A dfd shows the structure and detailed contents of a data flow.

Chapter 5: Data and Process Modeling

Kent Institute Australia Pty. Ltd.

ABN 49 003 577 302 CRICOS Code: 00161E RTO Code: 90458 TEQSA Provider Number: PRV12051

Version 2 – 18th December 2015

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Prescribed Text and recommended readings

Rosenblatt, H. J. (2016), Systems Analysis and Design.11th Edition, Cengage Learning, Boston MA

Robertson, S. and Robertson, J. (2013), Mastering the Requirements Process: Getting Requirements Right, 3rd Edition, Addison Wesley, Upper Saddle River, NJ

IIBA (2015), Guide to the Business Analysis Body of Knowledge, BABOK Version 3.0, International Institute of Business Analysis, http://www.iiba.org/BABOKGuide.aspx

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Chapter Objectives

Describe data and process modeling concepts and tools, including data flow diagrams, a data dictionary, and process descriptions

Describe the symbols used in data flow diagrams and explain the rules for their use

Draw data flow diagrams in a sequence, from general to specific

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Chapter Objectives

Explain how to level and balance a set of data flow diagrams

Describe how a data dictionary is used and what it contains

Use process description tools, including structured English, decision tables, and decision trees

Describe the relationship between logical and physical models

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Introduction

Logical Model: Shows what the system must do, regardless of how it will be implemented physically

Physical Model: Describes how the system will be constructed

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Overview of Data and Process Modeling Tools

Systems analysts use graphical techniques to describe an information system

Data flow diagram (DFD) - Uses various symbols to show how the system transforms input data into useful information

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Data Flow Diagrams

A data flow diagram (DFD) shows how data moves through an information system but does not show program logic or processing steps

A set of DFDs provides a logical model that shows what the system does, not how it does it

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Data Flow Diagrams (Cont. 1)

DFD Symbols

Four basic symbols represent processes, data flows, data stores, and entities

Gane and Sarson: Used in data

flow diagrams

Processes, data flows,

data stores, and external

entities all have a unique symbol

Yourdon: Used in data flow

diagrams

Processes, data flows, data stores, and external entities each have a unique symbol

FIGURE 5-1 Data flow diagram symbols, symbol names, and examples of the Gane and Sarson and Yourdon symbol sets

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Data Flow Diagrams (Cont. 2)

Process Symbol

Must have at least one input and at least one output

Contains business logic that transforms the data

Process name identifies its function (verb)

Examples” : “apply rent payment” or “calculate commission

In DFDs, a process symbol can be referred to as a black box

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Data Flow Diagrams (Cont. 3)

Data Flow Symbol

Represents one or more data items

The symbol for a data flow is a line with a single or double arrowhead

FIGURE 5-3 Examples of correct combinations of data flow and process symbols

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Data Flow Symbol

Following data flow and process combinations must be avoided

Spontaneous generation

Black holes

Gray holes

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Data Flow Diagrams (Cont. 4)

FIGURE 5-4 Examples of incorrect combinations of data flow and process symbols. APPLY INSURANCE PREMIUM has no input and is called a spontaneous generation process. CALCULATE GROSS PAY has no outputs and is called a black hole process. CALCULATE GRADE has an input that is obviously unable to produce the output. This process is called a gray hole

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Data Flow Diagrams (Cont. 5)

Data Store symbol

Represent data that the system stores

A DFD does not show the detailed contents of a data store — the specific structure and data elements are defined in the data dictionary

A data store must be connected to a process with a data flow

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Data Flow Diagrams (Cont. 6)

FIGURE 5-5 Examples of correct uses of data store symbols in a data flow diagram

FIGURE 5-6 Examples of incorrect uses of data store symbols: Two data stores cannot be connected by a data flow without an intervening process, and each data store should have an outgoing and incoming data flow

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Data Flow Diagrams (Cont. 7)

Entity Symbol

Shows how the system interfaces with the outside world

A DFD shows only external entities that provide data to the system or receive output from the system

DFD entities also are called terminators because they are data origins or final destinations

Each entity must be connected to a process by a data flow

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Data Flow Diagrams (Cont. 8)

FIGURE 5-7 Examples of correct uses of external entities in a data flow diagram

FIGURE 5-8 Examples of incorrect uses of external entities. An external entity must be connected by a data flow to a process, and not directly to a data store or to another external entity

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Data Flow Diagrams (Cont. 9)

FIGURE 5-9 Examples of correct and incorrect uses of data flows

Keep in mind:

All flow lines must be labeled

Large processes can be broken down into smaller components

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Creating a Set of DFDs

Create a graphical model of the information system based on your fact-finding results

First, you will review a set of guidelines for drawing DFDs

Then you will learn how to apply these guidelines and create a set of DFDs using a three-step process

