Chapter 7 Process Modeling and Data Flow Diagrams...

Chapter 7Process Modeling and Data Flow Diagrams (DFD)

Copyright 2005, Merrill Warkentin page 1

Process Modeling and Dataflow Diagrams (DFDs)

Merrill WarkentinMississippi State University

© Merrill Warkentin, 20052

Major Topics

process modeling

data flow diagram symbols

data flow diagram levels

creating data flow diagrams

physical and logical DFDs

partitioning

(event driven modeling)

(use case and data flow diagrams)


Process Modeling

Graphically representing the functions, or processes, which capture, manipulate, store, and distribute data between a system and its environment and between components of a systemData Flow Diagram (DFD)


Data Flow Diagrams (DFD)graphical representation of data processesand flows throughout the organization

emphasizes logic of underlying system

based on general systems model

Input Process Output


Why Use DFD?

logical information flow of the system

determination of physical system construction requirements

simplicity of notation

establishment of manual andautomated systems requirements


Advantages of DFD

early understandingof system technical implications

establishment of inter-relatednessbetween business and systems

user involvementwith system development

determine if data and processes have been defined properly




DFD - Four Symbols


DFD - Four Symbols

Process - work or actions performed on data so that they are transformed, stored, or distributed

Date Store - data at rest, notebook, file folder, database

Source/Sink - origin and/or destination of the data (external entities)

Data flow - data in motion


DFD Formatting Symbols

Entity

D3 DataStore

2.1

Process

Data Flow

external to boundaries of systemsources and destinations of data

data movementelectronic exchangepaper documents(sometimes shown as)

transformation of datafrom input to output

data depository forelectronic filespaper files(sometimes shown as)


External Entities

supply data to the system

receive data/information from the system

but not considered part of the system

Entity

Purchase order

2.1

ProcessEnterorders

Entity


Data Flowsrepresent data being moved to or from

an external entitya processa data store

shown as a line with arrowheadslabeled with description ofthe type of data, document, or report

Order data2.1

ProcessEnterorders D3 Data

StoreOpen

order file


Processesset of tasks performed on datato convert inputs to outputs

labeled as a verb/noune.g., ENTER ORDERS

data flow out always labeled differently than data flow in

2.1

ProcessEnterorders

Orderdata D3 Data

StoreOpen

order filePurchase

orderEntity

2.1

Process




Processes - 4 Types

create new data change quality but not content of input

e.g., verify customer numberoutput = input = (verified) customer number

reorganize inputsortreformatfilter/select

convey input without any change


Data Storesdata at restcorresponds to data files or tablescan be electronic or paper filesshown as a rectangle (electronic)arrow into file means data writtenarrow out means data read

D3 DataStore

Orders

2.1

ProcessEnterorders

Orderdata D3 Data

StoreOpen

order file

D1 DataStore

Customer data

D2 DataStorePOFile

1.0


Balancing DFDs

Inputs and outputs must be conserved between levels of DFDsLevel n & n+1 must have the same inputs and outputs


Five guidelines for drawing DFDs

Completeness - all necessary components are included and describedConsistency - information on one level is included on other levelsTiming - a DFD does NOT indicate when the system is flowingIterative development - it may take 3 or more DFDs to get it rightPrimitive DFDs (the lowest level DFD) - stop decomposing when the lowest logical level is reached


Problems with DFDs

Identify problems on the DFD(see handout)


Describe problems (handout)




DFD Development/Levels

context diagramgeneral highest level diagramone process representing entire system

diagram 0exploded view of the entire systemup to nine processesall external entities and data stores

child diagramsexploded view of each individual processoutlined in Diagram 0


DFD Organization

Process

Input B

ExternalEntity

1

1.0

Name ofSystem

Input A

ExternalEntity

2

ExternalEntity

3Output C

Input B

ExternalEntity

1

D1 Data Store1

1.0

ProcessA

Input A

ExternalEntity

2

ExternalEntity

3

2.0

ProcessB

Record A

Record A

D2 Data Store2

4.0

ProcessD

Output C

3.0

ProcessC

Record E

Record E

Data Flow B

Data Flow D

Data Flow C

3.1

DetailedProcess

C-A

3.2

DetailedProcess

C-BD5 RecordStore 1Record 1 Record 1

Record E

Data Flow D


Creating the Context Diagramoriginal should be an overview

basic inputsgeneral systemoutputs

highest level

contains only one process


Context Diagram Example


Drawing Diagram 0explode context diagram

more detailclose-ups of processesdata storeslower-leveldata flows

may includeup to 9 processes


Drawing Diagram 0

start with data flowfrom an external entitywork backwardsfrom an output data flowexamine data flowanalyze well defined processtake note of problem areas




Diagram 0 Example


Creating Child Diagrams

more detailed levelseach process exploded further

parent processexploded diagram 0 process

child diagramresulting diagramentities usually not shown at this levelmay have error lines here only


Child Diagram Examples






Rules & Guidelines for DFD

A data store must always be connected to a process.

