Understanding Data Flow Diagram

An overview of What & How
04-04-2016Ankur Biswas
1

A process model is a formal way of representing how a
business operates
Data flow diagramming shows business processes and the
data that flows between them
Logical process models describe processes without
suggesting how they are conducted
Physical models include information about how the
processes are implemented
04-04-2016Ankur Biswas 2

Creating DFDs is a highly iterative process of gradual refinement.
General steps:
1. Create a preliminary Context Diagram
2. Identify Use Cases, i.e. the ways in which users most commonly
use the system
3. Create DFD fragments for each use case
4. Create a Level 0 diagram from fragments
5. Decompose to Level 1,2,…
6. Go to step 1 and revise as necessary
7. Validate DFDs with users.

• Context Diagram is showing
whole system as a process.
• Level 0 has added a data store
and major modules are identified
• Note the data flow among
entities are restored.
• Numbering has changed to
1,2,3…
• Level 1 & 2 for process 2 & 2.2
are restoring all the data flows
from previous level
• Also note the numbering scheme.

Top-level view of IS
A data flow diagram (DFD) of the scope of an
organizational system that shows the system boundaries,
external entities that interact with the system and the
major information flows between the entities and the
system
Example: Order system that a company uses to enter
orders and apply payments against a customer’s balance

One process, numbered 0.
Sources and sinks (external entities) as squares
Main data flows depicted
No internal data stores are shown
 They are inside the system
 External data stores are shown as external entities
 How do you tell the difference between an internal and external
data store?

• The whole system is shown as a
single process
• All the Entities are displayed
along with respective data flow
• The number of the process is
numbered as 0.

In general, a system could be too complex to understand
when viewed as a single Process
 We need a Process Decomposition scheme
i.e., to separate a system into its subsystems (sub-
processes), which in turn could be further divided into
smaller subsystems until the final subsystems become
manageable units (i.e., primitive processes!)
A divide and conquer strategy!!

A DFD that represents the primary functional
processes in the system at the highest possible
level
An Example ...

Functional Decomposition
Balancing

Functional decomposition
 An iterative process of breaking a system description down into
finer and finer detail
 Uses a series of increasingly detailed DFDs to describe an IS
 Act of going from one single system to many component processes
 Repetitive procedure
 Lowest level is called a primitive DFD
Level-N Diagrams
 A DFD that is the result of n nested decompositions of a series of
subprocesses from a process on a level-0 diagram

Generic Decomposition Rules:
A process in a DFD could be either a parent process or a
child process, or both.
A parent process must have two or more child processes.
A child process may further be decomposed into a set of
child processes.

Context Diagram
Level-0
Diagram

When decomposing a DFD, you must conserve inputs to
and outputs from a process at the next level of
decomposition
Ensures that the input and output data flows of the parent
DFD are maintained on the child DFD
This is called balancing

Example: Hoosier
Burgers
• In Figure 1,
notice that there
is one input to
the system, the
customer order
• Three outputs:
• Customer
receipt
• Food order
• Management
reports
Figure 1
Context diagram of Hoosier Burger’s Food ordering
system

• Notice Figure 2. We
have the same inputs
and outputs
• No new inputs or
outputs have been
introduced
• We can say that the
context diagram and
level-0 DFD are
balanced
Figure 2
Level-0 DFD of Hoosier Burger’s food
ordering system

• Figure 3:
• In context diagram,
we have one input
to the system, A and
one output, B
• Level-0 diagram
has one additional
data flow, C
• These DFDs are
not balanced
SOURCE SINK
(a) Context diagram
(b) Level-0 diagram
SOURCE 1
SINK
0
SOURCE 2
1.0
2.0
A B
A
C
B
Figure 3
An unbalanced set of data flow diagrams

We can split a data
flow into separate
data flows on a lower
level diagram (see
Figure 4)
Balancing leads to
four additional
advanced rules
x.0Payment & coupon
x.1Payment
x.2Coupon
Composite data
flow
Disaggregated
data flow

Function Process - A function is a set of related activities
of the business (e.g., Marketing, Production, etc.)
Event Process - An event process is a logical unit of work
that must be completed as a whole. (e.g., Process customer
credit verification)
Primitive Process - a primitive process is a discrete,
lowest-level activity/task required to complete an event.
(e.g., Check the credit card balance)

Function Process - use a Noun!
Event Process - Use a general action verb
 Process Student registration.
 Respond to ...
 Generate ...
Primitive Process - use a strong action verb
 Validate Student ID
 Check ...
 Calculate ...

