1. DFD(Data Flow Diagram)

2. Introduction to DFD What is DFD?
A data flow diagram is a graphical depiction of flow of data through intended software system and is used as 1st step to create an overview of system.
It’s really useful as it provides overview of data as well as functionality to software designers

3. Components Of DFD: Entity
External Entities:
They could be a person (facebook users), another software( like facebook) or a hardware ( sensors) which provide to or consume information from the intended software.
Represented by rectangle:
Must be named
No direct data flow between two entities ever.
User

4. Components of DFD: Process
A circle (sometimes called a bubble) represents a process or transform that is applied to data and changes it in some way.
The basic rules:
It must be properly labeled
It must not be repeated in a diagram
Ticket Booking

5. Components of DFD: Data Flow
The basic rules:
Data flows can’t be bidirectional, i.e the input data flow and the output data flow for a process, data store or for an entity should always be different.
The data flows should always be labeled
The labels should be precise and informative
You can join two similar input data flows(join) or two similar output data flows (fork)
User Login
Login info
WRONG
Login status
Login info
User Login
Login status
RIGHT =
Join
Fork

6. Components of DFD: Data Store
Data stores are places where data may be stored. This information may be stored either temporarily or permanently by the user.
They are internal to the system.
The basic rules:
Never shown in context level diagram
No direct data flows between two data sources
Symbol:
User info
Order info

7. DFD General rules Basic rules that apply to all DFDs:
No internal logic should be shown like loops, if-else, this is not a flow chart
In order to keep the diagram uncluttered, you can repeat data stores and external entities
No process can have only output data flows (a miracle).
No process can have only input data flows (black hole).
Data cannot be moved directly from one store to another without a process.
Data cannot move directly from an external entity to a data store without a process.
Data stores can’t be sink( only input data flows) or source ( only output data flows) in level 1 DFD
BlackHole
Miracle
Sink
Source

8. Context Level Diagram – Level 0
A level 0 DFD, also called a fundamental system model or a context model.
It represents the entire software element as a single process with input and output data.
All the external entities should be identified and shown.
Rule:
Only one process
Data flows should be labeled.
No data store can be shown in context diagram as they are internal to the system only.

9. Course Registration System
Context Diagram for Course Registration System
Courses & other info.
Faculty
Students
Registration
Process
Class schedule
Class list
Registrar

10. Level 1 DFD
The level 1 DFD we construct is a more refined version of the context diagram.
It covers the entire system, all the main processes are shown
The DFD should be balanced with respect to context diagram
No new external entities should be there
The data flows from context diagrams should be visible
Rules:
It should consists of 5-9 processes(bubbles).
Repetition of data sources is allowed.
Process can not be repeated.

11. DFD: Course Registration System
Level 1 DFD
Students
Individual Course
Registration
Information
Courses &
Other info.
2. Compile &
Distribute
Information
1. Enroll
Students
Registrar
Schedules
Class Lists
Students
Faculty
Note: External entity Students is replicated to avoid crossing lines

12. Level 2 DFD
Only those processes that merit being expanded need to have level 2 DFDs .
Level 2 DFD completely describes any one process from the level 1 DFD.
Rules:
All the data flows into and out of selected process on the level 1 DFD also appear on the level 2 DFD
Repetition of data sources is allowed.
A Data store can appear as a sink or source within level 2 DFD

13. DFD: Course Registration System
Level 2 DFD
Courses &
Other info.
Student Records
Course Prereqs
1.1 Obtain
Student
Preferences
1.2 Check
Eligibility
List of
Preferences
Eligible Students
1.3 Enroll Students
in Classes
Individual course registration
information

14. Check List
There are many errors that may occur when drawing data flow diagrams.
External entities must be people or systems that send information to or accept information form the system to be engineered
Check the direction of data flows to and from data stores
Data flows must always be labelled with the data they contain. Do not put verbs in the data flow description as this implies a process

15. Continued..
Parent and child diagrams should be consistent. Do not show a data flow coming from or to an external entity on a level 1 DFD that isn’t shown on the context diagram (and vice versa).
Make sure each process has at least one input and one output.
Each data store should have at least one input and one output on the DFDs somewhere.
Each process name should start with a verb
Where a process has only two data flows (one input and one output) then check it. Usually a data flow has been omitted.

16. System Context Diagram
Inputs
from
Customer
ATM
Customer
Bank
Requests
Cash
ATM
System
Outputs to
Customer
Receipt
Bank
Operator
Instructions
Bank
Responses
ATM
Operator
Operator
Responses

17. Data Flow Diagram – Level 1
Inputs
from
Customer
ATM Card
and
Transactions
Messages
to Bank
Interface
Bank
Requests
Cash 2
Interface
with
Customer
Messages
from
Customer
Interface 1
Control
ATM 4
Interface
with
Bank
Receipt
Bank
Responses
Messages to
Customer Interface
Messages
from Bank
Interface
Outputs to
Customer
Messages from
Operator Interface
Messages to
Operator Interface 3
Interface
with
Operator
ATM
Cash
Operator
Responses
Operator
Instructions

18. Level 2 DFD: Interface with Operator
Shutdown
ATM 1
Control
ATM
3.2
Display
Messages
Shutdown/Startup
Started
Display
Messages
3.1
Process
Operator
Inputs
Cash to be
Replenished
Replenish
Cash
Operator
Responses
Restart
ATM
3.3
Update
ATM Cash DB
ATM
Cash
Cash Amount
Cash Details

19. Level 2 DFD: Interface with Customer
ATM Card
and
Transactions
Card
Info
Cash
Eject
Card
2.1
Read and
Validate
Card
2.3
Dispense
Cash
Dispense
Cash
Confiscate
Card 1
Control
ATM
Card
Inserted
ATM
Cash
Transaction
Info
User Input
Read
Print
Receipt
Cancelled
2.4
Print
Receipt
2.2
Process
User Inputs
and Display
Display
Message
PIN
Cancel
Display
Messages
Receipt

20. Level 2 DFD: Interface with Bank
ATM Card
and
Transactions
Confiscate
Card
Validate
Card
Eject
Card
Card
Validation
Request
4.1
Validate
Card
Card
Valid
Dispense
Cash 1
Control
ATM
Transaction
Rejected
Invalid
Card
Card
Validation
Response
4.2
Process
Transaction
Transaction
Approved
Card
Inserted
Lost/Stolen
Card
Transaction
Request
Transaction
Response