Process Modelling

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• Process Modelling

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What we are to cover

• System modelling generally
• Structured Methods
• What is process modelling?
• Why use process modelling?
• Process Modelling Techniques

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System modelling
O u t p u t s
Source 1
Recipient 1
I n p u t s
And lo
Source 2
Recipient 2
a miracle happens!
Source 3
Recipient 3
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Structured modeling generally

• One of many approaches to system modeling
• based on premises that
• data will be stored in a relational database
• data will be made available from this database to
  code (programs) that will process it
• Therefore considers process and data
  independently
• BUT . . . .
• cross-relates them

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Structured techniques

• Process modelling
• Data modelling
• Time and Event modelling

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What is Process Modelling?

• Diagrammatic technique that
• does what it says on the tin!
• for any information system, models
• processes (at various levels of detail)
• participants
• data required/processed/stored/transmitted
• diagrams give framework for more detailed
  descriptions (more of this later)

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Process Modelling

• A series of tools and techniques to
• help you eat an elephant

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Process modelling

• Helps you eat an elephant
• . . . . . . . . . . . . . . . . . . . . .
• A little bit at a time
• By using a levelled approach

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Levelling

• A technique for those with very little brain

• Handles complexity through grouping of low level
  functions
• Handles complexity by decomposing high
  level functions
• Each level lower has successively more detail
  about higher level functions

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Levelling - graphically
Something complex
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Process modelling components

• System aims
• Context Diagram
• DFDs
• Data Dictionary
• Problem/Requirements Definition
• Beware non-functional requirements!
• can be used in many stages of methodology

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Aim of a system

• a (short) paragraph to define succinctly what a
  system does (and does not) do
• A retail system
• To support the correct administration and
  accounting of ordering, packaging and sales of
  goods through its shop and mail order operations

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Aim of a system - examples

• A payroll system
• To enable the correct calculation of gross pay
  and deductions for all weekly paid employees.
  Accounting and funds transfer will be automatic.
• A lift
• To transport a maximum of eight people between
  floors in a safe and timely manner

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Context Diagram and DFDs

• Symbols
• System (for context only)
• External entity
• Data Flow
• Process
• Data Store
• System boundary
• (Material Flow)
• Definitions in a moment

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Levelling - Graphically
System Aims
System Aim
Context
Level 1
Level 2
L e v e l 3
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When process models are used

• Current physical
• the how of now
• Current logical
• the what of now
• Proposed logical
• the what of the new
• Proposed physical
• the how of the new

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Current physical process model

• two volunteers please
• one student
• one bar steward

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External Entity
Student

• Person, organisation or system outside the system
  being considered

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Data Flow
Request for bottle of Bud

• The direction of flow of data from source to
  recipient
• Always has a description

• Key element in determining data needed by and in
  system

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Process

• where something happens to data (transformed,
  validated, stored )
• always has a number
• always has a description (strong verb, strong
  noun, maybe qualifier)
• always has a location/executor

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Data Store

• a location where data is stored
• may be manual or computer (M or D)
• always has a number and descriptor

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Material Flow

• A newish, non-purist representation of the
  movement of material

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System (context only)
UCAS Admissions System

• Represents the system as a whole
• may include non-computer functions
• shows no detail of internal processes or data
  stores
• used only for context diagrams

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Oversimple context diagram
Employee
Payslip
Statutory bodies
Payroll System
Money and account details
Statutory payments and account details
BACS
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Another Context Diagram
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Levelling

• Top Level - Context diagram (also known as Level
  0 DFD)
• Next level - Level 1 DFD
• Next level - Level 2 DFD
• and so on
• you are unlikely to need more than 3!
• Numbering of lower level processes links
  processes between levels

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Levelling - Graphically
Context
Level 1
Level 2
L e v e l 3
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Textual descriptions

• Every symbol represented on a process model
  should have a more detailed textual description
  linked to it
• Can be achieved in CASE tool
• Can be achieved with other, linked text (which is
  what you will do for your assignment)
• Provides explanatory detail

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Example of levelling

• Level 0
• Video Hire System
• Level 1
• Process 1 Administer Membership
• Level 2
• Process 1.1 Receive membership application

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Data dictionary

• A precise and unambiguous way of storing data
  about the data identified in data flows and data
  stores on DFDs
• Links DFDs and ERMs
• (you will understand more about this after Data
  Modelling lectures)
• Provides the basis for ER Modelling

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Problem/Requirements list

• A numbered list of all the problems with the
  current system and (usually hence) requirements
  of a new system
• Does not differentiate between whether issue is a
  problem or a requirement
• Serves as a check-list when conceiving and
  designing new system
• Helps to prevent problems and requirements
  becoming lost in the depths of analysis and
  design methods