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Chapter 7Structuring System Process

Requirements

2011 by Prentice Hall: J.A.Hoffer et.al., Modern Systems Analysis & Design, 6th Edition 2/68

Content9 Understand the logical (&physical) process modeling

by using data flow diagrams (DFDs)9 Draw DFDs & Leveling9 Balance higher-level and lower-level DFDs9 Explain the differences among four types of DFDs:

current physical, current logical, new physical, and new logical

9 Use DFDs as a tool to support the analysis of ISs9 Discuss process modeling for e-Commerce

applications9 Process Description **

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1. Process Modeling

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Process Modeling (Cont.)

Graphically represent the processes that capture, manipulate, store, and distribute databetween a system and its environment and among system components

Utilize information gathered during requirements determination

Processes and data structures are modeled

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1.1 Deliverable and Outcome Context data flow diagram (DFD) Scope of system

DFDs of current physical system Adequate detail only

DFDs of current logical system Enables analysts to understand current system

DFDs of new logical system Technology independent Show data flows, structure, and functional requirements of

new system DFDs of new physical system Description of each DFD component

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2. Data Flow Diagramming Mechanics Represent both physical and logical ISs Only four symbols are used

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Definitions and Symbols (Cont.)

Processwork or actions performed on data

Data storedata at rest (inside the system)

Source/sink (destination); terminator; actor; boundary external entity that is origin or destination of data

(outside the system) Data flowarrows depicting movement of data

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Data Flow Diagramming Mechanics (Cont.)

Useful for depicting purely logical information flows

DFDs that detail physical systems differ from system flowcharts which depict details of physical computing equipment

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Different between Sources/ Sinks and Processes

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2.1 Developing DFDs: an Example

Context diagram is an overview of an organizational system that shows the system boundariesexternal entities that interact with the systemMajor information flows between the entities

and the system Note: only one process symbol, and no data

stores shown

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Context Diagram

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Developing DFDs (Cont.)

Level-0 diagram is a data flow diagram that represents a systems major processes, data flows, and data stores at a high level of detailProcesses are labeled 1.0, 2.0, etc. These will be

decomposed into more primitive (lower-level) DFDs.

Leveling & Balancing

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Level-0 Diagram

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2.2 Data Flow Diagramming Rules

Two DFD guidelinesThe inputs to a process are different from

the outputs of that processProcesses purpose is to transform inputs into

outputsObjects on a DFD have unique namesEvery process has a unique name

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Data Flow Diagramming Rules (Cont.)

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Data Flow Diagramming Rules (Cont.)

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B2

Illegal Data Flows

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2.3 Decomposition of DFDs Functional decomposition is an iterative

process of breaking a system description down into finer and finer detail

PROJECTGOAL

0

PHASE

2PHASE

3PHASE

1

ACTIVITY

2.2ACTIVITY

2.1ACTIVITY

2.3

TASK

2.2.2TASK

2.2.1TASK

2.2.3

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Decomposition of DFDs (Cont.)

Primitive DFD is the lowest level of a DFD. Level-1 diagram results from decomposition

of Level-0 diagram. Level-n diagram is a DFD diagram that is the

result of a n nested decompositions from a process on a level-0 diagram.

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Level-1 DFD

Level-1 DFD shows the sub-processes of one of the processes in the Level-0 DFD.

This is a Level-1 DFD for Process 4.0

Processes are labeled 4.1, 4.2, etc. These can be further decomposed in more primitive (lower-level) DFDs if necessary.

Balancing!!!

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Level-n DFD

Level-n DFD shows the sub-processes of one of the processes in the Level n-1 DFD.

This is a Level-2 DFD for Process 4.3.

Processes are labeled 4.3.1, 4.3.2, etc. If this is the lowest level of the hierarchy, it is called a primitive DFD.

Not Necessary for this level!!

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2.4 Balancing DFDs

Conservation Principle: conserve inputs and outputs to a process at the next level of decomposition

Balancing: conservation of inputs and outputs to a data flow diagram process when that process is decomposed to a lower level

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Balancing DFDs (Cont.)

Balanced means (in general):Number of inputs to lower level DFD equals

number of inputs to associated process of higher-level DFDNumber of outputs to lower level DFD

equals number of outputs to associated process of higher-level DFD

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Balancing DFDs (Cont.)

This is unbalanced because the process of the context diagram has only one input but the Level-0 diagram has two inputs.

1 input1 output

2 inputs1 output

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Balancing DFDs (Cont.)

Data flow splitting is when a composite data flow at a higher level is split and different parts go to different processes in the lower level DFD

The DFD remains balanced because the same data is involved, but split into two parts

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Balancing DFDs (Cont.)

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Balancing DFDs: More DFD Rules

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Exercise

Document your project requirements by usingContext diagram (scope)L-0 DFD (main activities)++ Leveling & Balancing ++ Process description

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3. Four Different Types of DFDs

Current Physical Process labels identify technology (people or systems) used

to process the data Data flows and data stores identify actual name of the

physical media

Current Logical Physical aspects of system are removed as much as

possible Current system is reduced to data and processes that

transform them

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Four Different Types of DFDs (Cont.)

New Logical Include additional functionsRemove obsolete functions Reorganize inefficient data flows

New PhysicalRepresents the physical implementation of the new

system

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System Development Model (Classical Model)

AS IS (Current)

1 Physical Model

2 Logical Model

TO BE (New)

4 Physical Model

3 Logical Model

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4. Using Data Flow Diagram in the Analysis Process

4.1 Guidelines for Drawing DFDsCompleteness

DFD must include all components necessary for system Each component must be fully described in the project

dictionary or CASE repository

Consistency The extent to which information contained on one level

of a set of nested DFDs is also included on other levels

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Guidelines for Drawing DFDs (Cont.)

