DATA FLOW DIAGRAM

DEFINITION

A data flow diagram (DFD) is a graphical representation of the "flow" of data through a computer system.

ORA data flow diagram looks at how data flows through

a system. It concerns things like where the data will come from

and go to as well as where it will be stored. But you won't find information about the processing

timing (e.g. whether the processes happen in sequence or in parallel).

Flow chart shows “ flow of Control “ . DFD shows “ flow of Data The flowchart describes boxes that

describe computations, decisions, interactions & loops.

It is important to keep in mind that data flow diagrams are not flowcharts and should not include control elements .

DFD is not a “flow chart”

TYPES OF DFD

Data flow diagrams (DFDs) are categorized as either logical or physical. 1) LOGICAL DFD:- A logical DFD focuses on the business and how the business operates. It describes the business events that take place and the data required and produced by each event.

2) PHYSICAL DFD:- A physical DFD shows how the system will be implemented.

DFD COMPONENTS Data Flow Diagrams are composed of the four

basic symbols shown below:-

1)External Entity:-

The sharp cornered rectangles(or simply

boxes) in a DFD indicates entities.

The External Entity symbol represents

sources of data to the system or destinations

of data from the system.

Entities are people things, organizations etc

Entity

ENTITIES

Student

Doctor

TeacherManager

Cashier

UserCustomer

The rounded cornered rectangles in a DFD

indicate processes

The Process symbol represents an activity

that transforms or manipulates the data

(combines, reorders, converts, etc.).

2) Process:-

Process

Processes

Accounting System

Grading System

Reservation System

Patient Administration

System

Marketing System

3) Data Store:-

Opened sided rectangles in DFD indicates data store.

The Data Store symbol represents data that is not moving (delayed data at rest).

A Data Store is a repository of data.

Data can be written into the data store. This is depicted by an incoming arrow.

Two data stores cannot be connected by a data flow.

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.

Data Store

4) Data Flow:-

Arrow symbol in DFD indicate data flow

The Data Flow symbol represents

movement of data

Data

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

This diagram represents a banking process, which maintains customer accounts.

In this example, customers can withdraw or deposit cash, request information about their account or update their account details.

The five different symbols used in this example represent the full set of symbols required

to draw any business process diagram.

EXAMPLE 1

Level 0 DFD• The level 0 DFD (also known as the

context level DFD ) is the simplest DFD.• The outermost level (Level 0) is

concerned with how the system interacts with the outside world.

• This level basically represents the input and output of the entire system.

LEVELS OF DFD

1. Identify your main system

2. Identify the external people who interact with

the system

3. Decide what data these entities will enter into

the system

4. Determine what these entities expect as

output from the system

How to create Level 0 DFD

Context Level DFD for a Mail Order Business

ORDER PROCESSINGCUSTOMERORDER

DELIVERY

The basic module of the system are

represented in this phase and how data

moves through different module is shown.

The level 1 DFD provides a high –level view

of the system that identifies the major

processes and data stores.

Level 1 DFD

1. Focus on your process and break it into 2

or more sub-processes

2. Identify what data flows between these

processes and between the entities

3. Identify What permanent data files are

used in this system

4. Note that no new entities can be

introduced

How to create Level 1 DFD

Delivery

Credit

Status

Level 1 DFD for a Mail Order Business

CUSTOMERSALES

PROCESSING

ACCOUNTING SYSTEM

CUSTOMER DATABASE

Customer no.

Credit Status

ORDERS

Order

Order

Order

Each process from level 1 is exploded even

more into sub processes. This decomposition

continues for each level.

The number of levels possible depends on the

complexity of the system

2-Level DFD and other level of DFD

LEVELLING DFD 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

leveling.

1. With a datafl ow diagram, users are able to visualize how the system will operate, what the system will accomplish, and how the system will be implemented

2. Data fl ow diagrams can be used to provide the end user with physical idea of how the data they input ultimately has an effect upon the structure of the whole system.

3. The old system’s datafl ow diagrams can also be drawn up and compared with the new system’s datafl ow diagrams to draw comparisons in order to help implement a more efficient system.

The value of a DFD

Questions ?

1) In a DFD external entities are represented by a______ a. Rectangleb. Ellipsec. diamond shaped boxd. Circle

A

2) External Entities may be a_________a. source of input data onlyb. source of input data or destination of resultsc. destination of results onlyd. repository of data

B

3) A data store in a DFD represents a. a sequential fileb. a disk storec. a repository of datad. a random access memory

C4) A data cannot flow between a store and (i) a store (ii)a process (iii)an external entity a. i and iii b. i and iic. ii and iii d. ii

A

5) Data cannot flow from an external entity to an external entity because

a. it will get corruptedb. it is not allowed in DFDc. an external entity has no mechanism to read or writed. both are outside the context of the system

D

6) A data flow can a. only enter a data storeb. only leave a data storec. enter or leave a data stored. either enter or leave a data store but not both C

7) The following portion of a DFD is not correct as

a. there is no output data flow from the processb. there are three data flow inputs to the processc. there is no external entityd. there is no data store

Cost/unitQuantity

Billing Process

Discount

A

Quantity

Billing Process

Discount

8) The following portion of a DFD is not correct as

(a) there are many data flows out of the process (b) there are no input data flows to the process (c) the output does not go to an external entity (d) there is no data store

Order to vendor

Out of stock

Too much stock

Billing Proces

B

B10) By leveling a DFD we mean a. splitting it into different levels

b. make its structure uniform

c. expanding a process into one with more sub-processes giving more detail

d. summarizing a DFD to specify only the essentials

9) A context diagram

a. describes the context of a systemb. is a DFD which gives an overview of the systemc. is a detailed description of a systemd. is not used in drawing a detailed DFD

C

11 ) A physical DFD specifies

a. what processes will be used

b. who generates data and who processes it

c. what each person in an organization does

d. which data will be generated

B

12) Data flow in a DFD must have (i) an arrow showing direction of flow of data (ii)a meaningful name (iii)a label such as: xyz (iv)no arrows as they are confusing a. i and iiib. ii and iv c. iii and iv

A

Your Assignment

A general DFD for an airline reservation system

http://en.wikipedia.org/wiki/Physical_Data_Flow

http://www.visual-paradigm.com/product/bpva/tutorials/dfd.jsp

http://www.civil.utm.my/staff/file/141/file/CIMLectureNotes2011.pdf

http://spot.colorado.edu/~kozar/DFDtechnique.html

http://www.fbk.eur.nl/PRJ/MEETEYEES/dfd.htm

Http://ratandon.mysite.syr.edu/cis453/notes/DFD_over_Flowcharts.pdf

http://www.slideshare.net/mohit4192/dfd-examples

http://ecourses.vtu.ac.in/nptel/courses/Webcoursecontents/IIScBANG/System%20Analysis%20and%20Design/pdf/module5.pdf

References

Submitted byNidhi Sharma8503