D4 DataStore

This:

Not this:

Nor this: Entity

2.1

ProcessD3 Data

Store

Data Flow

Data Flow

Data Flow

D3 DataStore

D4 DataStore




External entity must always be connected to a process.

OK -

not OK -

Data may flow out of one process and into another.

OK - Data Flow

2.2

Process

Rules & Guidelines for DFD

Entity

Entity

Data Flow

Data FlowD4 Data

Store

2.1

Process

2.1

Process


Naming Rules for Data Flows

data flows must be nameduse a NOUN(what data are being used by a process?)

output flow from a processmust have a different name than the input flowoutput cannot be the same as the input

data traveling togethershould be shownand named as one data flowexample: orders and payments flowing togethershould be labeled as “orders and payments”


Data Stores’ Rules & Guidelines

if no input data flowing to them,may not be properly represented

where did the datain the data store come from?

should be uniquely numberedon the flow diagramdescription of the data attributes should be contained in a similarly numbered tablestructure / data attribute dictionary

you can create the dictionary by printing out the ACCESS data structures for each table


Rules & Guidelines for Processes

process performs one simple, well-defined task, activity or functionmust have at least one input andone output data flow

if no input, process is creating output from “thin air”(where is the data to be processedinto output coming from?if no output, process is “black hole” or “data sink”(why is data flowing to a processthat does not produce any output?)

data should be sent only to processesthat use them


should be uniquely named and numbered

description of the process should bein a similarly numbered and indexed“process dictionary”

data store to can be the only receiverof output from a process

data store to can be the only sourceof input to a process

Rules & Guidelines for Processes


Common Errorsnot including data flow placing arrowhead in the wrong directiondirectly connecting data stores & external entitieslabeling incorrectlyincluding more than 9 processesomitting data flowhaving unbalanced composition




Common DFD Errors

2.1

ProcessD3 Data StoreD3 Data Store

2.1

Process

Entity

D3 Data Store

Entity

Entity

2.1

Process

Dual/SplitData Flows

Entity to Entity/Data Store Communication No Data Output

No Data Input Data to Data Communication


B1 B2 B1 B1

B1 B1

B1 B1

DS1 DS2 DS1

a process is needed to

exchange data flows between

boundaries

a process is needed to update (or use) a data

store

a process is needed to

present data from a data

store

a process is needed to move data

from one data store to another

DS2

DS1

DS1 DS1

DS1

Illegal Data Flowsillegal data flows corrected data flows


Practice Question - THINKWhich is an error condition?


DFD for Hoosier Burger (food ordering system)

Context diagram - highest view of the system



Level-0 DFD -

systems major processes, data flows, and data stores

Level-n DFD -

n nested decompositions of sub-processes from the level-0 diagram



Level-1 - decomposes process 1.0 from level-0





Level-1 - decomposes process 4.0 from level-0



Level-2 Diagram Showing the Decomposition of Process 4.3 from the Level-1 Diagram for Process 4.0


Logical DFD

focus on how the business operatesprocesses that exist regardless of the typeof system implemented


Physical DFD

details physical system requirementscontains construction specifications(software, hardware, people)


Logical to Physical DFDcreate a logical DFD of the current systemadd all data and processes not in the current system which must be present in the new systemderive the physical data flow diagram forthe new systemlogical DFD of the current system will leadto appropriate logical DFD of the new systemphysical DFD will follow naturally ifthe logical DFD is accurate


Logical to Physical DFD




Logical DFD - Advantages

better communication

more stable systems

better understanding

flexibility and maintenance

elimination of redundancies


Physical DFD - Advantages

clarifying manual & automated processes

detailed process description

sequencing processes

identifying temporary data stores

specifying name of files and printouts

adding controls


Partitioning Data Flow Diagrams

identify separate user groupsprocess timingsimilar taskefficiencydata consistencysecurity

The process of separating activities into groupsbased on automation, manual, and user needs


Reasons for Partitioning

different user groups

timing

similar tasks

efficiency

consistency

security


Partitioning the DFD

Look . . .© Merrill Warkentin, 200554

Indicate with dotted line




Separate batch programs


Practice Question - THINK

Which of the following is NOT a reasonfor partitioning processes into separateprograms?


Summary

standard communication tool for design team

promotes better understanding of system

ensures that all components are included

leads to effective logical design

Copyright NoticeThis document may not, in whole or part,

be copied, photocopied, reproduced, translated, transmitted,or reduced to any electronic medium or machine readable form

without explicit permission and written authorization from Dr. Merrill Warkentin.

[email protected]://www.MISProfessor.com

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