TimingTime is not represented well on DFDsBest to draw DFDs as if the system has never

started and will never stop Iterative DevelopmentAnalyst should expect to redraw diagram several

times before reaching the closest approximation to the system being modeled

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Primitive DFDs

Lowest logical level of decomposition Decision has to be made when to stop

decomposition

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Rules for stopping decomposition When each process has been reduced to a single decision,

calculation or database operation When each data store represents data about a single entity When each user does not care to see any more detail When every data flow does not need to be split further to

show that data are handled in various ways When you believe that you have shown each business form

or transaction, online display and report as a single data flow

When you believe that there is a separate process for each choice on all lowest-level menu options

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4.2 Using DFDs as Analysis Tools

Gap Analysis is the process of discovering discrepancies between two or more sets of data flow diagrams or discrepancies within a single DFD

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4.3 Using DFDs in BPR

FIGURE 7-16IBM Credit Corporations primary work process before BPR(Source: Based on Hammer and Champy, 1993.) 38/68

Using DFDs in BPR (Cont.)

FIGURE 7-17IBM Credit Corporations primary work process after BPR(Source: Based on Hammer and Champy, 1993.)

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5. e-Commerce Application: Process Modeling using Data Flow Diagram

Process modeling for Pine Valley Furnitures Webstore

Completed JAD session Began translating the Webstore system

structure into data flow diagram Identified six high-level processes

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e-Commerce Application: Process Modeling using Data Flow Diagrams (Cont.)

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e-Commerce Application: Process Modeling using Data Flow Diagrams (cont.)

Figure 7-22 Level-0 data flow diagram for the WebStore42/68

6. Process Description 9 Use Structured English as a tool for representing

steps in logical processes in DFDs9 Use decision tables to represent the logic of

choice in conditional statements9 Select among Structured English and decision

tables for representing processing logic9 Understand how logic modeling techniques

apply to the development of your project study

2011 by Prentice Hall: J.A.Hoffer et.al., Modern Systems Analysis & Design, 6th Edition

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Logic Modeling: process description

Data flow diagrams (DFDs) do not show the logic inside the processes

Logic modeling involves representing internal structure and functionality of processes depicted on a DFD

Logic modeling can also be used to show when processes on a DFD occur

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Deliverable and Outcome

Structured English representation of process logic

Decision Tables representation OOT Interaction diagram Sequence diagram &

Communication diagramActivity diagram

Plus: decision tree & flowchart

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Structured English

Structured English: modified form of English language used to specify the logic of IS processes

Typically relies on action verbs and noun phrases and contains no adjectives or no specific standards

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Structured English

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Structured English

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Structured English Constructs (Part 1)

Whitten

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Structured English Constructs (Part 2)

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Process Description: Modeling Logic * * * Process #: Process NameDescription: .......................Created Date: .....................

InputActor

formP#: ...

attribute, ...D#: ...

(attribute, ...)

OutputActor

reportP#: ...

attribute, ...D#: ...

(attribute, ...)

Tasks and Activities (show logic not brief description)Use:

Structured English orActivity diagramFlowchartDecision table/ tree etc.

P# process

boundary

o

u

t

p

u

t

i

n

p

u

t

D# data store

p p

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Decision Table

Decision table: a matrix representation of the logic of a decision which specifies the possible conditions for the decision and the resulting actions

Best used for complicated decision logic

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Decision Table

1

2

3

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Decision Table

Condition stubs: list conditions that relevant to the decision

Action stubs: list actions that result for a given set of conditions

Rules: define which actions are to be followed for a given set of conditions

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Modeling Logic with Decision Table

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Logic Modeling (Cont.)

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Logic Modeling (Cont.)

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Summary9 Understand the logical process modeling by

using data flow diagrams (DFDs)9 Draw DFDs & Leveling9 Balance higher-level and lower-level DFDs9 Explain the difference among four types of

DFDs: current physical, current logical, new physical, and new logical

9 Process Description

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Questions & Answers

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Exercise

What is a data flow diagram? Why do system analysts use data flow diagrams (DFDs)?

Explain the rules of drawing good DFDs. What unique rules apply to drawing context

diagram?

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Case Study

A simple banking system1 Open new a/c2 Deposit and withdraw3 Close a/cDraw the context diagram and DFD of this

simple banking system.

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Main Activity New customers are registered; and accounts are

closed by bank manager When accounts are closed, customers are notified

by a system Cash deposits into an account are made by the

customer, and salary are paid by the employer The bank clerk performs a balance check before

allowing a withdrawal by a customer at the bank counter

List of System Logical ElementsWichian 62/68

List of system logical elements

Wichian

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Context Diagram :A simple banking system

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Data Flow Diagram : a simple banking system

Wichian

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Data Flow Diagram : a simple banking system

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Problems of System Development

Poor system thinking Poor system integration Poor verifying the modelFinding missing algorithmSyntax checkingTracing data elementCross-referencingTracing objective

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Physical Data Flow Diagrams (PDFDs)

Technical and human decisions to be implemented as part of an information system

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Example: Classify the following aspects into functional (F), non-functional (NF), or not be a requirement.

1. How information about flights, passengers and booking are entered.

2. What information appears on tickets and reports.

3. How fares are calculated.4. What information must be stored in the

database that travel agents and others access.5. The system should be designed so it can be

extended to handle a frequent flier plan.6. The system must be available at all times.

Only 2 minutes downtime a week is to be permitted.

7. A merge sort algorithm must be used to sort the flights by departure time.