Process Modelling: Introduction to Data Flow Modelling

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Process ModellingIntroduction to Data Flow
Modelling

• Information System AnalysisCOMM1B

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Outline

• Is the existing system important?
• How do we describe an existing system?
• Problems with narrative
• Data Flow Diagrams

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How important is the existing system?

• 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

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Current system model

• 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

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How do we describe an existing system?

• 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

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Example Data Flow Diagram Level 1
Customer
2
Despatch
Delivery note
Package
Order
Order
goods
acknowledgement
despatch

Sales
1
Enter Orders
Copy delivery
Despatch

note
instruction set
Weekly credit
limit list
Internal order
form
Accounts
Goods received
note
Warehouse
3
Accounts
Goods received
Control stock
note
Re-order card
Supplier
Order Processing System
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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

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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

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Data Flow Diagram Symbols
A data flow
A process
A data store
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Data Flow Diagrams
an external entity
a physical resource flow (rarely used, never in
logical DFDs)
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Data Flow Diagram Symbols
Labelling
A data flow
Order Details
Location
ID
A process
Name
A data store
M1
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Data Flow Diagram Symbols
Description (include a doing verb)
A process
b
an external entity
Name
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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

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Document Flow
Customer
Delivery note
Despatch
Order
Order acknowledgement
Sales
Copy delivery
Weekly credit limit list
Despatch
note
instruction set
Internal order
form
Goods received
Accounts
note
Warehouse
Accounts
Goods received
note
Re-order card
Supplier
Order Processing System
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Document Flow
Customer
Delivery note
Despatch
Order
Order acknowledgement
Sales
Copy delivery
Weekly credit limit list
Despatch
note
instruction set
Internal order
form
Goods received
Accounts
note
Warehouse
Accounts
Goods received
note
Re-order card
Supplier
Order Processing System Boundary
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Context Diagram (level 0)
Accounts
Customer
Goods received
note
Copy delivery
note
Delivery note
Re-order card
Weekly credit
limit list
Order
Order
acknowledgement
Order Processing System
Goods received note
Order Processing System
Supplier
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Level 1Data Flow Diagram
Order Processing System
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Level 2 DFDControl Stock
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Level 3 DFDMonitor Stock Levels
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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

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Components of a Data Flow Model
Level 0
Agency
Overall process
Agency
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Components of a Data Flow Model
Level 0 Level 1
Agency
1
Process
2
Overall process
Process
3
Agency
Process
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Components of a Data Flow Model
Level 0 Level 1 Level 2

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Agency
1
2.1
sub Process
Process
2
2.2
Overall process
sub Process
Process
3
2.3
sub Process
Agency
Process
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Components of a Data Flow Model
Level 0 Level 1 Level 2
Level 3
2
2.2
Agency
1
2.1
2.2.1
sub Process
sub-sub Process
Process
2
2.2
2.2.2
Overall process
sub Process
sub-sub Process
Process

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2.3
2.2.3
sub Process
sub-sub Process
Agency
Process
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Components of a Data Flow Model
Level 0 Level 1 Level 2
Level 3 Level 4
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2.2
2.2.2
Elementary Process Description Decision
Tree Decision Table Structured English
Agency
1
2.1
2.2.1
sub Process
sub-sub Process
Process
2
2.2
2.2.2
Overall process
sub Process
sub-sub Process
Process

3
2.3
2.2.3
sub Process
sub-sub Process
Agency
Process
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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

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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

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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

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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

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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