Process Modelling Using Data Flow Diagrams -

Building and Levelling Them;Process Modelling Using Function Decomposition

CSE1204 - Information Systems 1

Example system: a generic warehouse-inventory information system

Levelling Data Flow Diagrams any "real" system is too large to represent as a single data flow diagram

the solution is to decompose the system into a hierarchy of levels of processing

the process model of the system then consists of a set of levelled data flow diagrams

levelling of DFDs improves their readability and usefulness as a communication tool

Levelling of DFDs

Context diagram• Level 0 diagram

Level 1 diagrams Level 2 diagrams

.

.

. Level n diagrams

Levelling creates a hierarchical decomposition of the processing within the system

Context Diagram

• the highest level data flow diagram is the context diagram• the context diagram shows the interaction of the system

with its environment in terms of data flows• the context diagram defines the boundary of the system

(the scope of the system)• only the data flows which leave the system and the data flows which

come from outside the system are shown

Context Diagram• the entire system is represented as a single process

• all external agents (sources and sinks) are shown

• no data stores are shown: they are inside the boundary of the system

EASY GO HOTEL

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Example Context Diagram

InventorySystem

SuppliersSales Order

System

Warehouse

sales order

goods unavailable notice

purchase order

supplier delivery

shipping slip checked supplierinvoice Accounts

Department

goods returned

Level Zero Diagram• the level zero data flow diagram is the diagram at

the level immediately below the context diagram

• it "expands" the single process on the context diagram to show the major, high-level processes (or functions) within the system

1.0

2.0

3.0

4.0

Level Zero Diagram

• all external agents (sources and sinks) are included because the level zero diagram, like the context diagram, represents the entire system

• the number of each process ends in .0 which corresponds to the level of the diagram:

e.g. 1.0, 2.0, 3.0 etc.

often just 1, 2, 3 etc. are used

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Example Level Zero Diagram

Monitor

Stocklevels

SuppliersOrder Entry

System

Warehouse

sales order

out of stock notice

purchase order

supplierdelivery

shipping slip

checked supplierinvoice

AccountsDepartment

FillSales

Orders

Accept Deliveries

1.0

2.0

3.0

Stock file

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Levelling DFDs

12

3

1.1

1.2

Context diagram

Level zero diagram

Diagram 1 (at Level 1)

Level 1 diagrams

a set of data flow diagrams is created at Level 1 there is one Level 1 diagram for each of the processes

at Level zero each diagram decomposes a Level zero process into

several processes

2.1

2.2

2.32.4

2.5

Level 1 and lower level diagrams

each Level 1 diagram and diagrams at lower levels (e.g. Level 2, Level 3) show only a part of the processing

Level 1 processes are numbered 1.1, 1.2, 1.3, and 2.1, 2.2, 2.3 etc

that part is shown in more detail than on the Level zero diagram

no external agents are shown on a Level 1 or lower diagrams, as the entire system is not being represented

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Example Level zero diagram

CUSTOMERS

SALES ORDERS

INVENTORY

p.o. number

product

order qty

p.o. invoice

delivery qty

customerinvoice

Productdetails

product qty

Customer details

product holding

product order qty

product holding

product delivered qty

product order qty

Record and

Invoice Sales

1

Maintain Product

Inventory

2

SUPPLIERS

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Example Level 1 diagram

Customerdetails

Productdetails

product qty

SALES ORDERS

customer creditstatus

product qty

product

INVENTORY

productholding

available qty

INVOICES

customerinvoice

Check Customer

Credit

1.1

Record Sales

Demand

1.3

Check Stock

Available

1.2

Create Sales

Invoice

1.4

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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 DFDs

Guidelines for Levelling DFDs

External communication: external agents represent entities in the

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

information system we do NOT model interactions between

external agents we do NOT allow external agents to interact

directly with data stores

Guidelines for Levelling DFDs

• numbering:

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

• balancing of levelled DFDs:

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

• external agents:

are only included on the two diagrams which represent the entire 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

Guidelines for Levelling DFDs

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

2

1

1.1 1.21.3

Guidelines for Levelling DFDs

How many levels should be in a set of DFDs?

• each diagram usually has between 3 and 7 processes

• level the diagrams until bottom level or primitive processes are reached:

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

Guidelines for Levelling DFDs

• partition processes to minimise the data flows between them

• partition processes to form cohesive, related groups of activities

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

Guidelines for Levelling DFDs

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Payroll System

Line

Manager

EmploymentSystem

SuccessfulApplicant

Position Spec

Decision

Acknow-ledgement

An example - Context Diagram

Position

Address

Applicant name

Position

Applicant

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An 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

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Level 1- Diagram 1

Applicant name

Verify for Complete-

ness Acknow-ledge

Application

Acknow-ledgement

AcknowledgedApplication

VerifiedApplicationPosition

Address

1.2

1.1

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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

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Logical and physical DFDs

Models may focus on either:

• the “physical” view of the real world –

how things are done

OR• the “logical” view of the real world –

what things are done

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Physical DFDs

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

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

• they show how the processing takes place and how the data is implemented

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Logical DFDs

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

• they are technology independent• they show what processing, data

movements and data storage must occur in a system

• they show the essential aspects of a system

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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

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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

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• 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

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2.1

Billchecksform

2.1

Validatesalesorder

AZ104 formchecked AZ104 form

sales order

valid sales order

Master File

Sales orders

Physical to Logical DFDs

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Logical and Physical DFDs

Physical DFDs Logical DFDs

View How processing is implemented What the system does

Processes Actual sequence Essential sequence

Naming Forms, locations, people/roles Underlying data andactivities

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

implementation needs

Function Decomposition: a Process Modelling Technique

function decomposition is the decomposing of a system into its component functions and processes as a way of managing complexity

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

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

the decomposition of functions corresponds to levelling of processes in DFDs

Function Decomposition Diagrams

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

a process is a lower level activity that is repeatedly carried out

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

depend on the size and complexity of the system represented

Function Decomposition Diagrams

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

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

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

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

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

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Function Decomposition Diagrams

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

ABC Sales System

1. Sell Products

2. Manage Inventory

3. Control Finance

a top level function decomposition diagram

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Example Function Decomposition Diagram

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

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Function Decomposition

1.0 2.0 3.0

1.21.1

1.2.1 1.2.2

the decomposition of functions and processes

corresponds to the levelling of DFDs

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

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

they provide a useful overview of the processing within a system

Function Decomposition

References

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

HOFFER, J.A., GEORGE, J.F. and VALACICH (1999) 2nd ed., Modern Systems Analysis and Design, Benjamin/Cummings, Massachusetts.

Chapter 8