Practical Significance of DFDs Many older information systems (legacy systems):

Are mainframe-based (inaccessible to most non-IT users); Use non-relational (hierarchical) databases, and Require second-generation programming languages (e.g.

COBOL) to perform data input, queries, reporting.

Thus, many IT professionals focus on data flows and physical implementation, not data relationships or RDBM design principles.

Many business processes are so complex that E-R design alone will not give analysts insight into their businesses

1

DFDs allow the analyst to determine: What does the organization do? How does the organization do it?

Creating DFDs

2

Define Entities • External entities represent

persons, processes or machines which produce data to be used by the system or receive data that is output by the system

• Examples: Student, Customer, Client

Define Processes• Processes are discrete actions

that transform input data to output data

• Examples: Create Student Record, Calculate Purchase Cost, Register Client

Student

2.1

Create Student Record

Creating DFDs (cont’d)Define Data Stores

• Data stores are temporary or permanent repositories of information that are inputs to or outputs of processes

• Examples: Student Master, Client List

Define Data Flows• Data flows represent the

transfer of data over time from one “place” (entity, process, data store) to another

• Examples: New Student Information (from Student, to Student Master)

3

New Student Information

Student Master

D3

Creating DFDs (cont’d)Define the System

• A system is the collection of all business processes which perform tasks or produce outputs we care about. It is “what happens.”

• The system is a single process, connected to external entities

• Represented in a “Context Diagram”

Define Subsystems• A subsystem gives a more

detailed view individual processes contained in the context diagram

• Includes data stores, more elementary processes

4

(Figure 4.13, Shelly, Cashman and Rosenblatt)

DFDs Created by Top-Down Analysis

Create a narrative: description of system

Create a Context Diagram that contains a single process (“the system”) and all entities which share data with the system

Explode the “parent” context diagram to produce a Diagram 0 (“child”) DFD

Create Diagram 1, 2, …, n DFDs that represent “explosions” of Diagram 0, 1, …, n-1 DFDs until a diagram has only “primitive” processes

Create process descriptions to be implemented by application programs: queries, macros, reports, programming languages

5

Context Diagram

Diagram 0 DFD

Diagram 1 DFDs

Diagram n DFDs

Narrative

E-R Diagram Process Descriptions

Where to Begin Creating DFDs

Start with the data flow from an external entity and work forwards

Start with the data flow to an external entity and work backwards

Examine the data flows into or out of a data store

Examine data flows, entity connections and data stores associated with a particular process

Note fuzzy, ill-defined areas of the system for further clarification

6

What to Avoid in DFDs

Making the data flow diagram too cluttered (e.g. 9 processes)

7

4

Perform Repair

Processes with no outputs or no inputs

1 2 3Many processes with a single input and output (linear flow)

Processes whose inputs are obviously inadequate to yield

outputs

Having data flows terminate at data stores

Connecting data stores directly to each other

Courses StudentsClass List

Connecting entities to anything other than processes

Payroll Department

Employees

Process A

Process B

Process C

DFD Example: Bus Garage RepairsBuses come to a garage for repairs.

A mechanic and helper perform the repair, record the reason for the repair and record the total cost of all parts used on a Shop Repair Order.

Information on labor, parts and repair outcome is used for billing by the Accounting Department, parts monitoring by the inventory management computer system and a performance review by the supervisor.

Key process (“the system”): performing repairs and storing information related to repairs

External Entities: Bus, Mechanic, Helper, Supervisor, Inventory Management System, Accounting Department, etc.

Processes: Record Bus ID and reason for repair Determine parts needed Perform repair Calculate parts extended and total cost Record labor hours, cost

8

DFD Example: Bus Garage Repairs (cont’d) Data stores:

Personnel file Repairs file Bus master list Parts list

Data flows: Repair order Bus record Parts record Employee timecard Invoices

9

10

Bus

Mechanic

Helper0

Bus Repair Process

Supervisor

Accounting

Bus Garage Context Diagram

Mechanical problem to be repaired

Labor

Labor

Fixed mechanical problems

Inventory Management

System

Repair summary

List of parts used

Labor, parts cost details

Bus Garage Diagram 0 DFD

11

Bus Garage Diagram 0 DFD (cont’d)

12

Process Modeling(Chapter 8)

Today’s Lecture Outline

Where are we? Why Process Modeling? What is Process Modeling? Basic Symbols for Process Modeling System Concept vs. Process

Decomposition Decomposition Rules Basic Concepts about a Data Flow Rules for Data Flow

Where are we?

Project ID and Selection

Project Initiation & Planning

Analysis

Logical Design

Physical Design

Implementation

Maintenance

Data ModelingProcess ModelingLogic Modeling

1. Determine system requirements

2. Structure system requirements (ch. 8-10) 3. Generate alternatives for selection

Why Process Modeling?An Information Engineering Approach

Generic IE Approach:

STUDENT

FACULTY

SECTION

COURSE

ADVISING

Student roster

Advisees Listing

Trans-cript

Processenrollment

Processregistration

AssignAdvisor

Students

Courses

SectionRegistration

DataModel

ProcessModel

InfoSystem

What is Process Modeling?