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Creating a Set of DFDs (Cont. 1)

Guidelines for Drawing DFDs

Draw the context diagram so that it fits on one page

Use the name of the information system as the process name in the context diagram

Use unique names within each set of symbols

Do not cross lines

Provide a unique name and reference number for each process

Ensure that the model is accurate, easy to understand, and meets the needs of its users

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Creating a Set of DFDs (Cont.2)

FIGURE 5-10 Context diagram DFD for grading system

Step 1: Draw a Context Diagram

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Creating a Set of DFDs (Cont. 3)

FIGURE 5-11 Context diagram DFD for an order system

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Step 2: Draw a Diagram 0 DFD

If same data flows in both directions, you can use a double-headed arrow

Diagram 0 is an exploded view of process 0

Parent diagram

Child diagram

Functional primitive

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Creating a Set of DFDs (Cont. 4)

FIGURE 5-13 Diagram 0 DFD for the order system

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Step 3: Draw the Lower Level Diagrams

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Creating a Set of DFDs (Cont. 5)

FIGURE 5-14 Diagram 1 DFD shows details of the FILLORDER process in the order system

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Must use leveling and balancing techniques

Leveling examples

Uses a series of increasingly detailed DFDs to describe an information system

Exploding, partitioning, or decomposing

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Creating a Set of DFDs (Cont. 6)

FIGURE 5-15 This diagram does not show the symbols that connect to data flows entering or leaving FILL ORDER on the context diagram

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Creating a Set of DFDs (Cont. 7)

FIGURE 5-16 The order system diagram 0 is shown at the top of the figure, and exploded diagram 3 DFD (for the APPLY PAYMENT process) is shown at the bottom. The two DFDs are balanced because the child diagram at the bottom has the same input and output flows as the parent process 3 shown at the top

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Creating a Set of DFDs (Cont. 8)

FIGURE 5-18 In the next level of detail, the process 0 black box reveals three processes, two data stores, and four internal data flows — all of which are shown inside the dashed line

FIGURE 5-17 Example of a parent DFD diagram, showing process 0 as a black box

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Data Dictionary

A data dictionary, or data repository, is a central storehouse of information about a system’s data

An analyst uses the data dictionary to collect, document, and organize specific facts about a system

Defines and describes all data elements and meaningful combinations of data elements

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Data Dictionary (Cont. 1)

Data element: Smallest piece of data that has meaning within an information system

Called data item or field

Are combined into records, also called data structures

Record: Meaningful combination of related data elements that is included in a data flow or retained in a data store

Called data structures

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Data Dictionary (Cont. 2)

Using CASE Tools for Documentation

More complex the system, more difficult it is to maintain full and accurate documentation

Modern CASE tools simplify the task

A CASE repository ensures data consistency

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Data Dictionary (Cont. 3)

Documenting the Data Elements

Every data element in the data dictionary should be documented

Objective - To provide clear, comprehensive information about the data and processes that make up a system

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Documenting the Data Elements

Data element name and label

Alias

Type and length

Default value

Acceptable values - Domain and validity rules

Source

Security

Responsible user(s)

Description and comments

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Data Dictionary (Cont. 4)

FIGURE 5-20 A Visible Analyst screen describes the data element named SOCIAL SECURITY NUMBER.

Source: Visible Systems Corporation.

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Documenting the Data Flows

Data flow name or label

Description

Alternate name(s)

Origin

Destination

Record

Volume and frequency

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Data Dictionary (Cont. 5)

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Data Dictionary (Cont. 6)

Documenting the Data Stores

Data store name or label

Description

Alternate name(s)

Attributes

Volume and frequency

FIGURE 5-21 Visible Analyst screen that documents a

data store named IN STOCK

Source: Visible Systems Corporation.

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Documenting the Processes

Process name or label

Description

Process number

Process description

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Data Dictionary (Cont. 7)

FIGURE 5-22 Visible Analyst screen that describes a

process named VERIFY ORDER

Source: Visible Systems Corporation.

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Documenting the Entities - Data dictionary describes all external entities that interact with the system

Characteristics include

Entity name

Description

Alternate name(s)

Input data flows

Output data flows

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Data Dictionary (Cont. 8)

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Data Dictionary (Cont. 9)

Documenting the Records

Record or data structure name

Definition or description

Alternate name(s)

Attributes

FIGURE 5-23 Visible Analyst screen that documents a

record, or data structure named CREDIT STATUS

Source: Visible Systems Corporation.

