data flow diagrams levelling them; process modelling using function decomposition cse1204 -...

Data Flow Diagrams Data Flow Diagrams

Levelling Them;Levelling Them;

Process Modelling Using Function Process Modelling Using Function

DecompositionDecomposition

CSE1204 - Information Systems 1CSE1204 - Information Systems 1

1.3 4.2

1.1.1 1.1.2 3.2.1 3.2.2

Context

Level 0

Level 1

Level 2

1 2 3 4

1.1 1.2 2.1 2.2 3.1 3.2 4.1 4.3

Levelling of DFDsLevelling of DFDs

Guidelines for Levelling DFDs

External communication:External communication: external agents represent entities in the external agents represent entities in the

environment of our information system environment of our information system external agents are outside the scope of our external agents are outside the scope of our

information systeminformation system we do NOT model interactions between we do NOT model interactions between

external agentsexternal agents we do NOT allow external agents to interact we do NOT allow external agents to interact

directly with data storesdirectly with data stores


numbering: numbering:

when a process is decomposed, its diagram is given the when a process is decomposed, its diagram is given the same number as that processsame number as that process

balancing of levelled DFDs: balancing of levelled DFDs:

all data flows entering and leaving a process must appear all data flows entering and leaving a process must appear on the corresponding diagram which decomposes that on the corresponding diagram which decomposes that processprocess

external agents: external agents:

are only included on the two diagrams which represent the are only included on the two diagrams which represent the entire system, i.e. the context and level zero diagramsentire system, i.e. the context and level zero diagrams

the access to data stores across levels of diagrams must be consistent:

• the direction of accesses must match and all accesses on higher level diagrams must appear on corresponding lower level diagrams

• a data store is first shown on the highest level diagram where it is accessed by more than one process

• it can then appear on all lower level diagrams where it is accessed


the access to data stores across levels of diagrams must be consistent:

2

1

1.1 1.21.3


How many levels should be in a set of DFDs?How many levels should be in a set of DFDs?

each diagram usually has between 3 and 7 processeseach diagram usually has between 3 and 7 processes

level the diagrams until bottom level or level the diagrams until bottom level or primitiveprimitive processes processes are reached:are reached:

primitive processesprimitive processes have only 1 or 2 inputs and outputs, and have only 1 or 2 inputs and outputs, and cannot be further decomposed as a data flow diagramcannot be further decomposed as a data flow diagram


partition processes to minimise the data flows partition processes to minimise the data flows between thembetween them

partition processes to form cohesive, related partition processes to form cohesive, related groups of activitiesgroups of activities

not all parts of the system may need to be not all parts of the system may need to be decomposed to the same leveldecomposed to the same level


Payroll System

Line

Manager

EmploymentSystem

SuccessfulApplicant

Position Spec

Decision

Acknow-ledgement

An example - Context DiagramAn example - Context Diagram

Position

Address

Applicant name

Position

Applicant

An example - Level zero diagramAn example - Level zero diagram

Position Spec

Unsuccessfuls fileEvaluation

results

AcceptApplic-ation

Decision

SuccessfulApplicant

Acknow-ledgement

Ackn’dAppn

Line Manager

Applicant

AddressPosition

Applicant name

Applicant

Payroll System

Position

1

2

EvaluateApplicants

Level 1- Diagram 1Level 1- Diagram 1

Applicant name

Verify for Complete-

ness Acknow-ledge

Application

Acknow-ledgement

AcknowledgedApplication

VerifiedApplicationPosition

Address

1.2

1.1

Position Spec

Unsuccessfuls file Evaluation results

SuccessfulApplicant

Ackn’dAppn

unqualified applicant

Decision

Qualifiedapplicant

Decision

Level 1 - Diagram 2

2.1

2.2 2.3

ScreenApplicants

RejectUnsuitables

ScheduleEvaluation

Logical and physical DFDsLogical and physical DFDs

Models may focus on either: Models may focus on either:

the “physical” view of the real world –the “physical” view of the real world –

how things are donehow things are done

OROR the “logical” view of the real world –the “logical” view of the real world –

what things are donewhat things are done

Physical DFDsPhysical DFDs

represent a particular way of implementing represent a particular way of implementing the processes and data in a system the processes and data in a system

they are technology dependent – they they are technology dependent – they specify particular methods of doing tasksspecify particular methods of doing tasks

they show they show howhow the processing takes place the processing takes place and and howhow the data is implemented the data is implemented

Logical DFDsLogical DFDs

represent what a system must do represent what a system must do regardless of how it is implementedregardless of how it is implemented

they are technology independentthey are technology independent they show they show whatwhat processing, data processing, data

movements and data storage movements and data storage mustmust occur in occur in a systema system

they show the they show the essentialessential aspects of a aspects of a systemsystem

Using Logical and Physical DFDs

•Physical DFDs modelling current system: help systems analysts become familiar with how a business or system operates

•Physical DFDs modelling new systems: model the technical and human design decisions to be implemented

•users can relate to physical DFDs more readily because they contain implementation details:

landmarks e.g. people or roles, actual locations

Use of Logical and Physical DFDs

Systems analysts often begin with physical DFDs of current systems:• convert that physical DFD to a logical model in order to focus on essential elements• use the logical model to model a new logical solution (DFD)•Convert the logical solution into a physical DFD (implementation) model

