Unit 7 University of Sunderland COMM1B Information Systems Analysis Process Modelling: Introduction to Data Flow Modelling Information System Analysis

Unit 7University of Sunderland COMM1B Information Systems Analysis

Process Modelling:Introduction to Data Flow

Modelling

Information System AnalysisCOMM1B


• Is the existing system important?• How do we describe an existing

system?• Problems with narrative• Data Flow Diagrams

Outline


It can been argued:

“If we analyse the existing system we will eventually end up with a computerised version of the existing system with all of its problems and weaknesses”

How important is the existing system?


• Why bother?– Occasionally we do not

• Promotes understanding– Provides an accurate and complete view of the

existing system– Can reveal weaknesses and identify strengths– Can help identifies information requirements for

new system

• Examination of the whole picture can enable discussion between developers and users

Current system model


Narrative description• Can be ambiguous• Can be long-winded• Can be incomplete

Data Flow Modelling (DFM)• Diagrammatic• Readable

– Even by those without knowledge of the technique

How do we describe an existing system?


Accounts

Customer

Accounts

Supplier

*

Enter Orders

Sales1

*

Packagegoods

& despatch

2

Control stock

Warehouse3

Delivery note

Orderacknowledgement

Order

Weekly creditlimit list

Goods receivednote

Internal orderform

Despatchinstruction set

Re-order card

Goods receivednote

Copy deliverynote

Example: Data Flow Diagram Level 1

Order Processing System

Despatch


Data Flow Diagrams

• Provide a complete model of the information system showing:– View of system focusing on its data– Where it arrives from

• external sources

– What the system does to it• processing

– Where it is kept• storage

– Who receives it– What it is used for


Data Flow Diagrams

• A major generic technique– Powerful and useful– Not just in computer based systems development

• Notation– There are a number of variations in symbols used

(esp. methods used in USA)• Yourdon• Gane and Sarson

– We use SSADM notation• All share exactly the same construction


Data Flow Diagram Symbols

A data flow

A process

A data store


Data Flow Diagrams

an external entity

a physical resource flow(rarely used, never in logical DFDs)



A data flow

A process

Location ID

Order Details

Labelling:

A data storeM1 Name



A processDescription(include a ‘doing’

verb)

an external entityNameb


Example

• Following example shows a Data Flow Model of an order processing system

• An initial document flow diagram can be used as a starting point– Helps define the boundaries of the system and

therefore the agencies which are external


Accounts

Customer

Accounts

Supplier

Sales

Despatch

Delivery note

Order acknowledgement

Order

Weekly credit limit list

Goods receivednote

Internal orderform


Re-order card

Goods receivednote

Copy deliverynote

Document Flow


Warehouse


Accounts

Customer

Accounts

Supplier

Sales

Despatch

Delivery note

Order acknowledgement

Order

Weekly credit limit list

Goods receivednote

Internal orderform


Re-order card

Goods receivednote

Copy deliverynote

Document Flow

Order Processing System Boundary

Warehouse


Context Diagram (level 0)


AccountsCustomer

Supplier

Order

Delivery note



Copy deliverynote

Re-order card

Goods receivednote

Goods received note



Level 1Data Flow Diagram


Accounts

Customer

Accounts

Supplier

*

Enter Orders

Sales1

*

Packagegoods

& despatch

2

Control stock

Warehouse3

Delivery note


Order


Goods receivednote

Internal orderform


Re-order card

Goods receivednote

Copy deliverynote

Despatch


Level 2 DFD:Control Stock

Control stock3

AccountsAccounts

*

Packagegoods

& despatch

Despatch2

Supplier

*

OrderEntry

Sales1

*

Allocatestock

to order

Stock Clerk3.1

*

Type dispatch instruction sheetTypist3.2

Monitorstocklevels

W'h men3.3

*

Check goodsreceipt note

to stock recei-

Goods Rec3.4

StockM3/1

Accounts

Re-order card

Goods receivednote


Goods receivednote

Internal orderform

read & updatestock

stock levelstock additions

Internal orderform

Re-order card


Level 3 DFD:Monitor Stock LevelsMonitor stock levels3.3

Accounts

StockM3/1

*

Monitor stocklevels andreport low

stock

Stock Clk3.3.1

*

Replacere-order

card

S/visor3.3.2

*

Extractappropriate

re-ordercard

S/visor3.3.3

Re-ordercardsM3.3/1

Re-order card

stock level

Stock receiptnote

Low stockdetails Re-order card

Re-order card

Re-order card


Components of a Data Flow Model

• Hierarchic structure– At highest level shows an overview of the

system– Level 1 is the most important– Gradually refined into further detail

• Level 2, 3 etc

– Until system processing is described in the utmost detail:

• Elementary process descriptions


Overallprocess

Agency

Agency

Level 0



Overallprocess

Agency

Agency

Level 0 Level 1

Process

Process

Process

1

2

3



Overallprocess

Agency

Agency

Level 0 Level 1 Level 2

Process

Process

Process

1

2

3

2

2.1

2.2

2.3

subProcess

subProcess

subProcess



Overallprocess

Agency

Agency

Level 0 Level 1 Level 2 Level 3

Process

Process

Process

1

2

3

2 2.2

2.1

2.2

2.3

2.2.1

2.2.2

2.2.3

subProcess

subProcess

subProcess

sub-subProcess

sub-subProcess

sub-subProcess


*


Overallprocess

Elementary Process

Description

Decision Tree

DecisionTable

StructuredEnglish

Agency

Agency

Level 0 Level 1 Level 2 Level 3 Level 4

Process

Process

Process

1

2

3

2 2.2 2.2.2

2.1

2.2

2.3

2.2.1

2.2.2

2.2.3

subProcess

subProcess

subProcess

sub-subProcess

sub-subProcess

sub-subProcess


*


Data Flow Diagrams

Basic Rules• Data flow lines must NOT cross each other• Data MUST pass through a process

– not between external entities– nor between data stores– nor external entity to data store (or vice-versa)

• Process descriptions MUST contain a verb and describe what is happening to DATA– They must be concise ie. not a list of sub-processes


Data Flow Diagrams

Basic Rules• Data stores used by only one process are

internal to that process and are not shown in that level diagram

• A guideline is max. of six processes in one diagram at a particular level– You will see examples of eight or more level 1

processes - this is not good practice• too much detail• difficult to read

– Key features - clarity, simplicity, completeness


Data Flow Diagrams

Basic Rules• DFDs are not Systems Flowcharts (an older

technique used to describe processing) and do NOT show:– the passage of time– or sequence of processing operations– Although it is common practice to label and draw

processes in approximate order for clarity and sense

• therefore order is sometimes implicit, but not explicit


Data Flow Diagrams: Summary

Powerful modelling tool used at:• Analysis phase

– Physical representation of current system– Transformed into Logical view of current system

• by removing physical circumstances

• Design phase– Logical view of required system

• by adding requirements.

– Physical representation of required system

• CASE tools are often used in their construction


Further Reading• Lejk and Deeks

– An Introduction to Systems Analysis Techniques– Chapter 4 Introduction to Data Flow Diagrams

• M Goodland and C Slater– SSADM version 4: a Practical Approach– McGraw Hill 1995

• Weaver, Lambrou and Walkley– Practical SSADM version 4+, 2nd Edition

• Chapter2

– Pitman Publishing 1998

Documents

Unit 7 University of Sunderland COMM1B Information Systems Analysis Process Modelling: Introduction to Data Flow Modelling Information System Analysis