1 14/08/00arcot sowmya software engineering comp3111/comp9008 data flow diagrams

1 14/08/00 Arcot Sowmya

Software EngineeringSoftware Engineering

COMP3111/COMP9008COMP3111/COMP9008

Data Flow DiagramsData Flow Diagrams


Tools of Structured Tools of Structured AnalysisAnalysis Much of SA consists of system modelingMuch of SA consists of system modeling modeling allows us to modeling allows us to

– focus on important featuresfocus on important features– change/correct user’s requirements at low change/correct user’s requirements at low

cost and riskcost and risk– verify that analyst’s understanding is correctverify that analyst’s understanding is correct

include: data flow diagrams, entity-include: data flow diagrams, entity-relationship diagram and state-transition relationship diagram and state-transition diagram.diagram.


Data Flow DiagramData Flow Diagram

What system functions, their What system functions, their interactions?interactions?

What transformations, what inputs What transformations, what inputs and outputs?and outputs?

What work, what information, what What work, what information, what results?results?

Data flow diagram models Data flow diagram models transformation of inputs to outputs.transformation of inputs to outputs.


A Data Flow DiagramA Data Flow Diagram

TapeTransaction

1

CustomerTransaction

Membernumber

Rentalcost

Prices

Library

CustomerGenerate

Report

3Reports

UpdatePricing

2

NewPrices


Components of DFD: Components of DFD: ProcessProcess Also called bubble, function, Also called bubble, function,

transformationtransformation shows part of system that shows part of system that

transforms inputs to outputstransforms inputs to outputs represented graphically as a circlerepresented graphically as a circle named with a single word, phrase named with a single word, phrase

or sentenceor sentence


Components of DFD: The Components of DFD: The FlowFlow Represented graphically by an Represented graphically by an

arrow into/out of aprocessarrow into/out of aprocess describes movement of describes movement of

information in the systeminformation in the system ““data in motion”data in motion” in non-computerized systems, in non-computerized systems,

could describe flow of materialscould describe flow of materials


The Flow- cntdThe Flow- cntd

must be named (with one must be named (with one exception)exception)

flow shows directionflow shows direction double-headed arrow stands for double-headed arrow stands for

dialogue- convenient packaging of dialogue- convenient packaging of two data packetstwo data packets

dalatflows can diverge or converge dalatflows can diverge or converge in a DFDin a DFD


Components of DFD: The Components of DFD: The StoreStore Used to model a collection of data Used to model a collection of data

packets at rest- store is passivepackets at rest- store is passive named, notation is two parallel linesnamed, notation is two parallel lines stored connected by flows to processesstored connected by flows to processes flow from a store is a read- store is not flow from a store is a read- store is not

changedchanged flow to a store is a write/update/delete- flow to a store is a write/update/delete-

store is changedstore is changed


Components of DFD: The Components of DFD: The TerminatorTerminator Represented graphically as a rectangleRepresented graphically as a rectangle represents external entities which represents external entities which

system communicates withsystem communicates with outside control of system being modeledoutside control of system being modeled flows between terminators and processes flows between terminators and processes

or stores is the system interfaceor stores is the system interface relationships between terminators will relationships between terminators will

not be modelednot be modeled


Guidelines for constructing Guidelines for constructing DFDDFD Choose meaningful namesChoose meaningful names

– try to identify functions of processestry to identify functions of processes– avoid vague names (`Do STUFF’)avoid vague names (`Do STUFF’)– Use words meaningful to userUse words meaningful to user

number the processesnumber the processes– be consistentbe consistent– easier to refer to aprocess by number!easier to refer to aprocess by number!– Useful in hierarchy of processesUseful in hierarchy of processes


Guidelines- cntdGuidelines- cntd

redraw for aestheticsredraw for aesthetics– size/shape of bubblessize/shape of bubbles– curved versus straight dataflowscurved versus straight dataflows– hand-drawn versus machine-generated hand-drawn versus machine-generated

diagramsdiagrams avoid overly complex DFDsavoid overly complex DFDs

– must be comprehensiblemust be comprehensible– not too many processes, flows, stores, not too many processes, flows, stores,

terminatorsterminators


Guidelines- cntdGuidelines- cntd

make sure the DFD is logically make sure the DFD is logically consistentconsistent– avoid infinite sinksavoid infinite sinks– avoid spontaneous generation bubblesavoid spontaneous generation bubbles– beware of unlabeled flows and beware of unlabeled flows and

processesprocesses– beware of read-only or write-only beware of read-only or write-only

storesstores


Hierarchical DFDsHierarchical DFDs

To handle large, complex systems To handle large, complex systems (DFDs),(DFDs),

organise them in a series of levelsorganise them in a series of levels each level provides more detail each level provides more detail

about a portion of the level above about a portion of the level above itit


DFD: A Hierarchical DFD: A Hierarchical SystemSystem

ContextDiagram

Level 0

Level 1

Level 2


Context Diagram- Level 0Context Diagram- Level 0

Single bubble represents whole systemSingle bubble represents whole system show show

– all people, organisations, systems with all people, organisations, systems with which our system communicates- as which our system communicates- as terminatorsterminators

– system inputs and outputssystem inputs and outputs– data storesdata stores– boundary between our system and boundary between our system and

outside world.outside world.


