1. Data Flow Diagrams
BCA Sem IV
K.I.R.A.S

2. What is A DFD?
Data flow diagrams (DFDs) are used to depict the flow and transformation of data in an information processing system.
DFDs give an overview to an analyst specifying where data originates, how it is processed and where the results go.
DFDs act as a graphical communication aid between a user and an analyst. It is also useful as a communication aid between an analyst and a system designer.
The procedure to develop a DFD starts with one DFD giving an overview of the system to be designed. This is called a context diagram.
The context diagram is expanded into a series of DFDs, each describing a specific function. This method of top down analysis and breaking down DFDs to give more and more detail is known as levelling.

3. What is A DFD?
The main merit of DFD is that it provides an overview of what data flows in a system, what transformations are done on the data, what files are used and where results flow.
It is a good documentation aid which is understood by both programmers and non-programmers (i.e., laypersons). As DFD specifies only what processes are performed and not how they are performed it is easily understood by a non-programming user.
A DFD allows you to identify the transformations that take place on data as it moves from input to output in the system.

4. Types of Diagrams Context Diagram Level-O Diagram
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
Level-O Diagram
A data flow diagram (DFD) that represents a system’s major processes, data flows and data stores at a high level of detail

5. Context diagram
A diagram giving an entire system’s data flows and processing with a single Process (circle) is called a context diagram.
A context diagram is expanded into a number of inter-related processes. Each process may be further expanded into a set of inter-connected sub processes. This procedure of expanding a DFD is known as levelling.

6. Elements of data-flow diagramsor

7. Show all processes
data flows must start or end in a process
beware of black holes - every process and data store must produce data

8. Elements of data-flow diagrams
There are four basic elements of data flow diagrams:
Data Transforms: where data is filtered, formatted or synthesized.
Information sources and sinks: essentially inputs and outputs to the system.
Information/Data stores: where information is kept for the duration of system activity.
Information Flows: data that is passed into and out of transforms, information stores.

9. Source
Source represent persons, processes which produce data to be used by the system or receive data that is output by the system
Reside outside the system, but interact with system
Either
a) receive info from system,
b) trigger system into motion,
c) provide new information to system
Examples: Student, Customer, Client

10. Data Stores
Data stores are temporary or permanent repositories of information that are inputs to or outputs of processes
Internal to the system
Information Stores represent locations where information can be store for the duration of the system activity
Examples: Student Master, Client List
Total Payroll Expense

11. Data Flows Taxable Income
Data flows represent the transfer of data over time from one “place” (entity, process, data store) to another
Information flows represent the information being passed into or out of a transform, source, sink or store. It is represented as a labeled arrow:
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
Examples: New Student Information (from Student, to Student Master)
Taxable Income

12. Data transforms
Data transforms labeled circles with one or more incoming and outgoing information flows:
Taxable Income
Tax owed
Calculate
Tax
Tax Tables

13. Processes
Processes are discrete actions that transform input data to output data
Examples: Create Student Record, Calculate Purchase Cost, Register Client
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 (to avoid black holes)
Should have sufficient inputs to create outputs

14. Define the System Define Subsystems
A system is the collection of all business processes which perform tasks or produce outputs we care about. It is “what happens.”
The system is a single process, connected to external entities
Represented in a “Context Diagram”
Define Subsystems
A subsystem gives a more detailed view individual processes contained in the context diagram
Includes data stores, more elementary processes

15. Creating Data Flow Diagrams
Creating DFDs is a highly iterative process of gradual refinement.
General steps:
1. Create a preliminary Context Diagram
2. Identify the ways in which users most commonly use the system
3. Create a Level 0 diagram from fragments
4. Decompose to Level 1,2,…
5. Go to step 1 and revise as necessary
6. Validate DFDs with users.

16. DFD Rules Data Store Data Flow
Data cannot be moved directly from one store to another
Data cannot move directly from an outside source to a data store
Data cannot move directly from a data store to a data sink
Data Flow
A data flow has only one direction of flow between symbols

17. DFD Rules—Context Diagram
One process, numbered 0.
Sources (external entities) as squares
Main data flows depicted
No internal data stores are shown
They are inside the system

18. DFD Example
A mail-order company distributes CDs and tapes at discount price to record-club members. When an order processing clerk receives an order form, he or she verifies that the sender is a club member by checking the Member file. If the sender is not a member, the clerk returns the order along with a membership application form. If the customer is a member, the clerk verifies the order item data by checking the Item file. Then the clerk enters the order data and saves it to the Daily Orders file. The clerk also prints an invoice and shipping list for each order, which are forwarded to Order Fulfillment.

