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Chapter 5

Understand the tools of structured analysis(DFD)

System Analysis Design

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Learning Goals

In this module we will learn

What are Data Flow Diagrams (DFDs)?
Why they are useful?
How are they developed?
How to level DFDs?
Good style conventions in developing DFDs
Difference between Logical and Physical DFDs
Tools available to draw DFDs

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Flow-Oriented Modeling

Represents how data objects are transformed at they move through the system.
Data flow diagram (DFD) is the diagrammatic form that is used.
Considered by many to be an “old school” approach, but continues to provide a view of the system that is unique—it should be used to supplement other analysis model elements.

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Why DFD?

If a DFD is too detailed it will have too many data flows and will be large and difficult to understand

Start from a broad overview. Expand to details - Idea similar to using procedures and linking these with a main program

Each DFD must deal with one aspect of a big system

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The Flow Model

Every computer-based system is an

information transform ....

computer

based

system

input

output

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WHAT ARE DATA FLOW DIAGRAMS?

DFDs models the system by depicting

External entities from which the data flows and where results terminate
Processes which transform data flows
Data stores from which the data are read or into which data are written by the processes.

Data Flow Diagrams

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Flow Modeling Notation

External Entity

Process

Data flow

Data store

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External Entity

A producer or consumer of data

Examples: a person, a device, a sensor

Another example: computer-based system

Data must always originate somewhere and must always be sent to something

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Example: External Entity

A Rectangle represents an external entity
They either supply data or receive data
They do not process data

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Process

A data transformer (changes input

to output)

Examples: compute taxes, determine area, format report, display graph

Data must always be processed in some

way to achieve system function

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Example: Process

A circle represents a process
Straight lines with incoming arrows are input data flows
Straight lines with outgoing arrows are output data flows
Processes are given serial numbers for easy reference
Labels are assigned to Data flow.

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Data Flow

Data flows through a system, beginning

as input and transformed into output.

compute

triangle

area

base

height

area

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Data Stores

Data is often stored for later use.

look-up

sensor

data

sensor #

report required

sensor #, type,

location, age

sensor data

sensor number

type,

location, age

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DFD Notation

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Each process should have at least one input and an output.
Each data store should have at least one data flow in and one data flow out.
Data stored in a system must go through a process.
All processes in a DFD go to another process or a data store.

DFD Rules and Tips

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Cont…

Data can flow from

-external entity to process

-process to external entity

-process to store and back

-process to process

Data cannot flow from

-external entity to external entity

-external entity to store

-store to external entity

-store to store

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Good style in drawing DFD

Use meaningful names for data flows, processes and data stores.
Use top down development starting from context diagram and successively levelling DFD
Only previously stored data can be read
A process can only transfer input to output. It cannot create new data
Data stores cannot create new data

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A data flow diagram can dive into progressively more detail by using levels and layers, zeroing in on a particular piece.
DFD levels are numbered 0, 1 or 2, and occasionally go to even Level 3 or beyond.

DFD levels and layers:

From context diagrams to pseudocode

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DFD Level 0 is also called a Context Diagram. It’s a basic overview of the whole system or process being analyzed or modeled.
It’s designed to be an at-a-glance view, showing the system as a single high-level process, with its relationship to external entities.
It should be easily understood by a wide audience, including stakeholders, business analysts, data analysts and developers.

DFD 0

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DFD Level 0 is also called a Context Diagram. It’s a basic overview of the whole system or process being analyzed or modeled.
It’s designed to be an at-a-glance view, showing the system as a single high-level process, with its relationship to external entities.
It should be easily understood by a wide audience, including stakeholders, business analysts, data analysts and developers.

DFD 0

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DFD Level 1 provides a more detailed breakout of pieces of the Context Level Diagram.
You will highlight the main functions carried out by the system, as you break down the high-level process of the Context Diagram into its sub-processes.

DFD-1

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DFD Level 2 then goes one step deeper into parts of Level 1.
It may require more text to reach the necessary level of detail about the system’s functioning.

DFD-2

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Progression to Levels 3, 4 and beyond is possible, but going beyond Level 3 is uncommon.
Doing so can create complexity that makes it difficult to communicate, compare or model effectively.

DFD 3, 4

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Leveling Rules

If process p is expanded, the process at the next level are labeled as p.1,p.2 etc.

All data flow entering or leaving p must also enter or leave its expanded version.

Expanded DFD may have data stores

No external entity can appear in expanded DFD

Keep the number of processes at each level less than

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DFD levels and layers

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Example: Billing System

Going to next process (Continued in next slide)

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Example: Billing System(Cont…)

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