Data Flow Diagrams Example

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System Analysis and Design/ Tools for systems analysts

Learning Objectives

Learning Objectives

What are Data Flow Diagrams (DFDs)?
Why they are useful?
How are they developed?
How to level DFDs?
Good style conventions in developing DFDs
Difference between Logical and Physical DFDs
Tools available to draw DFDs

V. Rajaraman/IISc. Bangalore

//V1/June 04/1

System Analysis and Design/ Tools for systems analysts

Motivation

Motivation

WHY DFD ?
Provides an overview of
-What data a system processes
-What transformations are performed
-What data are stored
-What results are produced and where they flow
Graphical nature makes it a good communication tool between
-User and analyst
-Analyst and System designer
Structure of DFD allows starting from a broad overview and expand it to a hierarchy of detailed diagrams

V. Rajaraman/IISc, Bangalore

//V1/July 04/1

DATA FLOW
DIAGRAMS
WHAT ARE DATA FLOW DIAGRAMS?
DFDs models the system by depicting
• External entities from which the data flows and where
results terminate
• Processes which transform data flows
• Data stores from which the data are read or into
which data are written by the processes.
V.Rajaraman

M5/LU1/V1/2004

5

SYMBOLS USED IN DFD
PROCESS
Stores demand
note

1.
STORES Issue Advice

Delivery slip

• A circle represents a process
• Straight lines with incoming arrows are input data flows • Straight lines with outgoing arrows are output data flows • Processes are given serial numbers for easy reference
• Labels are assigned to Data flow.These aid documentation V.Rajaraman

M5/LU1/V1/2004

2

SYMBOLS USED IN DFD
EXTERNAL ENTITIES

VENDOR

Invoice

Order
Customer
Bill

• A Rectangle represents an external entity
• They either supply data or receive data
• They do not process data

V.Rajaraman

M5/LU1/V1/2004

3

SYMBOLS USED IN DFD
DATA STORES
Inventory

Writing

Reading

• A Data Store is a repository of data
• Data can be written into the data store
This is depicted by an incoming arrow
• Data can be read from a data store
This is depicted by an outgoing arrow
• External entity cannot read or write to the data store
• Two data stores cannot be connected by a data flow
V.Rajaraman

M5/LU1/V1/2004

4

RULES OF DATA FLOW
• Data can flow from
-external entity to process
-process to external entity
-process to store and back
-process to process
• Data cannot flow from
-external entity to external entity
-external entity to store
-store to external entity
-store to store
V.Rajaraman

M5/LU1/V1/2004

5

DATA FLOW DIAGRAMS
An alternate notation is often used
3
Store
Issue

A Process

A Data store

DS1

Inventory

Label
Nam
e

Nam
e
Label

V.Rajaraman

M5/LU1/V1/2004

6

GOOD STYLE
IN DRAWING DFD
• Use meaningful names for data flows, processes and data stores. • Use top down development starting from context diagram and successively leveling DFD
• Only previously stored data can be read
• A process can only transfer input to output. It cannot create new data
• Data stores cannot create new data

V.Rajaraman

M5/LU1/V1/2004

7

DESCRIBING
A SYSTEM WITH A DFD
• An entire system is represented by one DFD which gives the system’s overview
• It is called a context diagram
• It gives little detail & is also known as the top level DFD • Context diagram of mess management is shown in the next
transparency

V.Rajaraman

M5/LU2/V1/2004

5

CONTEXT DIAGRAM OF
MESS MANAGEMENT SYSTEM
Extras Note

Students

Vendors

Requisitions

Payments
Payments
Bills

Perishable
Items

Mess manager

Supplies

Mess
Management
Daily rate
System

Mess secretary

Menu
Item needed
Each day

Overdue Bills
Overdue
Payments

Chief warden

• Observe this diagram gives very little detail
V.Rajaraman

M5/LU2/V1/2004

2

LEVELLING DFD
• A context diagram gives an overview
•...
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