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IFS310: Week 3BIS310: Structured Analysis and Design

Process Modeling and Data Flow Diagrams

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IFS310: Week 3

Overview

• Discussion– Systems Theory (Input-Process-Output)– Process Modeling and Mechanics of DFDs– Current System Process Modeling– Future System Process Modeling

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IFS310: Week 3

Systems Theory and Process Concepts

– A System is a Process• The simplest process model of a system is based on

inputs, outputs, and the system itself – viewed a process.

• The process symbol defines the boundary of the system.

• The system is inside the boundary; the environment is outside that boundary.

• The system exchanges inputs and outputs with its environment

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IFS310: Week 3

The System's Environment (constantly changing)

The System Process

input output

Feeback and Control Loop

inputinput

outputoutput

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IFS310: Week 3

System Models

• Logical models show what a system ‘is’ or ‘does’. They are implementation-independent; that is, they depict the system independent of any technical implementation. As such, logical models illustrate the essence of the system. Popular synonyms include essential model, conceptual model, and business model.

• Physical models show not only what a system ‘is’ or ‘does’, but also how the system is physically and technically implemented. They are implementation-dependent because they reflect technology choices, and the limitations of those technology choices. Synonyms include implementation model and technical model

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IFS310: Week 3

What is Process Modeling?

• Process modeling is a technique for organizing and documenting the structure and flow of data through a system’s PROCESSES and/or the logic, policies, and procedures to be implemented by a system’s PROCESSES.

• Process modeling originated in classical software engineering methods.

• A systems analysis process model consists of data flow diagrams (DFDs).

– A data flow diagram (DFD) is a tool that depicts the flow of data through a system and the work or processing performed by that system. Synonyms include bubble chart, transformation graph, and process model.

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IFS310: Week 3

Data Flow Diagram

• There are only three symbols and one connection:– The rounded rectangles represent processesprocesses or work to

be done.

– The squares represent external agentsexternal agents – the boundary of the system.

– The open-ended boxes represent data storesdata stores, sometimes called files or databases, and correspond to all instances of a single entity in a data model.

– The arrows represent data flowsdata flows, or inputs and outputs, to and from the processes.

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IFS310: Week 3

Logical Processes

• You should be left only with logical processes that:– Perform computations (e.g., calculate grade point

average)

– Make decisions (determine availability of ordered products)

– Sort, filter or otherwise summarize data (identify overdue invoices)

– Organize data into useful information (e.g., generate a report or answer a question)

– Trigger other processes (e.g., turn on the furnace or instruct a robot)

– Use stored data (create, read, update or delete a record)

Gane & Sarson Process Shape

Process Name

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IFS310: Week 3

Data Flows

– Data in Motion• A data flow is data in motion.

– A data flow represents an input of data to a process, or the output of data (or information) from a process. A data flow is also used to represent the creation, deletion, or update of data in a file or database (called a data store on the DFD).

– A data flow is depicted as a solid-line with arrow.

Gane & Sarson Data Flow Shape

Name of data-flow

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IFS310: Week 3

• External Agents– An external agent defines a person, organization unit, other

system, or other organization that lies outside of the scope of the project, but which interacts with the system being studied. External agents provide the net inputs into a system, and receive net outputs from a system. Common synonyms include external entity.

Gane & Sarson External Agent

Shape

System Concepts for Process Modeling

External Agent

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IFS310: Week 3

System Concepts for Process Modeling

• Data Stores– A data store is an ``inventory’’ of data. Synonyms

include file and database (although those terms are too implementation-oriented for essential process modeling).

Gane & SarsonData Store

Shape

D1 Data-store name

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IFS310: Week 3

Data Flow Diagramming Definitions

• Context Diagram– A data flow diagram (DFD) of the scope of an

organizational system that shows the system boundaries, external entities that interact with the system and the major information flows between the entities and the system

• Level-0 Diagram– A data flow diagram (DFD) that represents a system’s

major processes, data flows and data stores at a high level of detail

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IFS310: Week 3

Decomposition of DFDs

• Functional decomposition– Act of going from one single system to many

component processes

– Repetitive procedure

• Level-N Diagrams– A DFD that is the result of n nested decompositions of

a series of subprocesses from a process on a level-0 diagram

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IFS310: Week 3

Guidelines for Drawing DFDs

• Completeness– 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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IFS310: Week 3 Figure 3.2An unbalanced set of data flow diagrams

(a) Context diagram(b) Level-0 diagram

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IFS310: Week 3

Common Mechanical Errors

3.1.2 Create a new

member account

3.1.1 Generate an

employee bank statement

3.1.3 Freeze member account number

Accounts Receivable Department

Employee

Member Accounts Employees

Existing account

New account status

Employee status

Frozen account notification

Employee address

Bank statement

Membership application

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IFS310: Week 3Illegal Data Flows

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 flows

Corrected data flows

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IFS310: Week 3

Logical Processes

• Logical processes are work or actions that must be performed no matter how you implement the system. – Even for the future system, at the definition phase, the

process model should be logical.

• In the study phase, the current system was analyzed in three levels of models:– Context Diagram

– System Diagram

– Event Diagrams

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IFS310: Week 3

Context Diagram

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IFS310: Week 3

System Diagram

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IFS310: Week 3

Event (Child) Diagram(s)

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IFS310: Week 3

How to Model a New System

• Step 1: Identify the changes in functional requirements for the new system

• Step 2: Establish the context for the new system

• Step 3: Create a new system diagram

• Step 4: Create necessary child diagrams

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IFS310: Week 3Identify the changes in

functional requirements for the new system

• Source: Problem Statement (Cause/Effect)• Prioritize the “System Objectives” based on the

urgency/importance of the effects• Regroup the objectives into changes in functional

requirements e.g.,

– Operational requirements,

– Reporting/Inquiry requirements

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IFS310: Week 3

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IFS310: Week 3

Establish the context for the new system

• Analyze the new functional requirements in terms of necessary input/output of data

i.e., • Is there any new input?• Is it necessary to produce new output?• Is it going to allow new functions to external

entities?

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IFS310: Week 3

New Context Diagram

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IFS310: Week 3

Create a new system diagram

• Principles for creating new DFD– Keep the model simple

– Try to use data stores to connect all the processes

• Identify the functions that must be affected by the new functional requirements

• Determine the boundary of the new system

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IFS310: Week 3

New System Diagram

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IFS310: Week 3

Create necessary child diagrams

• Explode each function to depict the flow of data within the function

• A list of events within the function will be helpful• Add necessary data stores

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IFS310: Week 3A New Event (Child) Diagram

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IFS310: Week 3

Group Project (Assignment 2)

Objectives– The goal for this assignment is having all

leveled data flow diagrams (logical). Do your best to decompose the system (process) into as detail as you can, using assumptions from your common sense to the wildest imagination. You may use Visio, or any other graphical processors (e.g., Word, or PowerPoint) to create the diagrams.