Process modeling - a technique to organize and document the system structure and data flow between system’s PROCESSES and their relevant procedures to be implemented by a system.

Data Flow Diagramming - Another logical modeling tool to

support the process modeling the second step in the Analysis stage

of SDLC

Basic Symbols of Process ModelingData Flow Diagram and Its ComponentsDFD: A diagram about the data flow

between external agents (sources/ sinks) and the processes and data stores within a system

Key Components:

ExternalAgent

An Example - A Data Flow Diagram for a Banking System

An Information System A Generic View

In general, a system could be viewed as a single Process

InformationSystem

0

Source(Customer)

Sink(Mgmt)

Trans Data

Report

You could have multiple sources and sinks!

This generic diagram is called “Context Diagram”

A Context Diagram

An overview of an organizational system that shows the system boundary, sources / sinks that interact with the system, and the major information flows between the entities and the system

A Context Diagram addresses only one process.

An example ...

An Example - A Context Diagram for a Fast-Food IS

A Systematic Way for Process Modeling Process Decomposition

In general, a system could be too complex to understand when viewed as a single Process

We need a Process Decomposition schemei.e., to separate a system into its subsystems (sub-processes), which in turn could be further divided into smaller subsystems until the final subsystems become manageable units (i.e., primitive processes!)

A divide and conquer strategy!!

Level-0 Diagram

A DFD that represents the primary functional processes in the system at the highest possible level

An Example ...

An Example - A decomposed Context Diagram - Level 0 Diagram

An Example - A further decompositionA Level-1 Data Flow Diagram

Process Decomposition RulesGeneric Decomposition Rules: A process in a DFD could be either a

parent process or a child process, or both.

A parent process must have two or more child processes.

A child process may further be decomposed into a set of child processes.

Decomposition Overview

Context Diagram

Level-0 Diagram

Three Major Types of Process

Function Process - A function is a set of related activities of the business (e.g., Marketing, Production, etc.)

Event Process - An event process is a logical unit of work that must be completed as a whole. (e.g., Process customer credit verification)

Primitive Process - a primitive process is a discrete, lowest-level activity/task required to complete an event. (e.g., Check the credit card balance)

Naming Rules for Processes

Function Process - use a Noun! Event Process - Use a general action

verb Process Student registration. Respond to ... Generate ...

Primitive Process - use a strong action verb Validate Student ID Check ... Calculate ...

Rules for Processes

No process can have only outputs (a miracle!)

No process can have only inputs (a black hole!)

No process can produce outputs with insufficient inputs ( a gray hole!)

Can You Identify Errors in This Diagram?

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 What’s wrong?

Black Hole

Miracle

Gray Hole

Processes in a DFDCorrect vs. Incorrect

Incorrect Correct

It has two kinds of flow: a) Inflow to a Data Store (Create/Modify/Delete) b) Outflow from a Data Store (Read)

Orders

Process Order

Cencel Order

Change Order

Address

Summarize Unfilled Orders

OrderCancelled Order

Change of Address Summary of Orders

New Order

Address

Unfilled Order

l

New Order

Order to be

Deleted

2

1

2

2

Read

Delete

Create

Modify

Basic Concept About Data Flows ...

Rules for Data Stores

Data cannot move directly from one data store to another data store -- it must be moved by a process.

Data cannot move directly from an outside source to a data store -- it must be moved by a process.

Data cannot move directly to an outside sink from a data store -- it must be moved by a process.

You need to use a Noun phrase to label each data flow

Data Flows in a DFDIncorrect vs. Correct

Incorrect Correct

Naming Rules Data Flow

Use a singular noun phrase for each data flow Ex: customer data, shipping report, …, etc.

Carry logical meaning only, i.e., no implication on data form or data structure

Minimum flow (no data flooding!!) Should never be “Unnamed!!” -

otherwise, there might be a modeling error.

Naming Scheme for Other DFD Components

Process (Event) - Use an Action Verb Phrase Process member order, Generate bank

statement, ... External Agent (Sink/Source) - Use a

singular descriptive noun Ex: Student, Customer, etc.

Data Store – Use a plural descriptive noun (Members,

Customers, etc.) Or use a noun + file (Inventory file, Goods sold

file)

Basic Rule in DFD Decomposition

Balancing Principle -- the decomposed DFD (I.e., the next lower level DFD ) should retain the same number of inputs and outputs from its previous higher level DFD (I.e., No new inputs or outputs when a DFD is decomposed)

Balancing Principle

Context Diagram

Level-0 Diagram

Unbalanced DFD