DFD: Context Level DFD: Context Level DiagramDiagram

VideoShop

System

Member Manager

New Memberdetails

Rentaldetails

Rentalcost

Membernumber

NewPrices

Reports


DFD: Level 0 DiagramDFD: Level 0 Diagram

TapeTransaction

1

CustomerTransaction

Membernumber

Rentalcost

Prices

Library

CustomerGenerate

Report

3Reports

UpdatePricing

2

NewPrices


DFD: Numbering DFD: Numbering ProcessesProcesses

ContextDiagram

Level 0

Level 1

Level 2


Numbering ProcessesNumbering Processes

Number all processes as 1, 2, 3, .. in Number all processes as 1, 2, 3, .. in Level 0Level 0

If parent process at Level 0 was If parent process at Level 0 was numbered 1, then its child processes at numbered 1, then its child processes at Level 1 will be 1.1, 1,2,….Level 1 will be 1.1, 1,2,….

Processes in context diagram are not Processes in context diagram are not numberednumbered

not all processes are decomposed to not all processes are decomposed to the same depththe same depth


Points to PonderPoints to Ponder

How many levels in a DFD?How many levels in a DFD? decompose to same level of detail?decompose to same level of detail? How to present to user?How to present to user? How to ensure consistency in levels?How to ensure consistency in levels? How to show stores at different levels?How to show stores at different levels? How to DO the levelling?How to DO the levelling?


DFDs: Common MistakesDFDs: Common Mistakes

Customer VSS

Name

Address

Phone

Order

Invoice

Should be rationalisedinto a single data flow: “Customer Detail”



CustomerDetails

Name

Address

ValidateCustomer

1.1 1.2

InvoiceGenerateInvoice

This is okay becauseit is still the same data flow



CustomerDetails

Name

Address

ValidateCustomer

1.1 1.2

InvoiceGenerateInvoice

This is NOT okay becausewe have split the data flowinto 2 new flows. Only processesare allowed to split data flows.



Items

Costs

You cannot mergedata flows. Only processesare allowed to merge dataflows.



PersonalInfo

Transactioninterface

Return-Key

PersonalDetails

Control Signalsshould be avoidedin logical DFDanalysis.



GetNextByte

CompilePacket

Ask for more

Byte

Packet

Avoid control loopsin logical DFDanalysis.


DFD: ExampleDFD: Example

A simple Video Shop System.A simple Video Shop System. General Requirements:General Requirements:

– Facility to add new members and Facility to add new members and issue member numbers.issue member numbers.

– Borrow and return tapes.Borrow and return tapes.– Change rental prices.Change rental prices.– Generate usage reports.Generate usage reports.



AnalysisAnalysis– From the requirements, you should have From the requirements, you should have

identified the external entities to be the identified the external entities to be the ManagerManager and the and the Customer.Customer.

– Further analysis might suggest what data Further analysis might suggest what data is fed into and produced by the system.is fed into and produced by the system.

The following DFD is an interpretation The following DFD is an interpretation of the given requirements.of the given requirements.



VideoShop

System

Customer Manager

CustomerTransaction

Rentalcost

Membernumber

NewPrices

Reports

Context Diagram



TapeTransaction

1

CustomerTransaction

Membernumber

Rentalcost

Prices

Library

CustomerGenerate

Report

3Reports

UpdatePricing

2

NewPrices

Level 0 Diagram



DetermineMembership

1.1

Customer

CuntomerTransaction

New Member

TapeTransaction

RentalCost

Membernumber

AddNew

Member

ProcessBorrow/Return

1.2

1.3 Customer

Level 1 DiagramProcess 1



Prices Customer

Library

TapeTransaction

TapeReturn

TapeBorrow

RentalCosts

FilterTransaction

Return

Borrow

1.3.1

1.3.2

1.3.3

Level 2 DiagramProcess 1.3

1 14/08/00arcot sowmya software engineering comp3111/comp9008 data flow diagrams

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