No process can have only outputs (a miracle!)
No process can have only inputs (a black
hole!)
No process can produce outputs with
insufficient inputs ( a gray hole!)

3.1.2
Create a new
member account
3.1.1
Generate an
Employee bank
statement
3.1.3
Freeze member
account number
Accounts
Receivable
Department
Employee
Member Accounts Employees
Existing account
New account status
Employee
status
Frozen account notification
Employee address
Bank statement
Membership
application
What’s wrong?
Black Hole
Miracle
Gray Hole

Incorrect Correct

IT HAS TWO KINDS OF
FLOW:
1) Inflow to a Data
Store
(Create/Modify/D
elete)
2) Outflow from a
Data Store (Read)
Orders
Process
Order
Cencel
Order
Change
Order
Address
Summarize
Unfilled
Orders
Order
Cancelled Order
Change of Address Summary of Orders
New
Order
Address
Unfilled
Order
l
New
Order
Order
to be
Deleted
2
1
2
2
Create
Delete
Modify
Read
1
1
1
2

Data cannot move directly from one data store to
another data store -- it must be moved by a
process.
Data cannot move directly from an outside source
to a data store -- it must be moved by a process.
Data cannot move directly to an outside sink from
a data store -- it must be moved by a process.
You need to use a Noun phrase to label each data
flow 04-04-2016Ankur Biswas 30

Incorrect Correct

Use a singular noun phrase for each data flow
Ex: customer data, shipping report, …, etc.
Carry logical meaning only, i.e., no implication on
data form or data structure
Minimum flow (no data flooding!!)
Should never be “Unnamed!!” - otherwise, there
might be a modeling error.

Process (Event) - Use an Action Verb Phrase
 Process member order, Generate bank statement, ...
External Agent (Sink/Source) - Use a singular descriptive
noun
 Ex: Student, Customer, etc.
Data Store –
 Use a plural descriptive noun (Members, Customers, etc.)
 Or use a noun + file (Inventory file, Goods sold file)

 A composite DF on 1 level can be split into component DF as the
next level, but no new data can be added n all data in the composite
must be accounted for in 1 or more subfollows
 The inputs to a process must b sufficient to produce the outputs
from the process
 At the lowest level of DFD, new data flows may be added to
represent data that r transmitted under exceptional condition, these
data flows typically represent error message
 To avoid having data flow lines cross each other, u may repeat data
strores or sources/sinks on a DFD. Use an additional symbol, like a
double line on the middle vertical line of data strore symbol, or a
diagonal line in a corner of a sink/source square to indicate a
repeated symbol

Top-down strategy
Create the high-level diagrams (Context Diagram), then
low-level diagrams (Level-0 diagram), and so on
Bottom-up strategy
Create the low-level diagrams, then higher-level
diagrams

Made by your seniors –
GPRS – Gunjita, Pooja, Ritwika & Sayanil (AIT – IT – 2012 Batch)
04-04-2016Ankur Biswas
36

• The whole system is
shown as a single
process
• All the Entities are
displayed along with
respective data flow
• The number of the
process is numbered as
0.
©2011-2012. All rights reserved
Fig_DFD_1(CONTEXT LEVEL DFD):COLLEGE AUTOMATION SYSTEM

Only one mistake is
there – Inflow of
Admin is missing.
You should take care
of such silly mistakes
in your projects…
04-04-2016Ankur Biswas 38©2011-2012. All rights reserved
Fig_DFD_2(LEVEL 0 DFD):COLLEGE AUTOMATION SYSTEM

Always keep in mind to
increment the levels on
the basis of processes and
not entities…
Fig_DFD_3(LEVEL 1 DFD):ADMINISTRATION SYSTEM

Fig_DFD_4(LEVEL 2 DFD):ATTENDANCE PROCESSING SYSTEM

Fig_DFD_5(LEVEL 1 DFD):STUDENT DETAILS PROCESSING SYSTEM

In case the inflow and
outflow is to or from
another process outside the
considered process, then
only arrow need to be
shown.
If processes are interacting
with entities then that
should be show in the
diagram.
Fig_DFD_6(LEVEL 2 DFD):STUDENT REGISTRATION SYSTEM

Understanding Data Flow Diagram

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