19. DFD Example - Step 1 Verify Member data Member Record status Members
SECOND SENTENCE:
When an order processing clerk receives an order form, he or she verifies that the sender is a club member by checking the Member file.
RESULT:
Order Processing System
Verify
Member
status
Member data
Record
Club
Member
Members
order
Customer
(Non
Member)

20. DFD Example - Step 2 continued
SECOND SENTENCE:
When an order processing clerk receives an order form, he or she verifies that the sender is a club member by checking the Member file.
RULES FOLLOWED:
data flows start or end in a process

21. DFD Example - Step 3 THIRD SENTENCE:
If the sender is not a member, the clerk returns the order along with a membership application form.
RESULT: Order Processing System
Verify
Member
status
Member data
order
Record
Club
Member
Members
Customer
(Non
Member)
application form

22. DFD Example - Step 3 continued
THIRD SENTENCE:
If the sender is not a member, the clerk returns the order along with a membership application form.
RULES FOLLOWED:
shows all processes without conditions

23. Order Processing System
DFD Example - Step 4
FOURTH SENTENCE:
If the customer is a member, the clerk verifies the order item data by checking the Item file.
RESULT:
Order Processing System
Verify
Member
status
Member data
order
Record
Club
Member
Members
Verify
Order item
data
Customer
(Non
Member)
Items
Item details
application form

24. DFD Example - Step 4 continued
FOURTH SENTENCE:
If the customer is a member, the clerk verifies the order item data by checking the Item file.
RULES FOLLOWED:
data flows start or end in a process
shows all processes without conditions

25. DFD Example - Step 5 FIFTH SENTENCE:
Then the clerk enters the order data and saves it to the Daily Orders file.
Record
Club
Member
Verify
Member
status
Members
order
Member data
Member order
Verify
Order item
data
Item details
Customer
(Non
Member)
Items
application
form
Daily
Orders
order
Daily Orders
Verified member order

26. DFD Example - Step 5 continued
FIFTH SENTENCE:
Then the clerk enters the order data and saves it to the Daily Orders file.
RULES FOLLOWED:
data flows start or end in a process

27. DFD Example - Step 6
SIXTH SENTENCE:
The clerk also prints an invoice and shipping list for each order, which are forwarded to Order Fulfillment.

28. DFD Example - Step 6 Record Club Member Member data Verify Member
status
Members
order
Member order
Verify
Order item
data
Customer
(Non
Member)
Item details
Items
order
application
form
Daily
Orders
Verified member order
Order
Fulfillment
Print
Invoice
& shipping
list
order
Order data
shipping
list
Daily Orders

29. DFD Example - Step 6 continued
SIXTH SENTENCE:
The clerk also prints an invoice and shipping list for each order, which are forwarded to Order Fulfillment.
RULES FOLLOWED:
data flows start or end in a process
shows all processes without conditions
beware of black holes - every process and data store must produce data

30. University Course Registration System
Student
Course
details
Student
details, marks details
Acceptance / Denial note
University
Registration
Level 0 (Context Model)
Student
details
Course
details
University Administration

31. University Course Registration
Student
Student
details, marks details
Course
details
Denial
note
Acceptance
note
Validate
inputted
details
Level 1
Student
details
Course
details
Process
Application
University Administration
Student database

32. Publisher's present ordering system
Process
Orders

33. Publisher's present ordering system
Level 1

34. Level 2: Showing Order Verification & credit check

35. ATM User Card details Transaction amount PIN Invalid PIN ATM System
Invalid Card
Receipt

36. ATM Card details Valid Card Details Verify Card User PIN Verify PIN
Invalid Card
Invalid PIN
Card Num, PIN
Card
status
Card
status
Cards
Overdraw
Validate
Amount
amount
Account
balance
Daily Balance
Receipt
Transact
Withdrawal
Validated Transaction details
Accounts
Transaction details

37. Level 2 Validate Amount User Card status Transaction details
Daily Balance
Accept
Amount
amount
Check
Daily
limit
Accounts
Daily O.K
Weekly
Balance
Daily limit exceeded
Overdraw
Check
Weekly
limit
Notify
Invalid
amount
Weekly limit exceeded
Weekly O.K
Account
balance
Insufficient funds
Check
Account
Balance
Validated Transaction details

38. Payroll Management : Level 0
Employee
Employee No,
Hours worked
cheque
& payslip data
Payroll
Management
Payroll summary data
Accounts

39. Payroll Management : Level 1
Employee Table
Employee
Employee No, hours worked
Employee No, hours worked
Invalid
employee
data
Validate
Data
Payroll Details
Name, pay rate,
tax code, etc.
Accounts
Prepare
Payroll
Employee no,
pay, tax, etc.
cheque & payslips
Print
Payroll

40. Decomposition of DFDs Functional decomposition Level-N Diagrams
Act of going from one single system to many component processes
This is a repetitive procedure allowing us to provide more and more detail as necessary
The lowest level is called a primitive DFD
Level-N Diagrams
A DFD that is the result of n nested decompositions of a series of sub-processes from a process on a level-0 diagram

41. Uses of DFD
Used to analyze the system to ensure that the design is complete
Used to partition the system into programs
Used for system documentation
Better communication with users
Better understanding of the business by analysts