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Data Dictionary Reports - Following can be obtained

Alphabetized list of all data elements by name

Report describing each data element and indicating the user or department that is responsible for data entry, updating, or deletion

Report of all data flows and data stores that use a particular data element

Detailed reports showing all characteristics of data elements, records, data flows, processes, or any other selected item stored in the data

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Data Dictionary (Cont. 10)

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Process Description Tools

Process description: Documents the details of a functional primitive and represents a specific set of processing steps and business logic

Tools - structured English, decision tables, and decision trees

Used in object-oriented development

O-O analysis - combines data and the processes that act on the data into things called objects, and similar objects can be grouped together into classes

O-O processes are called methods

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Process Description Tools (Cont. 1)

Modular Design

Based on combinations of three logical structures, sometimes called control structures, which serve as building blocks for the process

Sequence

Selection

Iteration - looping

FIGURE 5-26 Iteration structure

FIGURE 5-25 Selection structure

FIGURE 5-24 Sequence structure

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Process Description Tools (Cont. 2)

Structured English

Rules

Use only the three building blocks of sequence, selection, and iteration

Use indentation for readability

Use a limited vocabulary

standard terms used in the data dictionary

Specific words that describe the processing rules

FIGURE 5-27 The VERIFY ORDER process description

includes logical rules and a structured English version of

the policy. Notice the alignment and indentation of the

logic statements

Source: Visible Systems Corporation.

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Process Description Tools (Cont. 3)

Decision Tables

Show a logical structure, with all possible combinations of conditions and resulting actions

Every possible outcome should be considered to ensure that nothing has been overlooked

Number of rules doubles each time a condition is added

Can have more than two possible outcomes

Are the best way to describe a complex set of conditions

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Process Description Tools (Cont. 4)

FIGURE 5-29 Example of a simple decision table showing the processing logic of the VERIFY ORDER process

FIGURE 5-28 The Verify Order business process has two conditions. For an order to be accepted, the product must be in stock and the customer must have an acceptable credit status

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Process Description Tools (Cont. 5)

FIGURE 5-31This table is based on the Verify Order conditions shown in Figure 5-30. With three conditions, there are eight possible combinations, or rules

FIGURE 5-30 A third condition has been added to the Verify Order business process. For an order to be accepted, the product must be in stock and the customer must have an acceptable credit status. However, the credit manager now has the authority to waive the credit status requirement

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Process Description Tools (Cont. 6)

FIGURE 5-32 In the first table, dashes have been added to indicate that a condition is not relevant. In the second version, rules have been combined. Notice that in final version, only four rules remain. These rules document the logic, and will be transformed into program code when the system is developed

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Process Description Tools (Cont. 7)

FIGURE 5-34 This decision table is based on the sales promotion policy in Figure 5-33. This is the initial version of the table, before simplification

FIGURE 5-33 A sales promotion policy with three conditions. Notice that the first statement contains two separate conditions – one for the 5% discount, and another for the additional discount

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Process Description Tools (Cont. 8)

FIGURE 5-35 In this version, dashes have been added to indicate that a condition is not relevant. At this point, it appears that several rules can be combined

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Process Description Tools (Cont. 9)

FIGURE 5-36 This example is based on the same Sales Promotion Policy shown in the decision tables in Figures 5-34 and 5-35 on the previous page. Like a decision table, a decision tree shows all combinations of conditions and outcomes. The main difference is the graphical format, which many viewers find easier to interpret

Decision Trees

Graphical representation of the conditions, actions, and rules found in a decision table

Show the logic structure in a horizontal form that resembles a tree

Provide the same results as decision tables, but in different forms

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Logical versus Physical Models

While structured analysis tools are used to develop a logical model for a new information system, such tools also can be used to develop physical models of an information system

A physical model shows how the system’s requirements are implemented

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Logical versus Physical Models (Cont. 1)

Sequence of Models

Systems analysts create a physical model of the current system and then develop a logical model of the current system before tackling a logical model of the new system

Performing extra step allows to understand the current system better

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Logical versus Physical Models (Cont. 2)

Four-Model Approach

Develop:

A physical model of the current system

A logical model of the current system

A logical model of the new system

A physical model of the new system

Disadvantage - Additional time and cost

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Chapter Summary

During data and process modeling, a systems analyst develops graphical models to show how the system transforms data into useful information

The end product of data and process modeling is a logical model that will support business operations and meet user needs

Data and process modeling involves three main tools: data flow diagrams, a data dictionary, and process descriptions

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Data flow diagrams (DFDs) graphically show the movement and transformation of data in the information system

DFDs use four symbols

A set of DFDs is like a pyramid with the context diagram at the top

The data dictionary is the central documentation tool for structured analysis

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Chapter Summary (Cont. 1)

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Each functional primitive process is documented using structured English, decision tables, and decision trees

Structured analysis tools can be used to develop a logical model during one systems analysis phase, and a physical model during the systems design phase

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Chapter Summary (Cont. 2)

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