COM 117 Introduction to Application Building

• Introduction to the module

• Learning outcomes

• Assessment

• Lecture on DFDs

• Practical activity

• Tutorial activity

Assessment Strategy

• Assessment Strategy• TCT on design techniques

(multiple choice and short answer) 20%

• TCT lab test on further Access Database Skills20%

• Group project 60%••

Group project 60%• Good interface (Input/output design)

10%

• Modelled requirements’ documentation

20%

• Level of complexity and accuracy in application

30%

• Ease of use of system

10%

• Choice of sample data

10%

• Reports

10%

• Testing documentation

10%

Timetable

• http://osiris.sunderland.ac.uk/~cs0lhu/Lynne/com117.htm

Data Flow Diagrams

IntroductionSSADMWhat is a Data Flow Diagram?

Why do we use DFDs?Levelling ConventionsDecomposition and AbstractionThe Elements

Process and Data StoresExternal EntityData FlowThe LevelsRules

Constructing DFDs

The Document Flow DiagramThe Context Diagram

Draw the external entities and data stores

Level 1 Physical DFDExample: Hairdressing Salon Level

1 Physical DFDSummary

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?

• Known as DFDs

• A way to model a real world situation

• They model the real world situation which aids discussion and clarity amongst users, analysts and designers.

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

Levelling• Levels determine the amount of

information shown• Context diagrams show environment• Each level shows more info. than the last • DFDs are expanded or decomposed into

levels.• Separating each process into sub

processes• Uncovers more and more detail

ConventionsBalancing

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

Decomposition and Abstraction

• Decomposition - Divide and subdivide into manageable size problems

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

The ElementsThe 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)

External entities, outside sources of data

Process and Data Stores

A process is made up of

Data Stores

Process Number

Destination (Place or Name)

Process description Should be descriptive, starting with a verb.

M1

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

Name of Store

External Entity

Is anything outside the system that is of interest to the system. Can be a person, a company or another system.Outside entity shows the Name and a lowercase alpha character is used to uniquely identify it.

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

Customer a

Customer a

Data Flow

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.

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.

RulesSequence 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 stopRule of ThumbIf there are more than 8 data flows

break itProcess of Identifying major

Processes

The Procedure forConstructing DFD’s

Draw a document flow diagramof the current situation

Draw a systems boundaryaround the agencies that arepart of the system

Draw a Context Diagram

Identify processes in the system

Complete the level 1 CurrentPhysical DFD

The Document Flow Diagram

The task of modelling a business situation can be daunting at first. Itis best to start with something simple such as a document flowdiagram.

Production Planning

Stock Control

FactoryDesign

Purchasing

Supplier

Stock

NoteWithdrawal

Production Plan Purchase

Order

Delivery Note

Material Requirements List

Bill ofMaterials

Supplier Details Update Form

DeliveryNote

The Context Diagram

You decide which agencies are to be part of the system thatyou are examining.

These agencies fall inside the system boundary and arereduced to one box in the centre.

This is a Context Diagram

Production Planning

Stock Control

Factory

Design Purchasing

Supplier

Stock

NoteWithdrawal

Production Plan

Delivery Note

Material Requirements List

Bill of Materials

Supplier Details Update Form

DeliveryNote

MaintainStock System

a b

c

d

e

(Lejk & Deeks)

All data flows going into the system must be received by a process. All data flows going out of the system must be generated by process. The first task is therefore to identify these processes:

Stock clerk

Maintain

1

2

3

Stock clerk

Stock clerk

planned call-off

Maintainstock cards

Preparematerial reqmnts

list

Draw the external entities and data stores.

Production

Stock clerk

Maintain

a Bill of materialsM11

2

3

Stock clerk

Stock cardsM2

Stock clerk

planned call-off

Maintainstock cards

Preparematerial reqmnts

list

Planning

Supplier

b

Factory

c

Purchasing

d

Level 1 Physical DFD - Complete

Finally draw in the data flows to give acompleted diagram. Note that a data flow musthave a process at the end .

Production

Stock clerk

Maintain

a Bill of materialsM11

2

3

Stock clerk

Stock cardsM2

Delivery note

B O M details

Planned call-off

details

Stock details

Stock clerk

planned call-off

Maintainstock cards

Preparematerial reqmnts

list

Planning

Production

Plan

Supplier

b

Factory

c

Stock withdrawal note

Purchasing

d

Material requirementslist

Updated supply details

Deliverynote

Stock details

(Lejk & Deeks)

Hairdressing Salon Level 1Physical DFD

Client

Receptionist

Register

a

Client card indexM1

1

Confirm

2

Hairdresser/Rcptnst

Conduct

3

arrival

Receptionist

Appointment

Appointment diaryM2

appointment

Appointmentdetails

Change ofhairstyle etc.

Appointmentdetails Confirmation

Request

ConfirmationDetails

New clientdetails

Existing clientdetails

Appointmentdetails

Confirmation ofarrival

Change ofhairstyle etc.

Appointmentdetails

(Lejk & Deeks)

(Lejk & Deeks)

Process 3 Level 2

3 Hair/Reception

Cliea

Hair Details

Clienta

3.1 Hairdresser

Conduct Appointment

3.2 Hairdresser

Inform Reception

3.3 Receptionist

Complete Appointment

M2 Diary

Appointment Details

M3 Client Card

Change of Hair Details

Naming of DFD processesLevel 0 Level 1 Level 2 Level 3 Level 4

Client

OverallProcess

Client

Process

Process

Process

1

2

3

2

2.1

2.2

2.3

SubProcess

SubProcess

SubProcess

SubProcessSubProcess

Process

2.2

Sub - SubProcess

2.1.1

2.1.2

Sub - SubProcess

Elementaryprocessdescriptions.

Decision treesDecision tableStructuredEnglish

2.1.1

There must be consistency between levels, with allthe data appearing on the higher level DFD. If a datastore is used only for one process it is placed withthat process. Outside entities are always shownoutside the boundary of a lower level DFD process,even if they only communicate with that one process.

What happens if part of the system is on computer?

• Electronic datastores • The above example was based on a manual system. If

the datastores are computer records rather than manual files the convention is to label the datastore D1, D2 etc.

D1

Summary

SSADMWhat a DFD is & Why we use themThe conventionsWhat the elements are