data flow diagrams cape computer science unit 2 t.batchelor in collaboration with y.c.lemard

DATAFLOW

DIAGRAMS

CAPE COMPUTER SCIENCE UNIT 2

T.Batchelor in collaboration with Y.C.LeMard

Introduction

We’ve learnt that a computer takes in data, processes it, and produces information as output

Data input process Information output

We also know that the processing is really done by a program.

Therefore a program (or a set of programs) receives data, does something to it, and produces information for the user

Introduction

process = program

Introduction

If you think about it carefully you will acknowledge that this is what all the programs you have written up to this point do.

And hopefully this is what your I.A. program will do too.

Introduction

Data input

PROGRAM

Information output

Data Flow Diagrams

A Data flow Diagram (DFD) is a pictorial representation of this flow of data through a computer system.

It is an essential systems analysis and design tool

DFDs

Here is an example of a Data Flow Diagram

DFDs

Here is another example

DFDs

Luckily our DFDs will not need to be so complex.

But from these examples we can isolate the components of a DFD

DFDs

Data Flow Diagrams have 4 symbols

1. Sharp cornered, shadowed rectangles

2. Rounded cornered rectangles

3. Opened sided rectangles

4. Arrows

1. External Entities

The sharp cornered rectangles (or simply boxes) in a DFD indicate entities.

Entities are people, things, organizations, etc outside of the system.

Entities are the source of data and the destination of data.

External EntityExternal Entity

Entities

Student

Teacher

Customer

Doctor

Patient

User

Manager

Cashier

External Entities

As you will have guessed, all external entities must be labeled.

It must be obvious too that every system must have at least one entity

Ms LeMard

2. Processes

The rounded cornered rectangles in a DFD indicate processes.

Processes are the changes that data go through causing it to be transformed gradually into information.

Process

Every computer program is a process

Processes

Accounting system

Marketing system

Shipping system

Grading system

Patient Administration system

Reservation system

Processes

Just like entities all processes must have a name.

Medical Expert System

Also every computer system has at least one process

3. Data Store

File Name

This is a generally accepted sign for a data store (file)

Data Store

Employees’ Data

Accounts Receivable File

Order Details

Students’ Grades

Data Store

A ‘data store’ is a secondary storage device which holds a file.

Most meaningful systems store permanent data in one or more master files. Transactions files are also used in batch processing systems

Data Store

Not every system has a file however, some are totally interactive.

In addition very simple DFDs [ Context Level DFDs ] do not include any data stores

4. Data

The arrows in a DFD show the data. Data must come from somewhere and go to somewhere else. The direction of the arrow indicates this.

All arrows must be labelled to show what particular piece of data is being referred to.

Data

Data

Name

Date

Grade

Amount

Symptom

• Some texts use a circle for the process

• Some texts also use a different sign for the file depending on the type of media on which it is stored.

FYI

Process

We will stick to the symbols below …

Types of DFD

The Top-Down approach is taken to the design of DFDs.

Data flow diagrams are therefore classified by levels. Each subsequent level indicates greater details.

Levels

Level 0 DFD

The Level 0 DFD (also known as the Context Level DFD) is the simplest DFD. This diagram shows a single process and all external entities which interact with this process, and the data flowing to and from the process.

Context Level DFD for a Mail Order Business

Customer Order Processing

Order

Delivery

Decomposition

Level 1 DFDs

A Context Level DFD is broken down (exploded) into a set of processes, the data moving between these processes, the entities interacting with these processes and the data stores.

Level 1 DFD for a Mail Order Business

Customer Sales processing

Order

Delivery

Accounting System

Customer Database

OrderCredit Status

Orders

Order

Credit Status

Customer number

Decomposition

Level 2 and other Level DFDs

Each process from Level 1 is exploded even more into sub processes. This decomposition continues for each subsequent level. The number of levels possible depends on the complexity of the system.

The Value of a DFD

With a dataflow diagram, users are able to visualize how the system will operate, what the system will accomplish, and how the system will be implemented.

Dataflow diagrams can be used to provide the end user with a physical idea of how the data they input ultimately has an effect upon the structure of the whole system.

The Value of a DFD

The old system's dataflow diagrams can also be drawn up and compared with the new system's dataflow diagrams to draw comparisons in order to help implement a more efficient system.

Context Level DFD for a Payroll System

SupervisorSupervisor

Payroll System

Hours worked

Pay cheque

EmployeeEmployee

Personal Data

Level 1 DFD for a Payroll System

SupervisorSupervisor

EmployeeEmployee

Batching system Personnel system

SalaryGeneration

System

Employee Database

Accounting data

Hours

worked

PersonalData

Employeerecord

Hoursworked

Paycheques

Accountingdetails

Accountingdetails

Employeepersonaldata

How to Create A Level 0 Data Flow Diagram

1. Identify your main system’s focus

2. Identify the external people/ organisations who interact with the system

3. Decide what data these entities will enter into the system

4. Determine what these entities expect as output from the system

How to Create A Level 1 Data Flow Diagram

1. Focus on your process and break it into 2 or more sub-processes

2. Identify what data flows between these processes and between the entities

3. Identify what permanent data files are used in this system.

4. Note that no new entities can be introduced

BEWARE

1. Data can not flow directly from file to file.

2. Data can not flow directly from entity to entity

3. No process can have no input data yet have output data, that would be a miracle.

4. No process can have input data yet have no output data, that would be a black hole.

Your Questions ?

Questions

1. What is the purpose of a data flow diagram?

2. What 4 symbols are used in a DFD?

3. Which symbols must appear on every DFD?

4. Every process must have data going in and data coming out. Why?

Exercise

1. Draw a Level 0 Data Flow Diagram for the following situation:

A travel agency takes calls from clients, checks seat availability on the flights they desire and makes a reservation for them if all is clear.

2. Explode your Context Level diagram into a Level 1 DFD

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