Gets items
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Gives them to builder
Builder signs docket
Takes copy as receipt
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Foreman
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Looks around yard and reorders
Records order in order book
takes delivery – checks
Foreman pays supplier
Staples receipt to order book
Joe
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Produces financial report
Level 1 current physical
20
Buy
supplies
Money
Payment
Supplier
receipt
22
Payment
Docket &
Payment
Payment
Restock
supplies
Docket
Supplier
Customer
M1
completed
docket copy
supplies
Supply order
Docket
completed
docket copy
required stock
stock
Signed docket
12
builder
signature
*
Supply order
get
items
11
Take
to
yard
Order
book
M3
required stock
completed
docket copy
Supplier
receipt
Joe
Supply needs
21
Reorder
supplies
Signed docket
Signed docket
5
Joe's Office
Produce
financial
report
Foreman
financial
report
*
Shareholders
Buy Supplies
20 Buy supplies
Customer
M1
Docket
20.1
Docket
Docket
Writes
Order
*
Payment
Docket
20.3
Stamp
Docket(signature)
20.2
Payment
Pays
Joe
*
*
Money
Get Items
12 get items
12.2
Foreman
Signed docket
12.1
Get builder
signature
Signed docket
*
Give items
to customer
Customer
*
required stock
builder
signature
completed
docket copy
completed
docket copy
stock
12.3
Give
copy
as receipt
*
Reorder supplies
21 Reorder supplies
21.1
Joe
Supply needs
Reorder
from
yard
*
Supply needs
21.2
Supply order
Record
order
*
M3
Order
book
Restock
22 Restock
M3
Order
book
Supply order
22.1
stock
supplies
Take
delivery
*
supplies
Supply order
Money
Payment
22.2
Supplier
receipt
Pay
supplier
*
Payment
Supplier
Creating Data Flow Diagrams
Steps:
1. Create a list of activities
2. Construct Context Level DFD
(identifies external entities and processes)
3. Construct Level 0 DFD
(identifies manageable sub process )
4. Construct Level 1- n DFD
(identifies actual data flows and data stores )
5. Check against rules of DFD
DFD Naming Guidelines
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External Entity  Noun
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Data Flow  Names of data
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Process  verb phrase
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a system name
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a subsystem name
Data Store  Noun
Creating Data Flow Diagrams
Lemonade Stand Example
Creating Data Flow Diagrams
Example
The operations of a simple
lemonade stand will be used
to demonstrate the creation
of dataflow diagrams.
Steps:
1. Create a list of activities
•
Old way: no Use-Case Diagram
•
New way: use Use-Case Diagram
2. Construct Context Level DFD
(identifies sources and sink)
3. Construct Level 0 DFD
(identifies manageable sub processes )
4. Construct Level 1- n DFD
(identifies actual data flows and data stores )
Creating Data Flow Diagrams
Example
1. Create a list of activities
Think through the activities
that take place at a lemonade
stand.
Customer Order
Serve Product
Collect Payment
Produce Product
Store Product
Creating Data Flow Diagrams
Example
1. Create a list of activities
Also think of the additional
activities needed to support
the basic activities.
Customer Order
Serve Product
Collect Payment
Produce Product
Store Product
Order Raw Materials
Pay for Raw Materials
Pay for Labor
Creating Data Flow Diagrams
Example
1. Create a list of activities
Group these activities in
some logical fashion,
possibly functional areas.
Customer Order
Serve Product
Collect Payment
Produce Product
Store Product
Order Raw Materials
Pay for Raw Materials
Pay for Labor
Creating Data Flow Diagrams
Example
2. Construct Context Level DFD
(identifies sources and sink)
Create a context level
diagram identifying the
sources and sinks (users).
Context Level DFD
Order
Customer Order
Serve Product
Collect Payment
Produce Product
Store Product
Order Raw Materials
Pay for Raw Materials
Pay for Labor
CUSTOMER
Sales Forecast
0.0
Lemonade Production Schedule EMPLOYEE
Pay
System
Product Served
Payment
Received Goods
Payment
VENDOR
Time Worked
Purchase Order
Creating Data Flow Diagrams
Example
Create a level 0 diagram
identifying the logical
subsystems that may exist.
3. Construct Level 0 DFD
(identifies manageable sub processes )
Level 0 DFD
1.0
Sale
Customer Order
Serve Product
Collect Payment
Product Ordered
Payment
CUSTOMER
Produce Product
Store Product
Pay for Labor
Product Served
Received Goods
VENDOR
Order Raw Materials
Pay for Raw Materials
Sales Forecast
Customer Order
Purchase Order
Production
Schedule
2.0
Production
EMPLOYEE
Inventory
3.0
Procurement
Payment
Order
Decisions
Pay
4.0
Payroll
Time Worked
Creating Data Flow Diagrams
Example
Create a level 1
decomposing the processes
in level 0 and identifying
data stores.
