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System Development Tools, Techniques and Methods

Structured Process Modelling

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What we will cover

Process Modelling in principle Data Flow Diagrams in principle Symbols and Semantics Process Modelling in action Data Flow diagrams in practice Other process modelling techniques

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Context Diagrams 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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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

Person, organisation or system outside the system being considered

Communicates only with system process via data flows

Student

Serve bottle of Bud

1 Bar stewardRequest for bottle of Bud

Student

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Data Flow

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

Serve bottle of Bud

1 Bar stewardRequest for bottle of Bud

Student

Request for bottle of Bud

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

1 Bar steward

Serve bottle of Bud

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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

Till M1

Serve bottle of Bud

1 Bar Steward

Till M1

Money offered

Change notification

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Material Flow

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

Student Serve bottle of Bud

1 Bar stewardBottle of Bud

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System (context only)

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

UCAS Admissions System

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Oversimple context diagram

BACS

Money and account details

Employee

Timesheet

Personal and account details

Money

Tax rules

NI rules

Other legal stuff

Statutory bodies

Payroll System

Payslip

Statutory payments and account details

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Another Context Diagram

Customer

Video HireSystem

Membership Application Request

Membership Details

Returned Video

Fee

Hire Video

Membership Application Response

Membership card

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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 before

Christmas) Provides the basis for ER Modelling Needed for SAD assignment 2 – see Britton and

Doake’s Structure Analysis book, Chapter 4

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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

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Things we haven’t covered

rules for building models “sensibleness” checks for DFDs

Britton and Doake Chapter 4 detail of textual descriptions

Tutorials, self guided study, Valachi, George and Hoffer and assignment

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Health warning

Process modelling only makes sense when you do it (it seems trivial otherwise)