Data Flow Modelling

Introduction to DFDs

What is a Data Flow Model ?

What is a Data Flow Diagram ?

A means of documenting the flow of

data through a system.

It is a graphical technique which helps: Provide the analyst in the development with an

understanding of the system.

Convey the analyst’s ideas to others.

Introduction to DFD’s

•A DFD is a diagrammatic representation of the

information flows within a system showing:

How the information enters and leaves the system;

What changes the information;

Where information is stored.

•Data Flow Diagrams are an important technique for

systems analysis and design.

What does a DFD give us ? Boundary definition

Checking the completeness of analysis

Basis of system design

Data Flow Diagram (DFD)

• Maps the route of data around a system

Data Source and Sink

Data Store

Flow Line

Process is easy to understand and communicate does not show procedures and control as a

flowchart does

D1 OrdersD1 Orders

1 Accounts

Check Customer Details and Create Order

1 Accounts

Check Customer Details and Create Order

CustomerCustomer

Data Flow Modeling

Customer andOrder Details

Valid Order Details

Modelling the way that data flows within a system

Looks at data in terms of Processes (which transfer

data from one form to another)

Data Stores (which are places where data is held)

External Entities (objects from which data comes, or objects which receive data)

Data Flows (routes by which the data can flow)

External Entities

An External Entity is whatever or whoever donates information to the system or receives information from it.

An external entity may be a user of the system, an external organization, a computer system, or any other source or recipient of information.

A Process

A process transforms or manipulates data within the system. Processes are represented by rectangles on a Data Flow Diagram.

Each process box contains the name of the process, an identifier, and possibly a location.

Process Boxes

The process name describes the process performed on the data received by the process.

Process identifiers are numerical. The location of the process is used to

denote either - The place where the process is performed Or the member of staff responsible for

performing the process.

Process Box Components

1 Manager

Process NewCustomerName

Location

Identifier

Data Stores

A data store is where information is held within the

system.

A data store is represented on a DFD by an open-

ended box.

D1 Student Details

Data store Components

D1 Student DetailsIdentifier

Description of Data Store

Data Flows

A package of information flowing between objects on the Data Flow Diagram.

Represented by a line and an arrow to denote the direction of the information flow.

Labeled with the name or details of the information represented by the data flow.

Example - Data Flows

Customer

A

Process New Customer

1 Manager

Customer Name

Customer Details

Customer Name

D1

Basic rules for a DFD

Data does not flow directly between processes

Data does not flow directly between data stores

Data cannot be transferred directly from store to sink or source to store

External entity Data store

D1 Student Details

Customer

A

Data store Data store

D1 Student Details

D3 Course Details

Drawing Data Flow Diagrams Identify

main inputs & outputs processes data flows

Draw Data Flow Diagram associate data stores internal data flows

Example

Process New Student

1 Academic

Student

AStudent Details

D1 Student Details

Student Details

Checking Data Flow Diagrams ?

Every process must have at least one data flow coming from it and two data flow flowing into it.

Every external entity and data-store must have at least one data flow either coming from it, or flowing into it.

For every data flow either its source is a process or its sink is a process.

A physical DFD represents HOW things are happening. It contains the problems of the current system, and is people or machine dependent. It tends to contain redundant data stores/ processing. It tends to mention names of departments, people, forms, devices used and where data is stored

A logical DFD is extracted from a physical DFD. It is a logical representation of the system, which indicates WHAT the system accomplishes. It is implementation-independent: it focuses on the flow of data between processes without regard to the specific devices, storage location or people in the system.

Types of DFDs

Create the context diagram (Level 0), which shows the external interfaces with the outside world, i.e. the global view of a system.

How to get a DFD

Dispatch note

WarehouseCustomer Invoice

Cheque

Order form Picking notes

Sales & Accountancy

System

0

DFD Level 1

How to get a DFD

Dispatch note

WarehouseCustomer

Invoice

Cheque

Order form Picking notesProcess Order

1 Sales Dept.

Prepare Invoice2 Accounts Dept.

Ensure all data flows are given a name

“A DFD cannot be considered to be completed unless all data flows and functions have been given a meaningful name”

Add Data Stores where needed

How to get a DFD

Example - Supermarket DFD A supermarket uses point of sale bar code

scanners at their checkout tills. The bar code on the product is scanned and is then checked to see if it is valid. If it is a valid bar code then the stock database is updated and an entry for that product is printed on the till receipt. If it is an invalid bar code then an alarm will sound on the checkout till and the operator will need to deal with the problem.

Draw the processes on the Supermarket Data Flow Diagram below.