4. Construct Level 1- n DFD
(identifies actual data flows and data stores )
Level 1 DFD
CUSTOMER
Customer Order
ORDER
Customer Order
Serve Product
Collect Payment
1.1
Record
Order
Severed Order
Produce Product
Store Product
Order Raw Materials
Pay for Raw Materials
Pay for Labor
Payment
1.2
Receive
Payment
PAYMENT
Request for Forecast
1.3
Produce
Sales
Forecast
Sales Forecast
Creating Data Flow Diagrams
Example
Create a level 1
decomposing the processes
in level 0 and identifying
data stores.
4. Construct Level 1 (continued)
Level 1 DFD
Product Order
ORDER
Customer Order
Serve Product
Collect Payment
Produce Product
Store Product
Order Raw Materials
Pay for Raw Materials
Pay for Labor
2.1
Serve
Product
Quantity Severed
RAW
MATERIALS
Production
Schedule
2.2
Produce
Product
Production Data
2.3
Store
Product
Quantity Used
INVENTORTY
Quantity Produced &
Location Stored
Creating Data Flow Diagrams
Example
Create a level 1
decomposing the processes
in level 0 and identifying
data stores.
Customer Order
Serve Product
Collect Payment
4. Construct Level 1 (continued)
Level 1 DFD
Order Decision
3.1
Produce
Purchase
Order
PURCHASE
ORDER
Quantity On-Hand
Quantity
Received
Received
Goods
3.2
Receive
Items
Produce Product
Store Product
Payment Approval
Order Raw Materials
Pay for Raw Materials
3.3
Pay
Vendor
Pay for Labor
Payment
RAW
MATERIALS
RECEIVED
ITEMS
VENDOR
Creating Data Flow Diagrams
Example
Create a level 1
decomposing the processes
in level 0 and identifying
data stores.
Customer Order
Serve Product
Collect Payment
4. Construct Level 1 (continued)
Level 1 DFD
Time Worked
4.1
Record
Time
Worked
TIME CARDS
Employee ID
EMPLOYEE
Payroll Request
4.2
Calculate
Payroll
Produce Product
Store Product
Unpaid time cards
PAYROLL
Payment Approval
Order Raw Materials
Pay for Raw Materials
4.3
Pay
Employe
e
Pay for Labor
Payment
PAYMENTS
Process Decomposition
0.0
Lemonade
System
Context Level
1.0
Sale
1.1
Record
Order
1.2
Receive
Payment
2.0
Production
2.1
Serve
Product
2.2
Produce
Product
2.3
Store
Product
3.0
Procurement
3.1
Produce
Purchase
Order
3.2
Receive
Items
3.3
Pay
Vendor
4.0
Payroll
4.1
Record
Time
Worked
4.2
Calculate
Payroll
4.3
Pay
Employe
e
Level 0
Level 1
DFD Example: Bus Garage
Repairs
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Buses come to a garage for repairs.
A mechanic and helper perform the repair, record
the reason for the repair and record the total cost of
all parts used on a Shop Repair Order.
Information on labor, parts and repair outcome is
used for billing by the Accounting Department, parts
monitoring by the inventory management computer
system and a performance review by the supervisor.
DFD Example: Bus Garage Repairs
(cont’d)
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External Entities: Bus, Mechanic, Helper, Supervisor,
Inventory Management System, Accounting
Department, etc.
Key process (“the system”): performing repairs and
storing information related to repairs
Processes:
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Record Bus ID and reason for repair
Determine parts needed
Perform repair
Calculate parts extended and total cost
Record labor hours, cost
DFD Example: Bus Garage
Repairs (cont’d)
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Data stores:
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Personnel file
Repairs file
Bus master list
Parts list
Data flows:
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Repair order
Bus record
Parts record
Employee timecard
Invoices
Bus Garage Context Diagram
Bus
Fixed
mechanical
problems
Mechanical
problem
to be repaired
Helper
Labor
Labor
Mechanic
Bus
Repair
Process
System
Labor,
parts cost
details
Repair
summary
List of
parts used
Supervisor
Inventory
Managemen
t System
Accountin
g
CSUB Burger’s Order Processing
System
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Draw the CSUB Burger’s context diagram
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System
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External entities
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Order processing system
Kitchen
Restaurant
Customer
Processes
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Customer order
Receipt
Food order
Management report