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Data Flow Diagrams. Introduction. SSADM What is a Data Flow Diagram? Why do we use DFDs? Levelling Conventions Decomposition and Abstraction The Elements Process and Data Stores Outside Entity Data Flow The Levels Rules The Procedure for Constructing DFD’s The Document Flow Diagram

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Introduction l.jpg
Introduction

SSADM

What is a Data Flow Diagram?

Why do we use DFDs?

Levelling

Conventions

Decomposition and Abstraction

The Elements

Process and Data Stores

Outside Entity

DataFlow

The Levels

Rules

The Procedure for Constructing DFD’s

The Document Flow Diagram

The Context Diagram

Draw the external entities and data stores

Level 1 Physical DFD - Complete


S s a d m l.jpg
S.S.A.D.M.

  • S.S.A.D.M. - Structured Systems Analysis and Design Method

  • Uses different techniques to model a system

    • Data Flow Diagrams

    • Entity Relational Model (Logical Data Stores)

    • Normalisation


What is a data flow diagram l.jpg
What is a Data Flow Diagram?

  • Known as DFDs

  • A way to model a real world situation

  • They are the interface between the real world activities and an understanding of how this can be converted into a computer system.


Why do we use dfds l.jpg
Why do we use DFDs?

  • It is a way of taking the physical view and converting it into a logical view.

  • The physical view - all documents involved

  • The logical view - the data they contain

  • Their main purpose is to communicate with the user, the analyst’s understanding of the scope of the required system


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Levelling

  • DFDs are expanded or decomposed into levels.

  • Separating each process into sub processes

  • Uncovers more and more detail


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Conventions

Balancing

Process at lower level should have identical data flows if they flow out of a process

Modelling Data Stores

Only use DATA STORES used within this process on the diagram

Numbering

1 - 1.1 - 1.1.1

1.2 - 1.2.1

Labels

Should carry as much meaning as possible


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Decomposition and Abstraction

  • Decomposition - Divide and subdivide into manageable size problems

  • Abstraction - Concentrate on the important issues and ignore the irrelevant


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The Elements

The four main elements of DFDs notation

  • Data Flows, with a label to indicate what data is flowing

  • Processes, that handle the data

  • Data stores, within the system (diary, filing cabinet or computer file)

  • Outside entities, outside sources of data


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Process and Data Stores

A process is made up of

Data Stores

Destination (Place or Name)

Process Number

Process description Should be descriptive, starting with a verb.

Can be M for manual or D for computer base data stores.

Name of Store

M1


Outside entity l.jpg
Outside Entity

Is anything outside the system that is of interest to the system. Can be a person, a company or another system.

Customer

a

Outside entity shows the Name and a lowercase alpha character is used to uniquely identify it.

Customer

a

If an outside entity is repeated for the purpose of neat layout a line is added across the top.


Data flow l.jpg
DataFlow

Is shown by a line with an arrowhead, indicating the direction of the flow of data. Each data flow should be named to indicate what data is being passed. Nouns or adjectives only no verbs are permitted.


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The Levels

  • Context - Overview - contains only one process

  • Level 1 - Utilises all four elements

  • Level 2 - A breakdown of a level 1 process

  • Level 3 - A breakdown of a level 2 process

  • There is no rule as to how many levels of DFD that can be used.


Rules l.jpg
Rules

Sequence not important - getting the Process correct is

  • Context or Level 0 - Identifies the system/ boundary/External Links

  • Level 1 - Overview of function

  • Level 2 - Breakdown to Understand

    Hard to know where to stop

    Rule of Thumb

    If there are more than 8 data flows break it

    Process of Identifying major Processes


Process 3 level 2 l.jpg
Process 3 Level 2

3

Hair/Reception

Client

a

Clie

a

3.1

Hairdresser

Appointment Details

Conduct Appointment

Hair Details

M2

Diary

3.2

Hairdresser

Inform Reception

Change of Hair Details

3.3

Receptionist

Complete Appointment

M3

Client Card



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Summary

SSADM

What a DFD is & Why we use them

The different conventions

What the elements are

Example

Next Week:- Entity Relational Model


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