•implementation details can be removed from physical DFDs

• use names for data flows and data stores which indicate their content, not their physical form or location

• use names for processes that indicate what, not how

Physical to Logical DFDs

2.1

Billchecksform

2.1

Validatesalesorder

AZ104 formchecked AZ104 form

sales order

valid sales order

Master File

Sales orders

Physical to Logical DFDs

Logical and Physical DFDsLogical and Physical DFDs

Physical DFDsPhysical DFDs Logical DFDsLogical DFDs

ViewView How processing is implemented How processing is implemented What the system doesWhat the system does

ProcessesProcesses Actual sequence Actual sequence Essential sequence Essential sequence

NamingNaming Forms, locations, people/roles Underlying data and Forms, locations, people/roles Underlying data andactivitiesactivities

Data flowsData flows Detailed/ specific/ duplicated Only essential inputs andDetailed/ specific/ duplicated Only essential inputs and data describing exact outputs of the processes data describing exact outputs of the processes

implementation implementation needsneeds

Function Decomposition: a Function Decomposition: a Process Modelling Process Modelling

TechniqueTechnique function decomposition is the decomposing of a system function decomposition is the decomposing of a system

into its component functions and processes as a way of into its component functions and processes as a way of managing complexitymanaging complexity

function decomposition creates a top-down view: it shows function decomposition creates a top-down view: it shows a hierarchy of levels of increasingly detailed processes a hierarchy of levels of increasingly detailed processes within a systemwithin a system

a function decomposition model (or diagram) is used to a function decomposition model (or diagram) is used to represent the hierarchical decomposition and structure of represent the hierarchical decomposition and structure of the processes of a systemthe processes of a system

the decomposition of functions corresponds to levelling of the decomposition of functions corresponds to levelling of processes in DFDsprocesses in DFDs

Function Decomposition Function Decomposition DiagramsDiagrams

a a functionfunction is a high-level set of related activities that are is a high-level set of related activities that are ongoing: a function is a broad, generic activityongoing: a function is a broad, generic activity

a a processprocess is a lower level activity that is repeatedly is a lower level activity that is repeatedly carried outcarried out

functions consist of groups of related processesfunctions consist of groups of related processes the depth and scope of function decomposition diagrams the depth and scope of function decomposition diagrams

depend on the size and complexity of the system depend on the size and complexity of the system representedrepresented


function decomposition diagrams showing only higher function decomposition diagrams showing only higher levels can be built early in systems analysislevels can be built early in systems analysis

more detailed process decomposition can be carried out more detailed process decomposition can be carried out as the system is studied in more detailas the system is studied in more detail

function decomposition diagrams show the structure of function decomposition diagrams show the structure of functions and processes within the systemfunctions and processes within the system

this structure may not correspond to the structure of the this structure may not correspond to the structure of the organisation shown in an organisation chartorganisation shown in an organisation chart

See Whitten et al (2001) p 5, p 338 for examplesSee Whitten et al (2001) p 5, p 338 for examples


each function and process should have a unique name each function and process should have a unique name which indicates which indicates whatwhat it does it does

ABC Sales System

1. Sell Products

2. Manage Inventory

3. Control Finance

a top level function decomposition diagram

Example Function Decomposition Example Function Decomposition DiagramDiagram

ABC Sales System

1. Sell Products

2. Manage Inventory

3. Control Finance

functions and processes are further decomposed

2.1 Deliver Product

2.2 Accept Delivery

2.3 CheckStock levels

Function DecompositionFunction Decomposition

1.0 2.0 3.0

1.21.1

1.2.1 1.2.2

the decomposition of the decomposition of functions and processes functions and processes

corresponds to the corresponds to the levelling of DFDslevelling of DFDs

function decomposition diagrams are an alternative function decomposition diagrams are an alternative representation of the hierarchy of functions and representation of the hierarchy of functions and processes within a systemprocesses within a system

they may be built using either a top down or a bottom they may be built using either a top down or a bottom up approachup approach

they provide a useful overview of the processing within a they provide a useful overview of the processing within a systemsystem

Function DecompositionFunction Decomposition

ReferencesReferences

WHITTEN, J.L., BENTLEY, L.D. and DITTMAN, K.C. (2001) 5th WHITTEN, J.L., BENTLEY, L.D. and DITTMAN, K.C. (2001) 5th ed., ed., Systems Analysis and Design MethodsSystems Analysis and Design Methods, Irwin/McGraw-HilI, , Irwin/McGraw-HilI, New York, NY. Chapters 8New York, NY. Chapters 8

HOFFER, J.A., GEORGE, J.F. and VALACICH (2005) 4th ed., HOFFER, J.A., GEORGE, J.F. and VALACICH (2005) 4th ed., Modern Systems Analysis and DesignModern Systems Analysis and Design, Benjamin/Cummings, , Benjamin/Cummings, Massachusetts.Massachusetts.

Chapter 7 Chapter 7

data flow diagrams levelling them; process modelling using function decomposition cse1204 -...

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