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Unit 7University of Sunderland COMM1B Information Systems Analysis
Process Modelling:Introduction to Data Flow
Modelling
Information System AnalysisCOMM1B
Unit 7University of Sunderland COMM1B Information Systems Analysis
• Is the existing system important?• How do we describe an existing
system?• Problems with narrative• Data Flow Diagrams
Outline
Unit 7University of Sunderland COMM1B Information Systems Analysis
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?
Unit 7University of Sunderland COMM1B Information Systems Analysis
• 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
Unit 7University of Sunderland COMM1B Information Systems Analysis
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?
Unit 7University of Sunderland COMM1B Information Systems Analysis
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
Unit 7University of Sunderland COMM1B Information Systems Analysis
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
Unit 7University of Sunderland COMM1B Information Systems Analysis
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
Unit 7University of Sunderland COMM1B Information Systems Analysis
Data Flow Diagram Symbols
A data flow
A process
A data store
Unit 7University of Sunderland COMM1B Information Systems Analysis
Data Flow Diagrams
an external entity
a physical resource flow(rarely used, never in logical DFDs)
Unit 7University of Sunderland COMM1B Information Systems Analysis
Data Flow Diagram Symbols
A data flow
A process
Location ID
Order Details
Labelling:
A data storeM1 Name
Unit 7University of Sunderland COMM1B Information Systems Analysis
Data Flow Diagram Symbols
A processDescription(include a ‘doing’
verb)
an external entityNameb
Unit 7University of Sunderland COMM1B Information Systems Analysis
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
Unit 7University of Sunderland COMM1B Information Systems Analysis
Accounts
Customer
Accounts
Supplier
Sales
Despatch
Delivery note
Order acknowledgement
Order
Weekly credit limit list
Goods receivednote
Internal orderform
Despatchinstruction set
Re-order card
Goods receivednote
Copy deliverynote
Document Flow
Order Processing System
Warehouse
Unit 7University of Sunderland COMM1B Information Systems Analysis
Accounts
Customer
Accounts
Supplier
Sales
Despatch
Delivery note
Order acknowledgement
Order
Weekly credit limit list
Goods receivednote
Internal orderform
Despatchinstruction set
Re-order card
Goods receivednote
Copy deliverynote
Document Flow
Order Processing System Boundary
Warehouse
Unit 7University of Sunderland COMM1B Information Systems Analysis
Context Diagram (level 0)
Order Processing System
AccountsCustomer
Supplier
Order
Delivery note
Orderacknowledgement
Weekly creditlimit list
Copy deliverynote
Re-order card
Goods receivednote
Goods received note
Order Processing System
Unit 7University of Sunderland COMM1B Information Systems Analysis
Level 1Data Flow Diagram
Order Processing 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
Despatch
Unit 7University of Sunderland COMM1B Information Systems Analysis
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
Despatchinstruction set
Goods receivednote
Internal orderform
read & updatestock
stock levelstock additions
Internal orderform
Re-order card
Unit 7University of Sunderland COMM1B Information Systems Analysis
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
Unit 7University of Sunderland COMM1B Information Systems Analysis
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
Unit 7University of Sunderland COMM1B Information Systems Analysis
Overallprocess
Agency
Agency
Level 0
Components of a Data Flow Model
Unit 7University of Sunderland COMM1B Information Systems Analysis
Overallprocess
Agency
Agency
Level 0 Level 1
Process
Process
Process
1
2
3
Components of a Data Flow Model
Unit 7University of Sunderland COMM1B Information Systems Analysis
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
Components of a Data Flow Model
Unit 7University of Sunderland COMM1B Information Systems Analysis
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
Components of a Data Flow Model
*
Unit 7University of Sunderland COMM1B Information Systems Analysis
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
Components of a Data Flow Model
*
Unit 7University of Sunderland COMM1B Information Systems Analysis
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
Unit 7University of Sunderland COMM1B Information Systems Analysis
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
Unit 7University of Sunderland COMM1B Information Systems Analysis
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
Unit 7University of Sunderland COMM1B Information Systems Analysis
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
Unit 7University of Sunderland COMM1B Information Systems Analysis
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