Data Flow Diagram(DFD)
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Introduction to DFD
When it comes to conveying how information data flows through
systems (and how that data is transformed in the process), data flow
diagrams (DFDs) are the method of choice over technical descriptions
for three principal reasons.
1. DFDs are easier to understand by technical and nontechnical audiences
2. DFDs can provide a high level system overview, complete with
boundaries and connections to other systems
3. DFDs can provide a detailed representation of system components
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DFD Symbol
Process
Data store
Source/Sink
Data flow
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Some Rules for External Entities
• External people, systems and data stores
• Reside outside the system, but interact with system
• Either
a) receive info from system,
b) trigger system into motion, or
c) provide new information to system
• e.g. Customers, managers
• Not clerks or other staff who simply move data
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External
Entity
Some Rules for Data Stores
• Internal to the system
• Data at rest
• Include in system if the system processes transform the data
– Store, Add, Delete, Update
• Every data store on DFD should correspond to an entity on an ERD
• Data stores can come in many forms:
– Hanging file folders
– Computer-based files
– Notebooks
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D1 Data Stores
Some Rules for Data Flows
• Data in motion, moving from one place to another in the system
– From external entity (source) to system
– From system to external entity (sink)
– From internal symbol to internal symbol, but always either start or end at a
process
Data Flow
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Some Rules for Processes
• Always internal to system
• Law of conservation of data:
#1: Data stays at rest unless
moved by a process.
#2: Processes cannot consume or create data
– Must have at least 1 input data flow
– Must have at least 1 output data flow
– Should have sufficient inputs to create outputs
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0.
Processes
Data Flow Diagram Types
Data Flow Diagram is of two types:
• 1. Physical Data Flow Diagram: Physical data flow diagrams
are implementation-dependent and show the actual devices,
department, people, etc., involved in the current system.
• 2. Logical or Conceptual Data Flow Diagram: Logical data
flow diagram represents business functions or processes. It
describes the system independently of how it is actually
implemented, and focuses rather on how an activity is
accomplished.
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Physical Data Flow Diagram
• Describes about “How a system works”.
• Describes the flow of physical data components between physical
operations in a system.
CUSTOMER
ORDER
Reject
1
Ordering
Processing
Clerk
Customer
Order
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2
Enter into
computer
file
3
Run
LOCATE
program
Delivery
node
Logical Data Flow Diagram
• Describes about “What a system does”.
• Describes the flow of logical data components between logical
operations in a system.
Reject
1
Receive
customer
order
Customer
order
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2
Find
Locations
of parts
3
Arrange
delivery
Delivery
order
Asynchronous operation
(a) Asynchronous operation
• The two processes are connected through a data store. Hence,
the operations of the two bubbles are independent.
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Synchronous operation
Synchronous operation
• The above two processes are directly connected by a data flow. This
means that the ‘validate-number’ process can start only after the
‘read-number’ process had supplied data to it.
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Data Flow Diagramming Rules
• Specific rules to DFD is as follows
–
–
–
–
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Context Diagram (or Level-0 DFD)
Level-1 and lower decompositions
Numbering of the processes
Balancing across levels
Context Diagram of College
Database System
User
Want to
view
records
about
students/fac
ulty
Show the
specified
records
Add a
record
The
specified
record is
added
issue the
specified
book for the
user if
available
College
Database
System
0
The
specified
record is
deleted
Administrator
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Want to
issue books
Delete a
record
Level-1 DFD College Database
System
User
Wants to
view
Records
about
students/
faculties
Shows the
specified
record
Wants to view
records about
books/want to
issue books
Educational
Dept
0.1
add or
delete
student
records
Add or
delete the
specified
data to
database
Administrator
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Shows
records/issue Wants to
the specified view
records
book if
available
Library
0.2
Shows
records as
specified
Accounts
section
0.3
Level-2 DFD College Database
System
User
Ask to
Give
records
Give
records
as
specified
Give
records as
specified
Student
0.1.1
Ask to
add or
delete
record
Add or
delete
record as
specified
Administrator
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Ask to
Give
records
Faculty
0.1.2
Balancing DFD
• The data that flow into or out of a bubble must match the data flow at
the next level of DFD.
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Balancing DFD
An example showing balanced decomposition
In the next level, bubble 0.1 is decomposed.
(a) Level 1 DFD
In the level 1 of the DFD, data items d1 and d3 flow out of the bubble
0.1 and the data item d2 flows into the bubble 0.1.
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Balancing DFD continued….
(b) Level 2 DFD
The decomposition is balanced, as d1 and d3 flow out of the level 2 diagram
and d2 flows in.
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What is a Good DFD?
• DFDs have a number of features which can be used to ensure that they
are self explanatory, complete and unambiguous. The features are:
•
•
•
The absence of flowchart structures
Conservation of data and
Good naming convention
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A good DFD should
•
•
•
•
Have no data flows that split up into a number of other data flows
Have no crossing lines
Not include flowchart loops of control statements and
Not include data flows that act as signals to activate processes
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Advantages of DFD
• DFD is a graphical text used for requirement analysis.
• DFD depicts information software without explicit representation of
procedural logic
• DFD also depicts real time systems with the help of some extended
notations
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