Data Flow Diagram
Shows
the flow of data through the system and the processing performed by the system.
Gane
and Sarson notation is widely popular
Process
A
Process is work performed by a system In response to incoming data flows or
conditions and it transforms incoming data flow into outgoing data flow.
External Entity
An
external Entity is an outside person (e.g. supplier, customer), organization
unit (e.g. other dept), system (other business systems), or organization (e.g.
Bank) that interact with the system.
Data Store
A
Data Store is an “inventory”of data. That is, stored data intended for later
use (data at rest).
Data Flow
Represent
inputs or outputs, to or from the processes. The arrow head indicates the
direction of data flow. Label the arrows with the name of the data that moves
through it.
Steps of DFD
Draw
Context Diagram
Level
0 (Top Level) Data Flow Diagram
Level 1 Data Flow Diagram
Continue
up to elementary functions
Draw Context Level Diagram
Ø Think the system
as a container
Ø Distinguish the
inside from the outside
Ø Ignore the inner
workings of the container
Ø Find out the net
inputs to the system
o
Business transactions a system must respond to
Ø For each net
input determine its source (External Agents)
Ø Find out the net
outputs from the system
o
Responses produced by the system
Ø For each net
output find the destination (External Agents)
Ø Identify any
external data stores,
Files or databases of other systems
Steps
for draw Context level Diagram
Ø Identify all sources and recipients of data to/from
the system.
Ø Identify major
data flows to and from the System
Ø Convert each
source or recipient into external entity
Ø Add the data flows
between each external entity and the process representing the entire system.
E.g.
Whenever an account holder wants to withdraw some cash, he pesents
a cheque or withdrawal slip.
The account is checked for the appropriate balance.
If
balance exists, the cash is paid and the account is updated.
Context diagram
Decomposition
Is
the act of breaking a system into its component subsystems, processes and sub
processes.
Steps for draw Data
Flow Diagram
Step1: Decompose the
system/sub system into sub process.
Step 2:
Step3: Identify Data stores & Data Flows
Level 0 DFD
