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DATA FLOW

DIAGRAM(PART 2)

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Four basic rules

The DFD must have at least one process, and must not

have any freestanding objects or objects connected to

themselves

A process must receive at least one data flow coming

into the process and create at least one data flow leaving

from the process

A data store should be connected at least one process

External entities should not be connected to each other.

Although they communicate independently, that

communication is not part of the system we design using

DFDs

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Creating Data Flow Diagrams

Creating DFDs is a highly iterative process ofgradual refinement.

General steps:

1. Create a preliminary Context Diagram

2. Identify Use Cases, i.e. the ways in which usersmost commonly use the system

3. Create DFD fragments for each use case

4. Create a Level 0 diagram from fragments5. Decompose to Level 1,2,

6. Go to step 1 and revise as necessary

7. Validate DFDs with users.

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Context Diagram

Top-level view of IS

A data flow diagram (DFD) of the scope of an

organizational system that shows the systemboundaries, external entities that interact with the

system and the major information flows between the

entities and the system

Example: Order system that a company uses toenter orders and apply payments against a

customers balance

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DFD RulesContext Diagram

One process, numbered 0.

Sources and sinks (external entities) assquares

Main data flows depicted

No internal data stores are shown

They are inside the system

External data stores are shown as externalentities

How do you tell the difference between aninternal and external data store?

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0

Order

System

SALESREP

CUSTOMER WAREHOUSE

BANKACCOUNTING

Order

Order

Reject

Notice

Picking

List

Completed

Order

Payment Invoice

Commission Bank

Deposit

CashReceipts

Entry

Context

Diagram of

OrderSystem

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Level-0 DFD

Shows the systems major processes, data flows,

and data stores at a high level of abstraction

When the Context Diagram is expanded into DFDlevel-0, all the connections that flow into and out of

process 0 needs to be retained.

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0

OrderSystem

SALES

REP

CUSTOMER WAREHOUSE

BANKACCOUNTING

Order

OrderReject

Notice

Picking

List

Completed

Order

Payment Invoice

Commission Bank

Deposit

Cash

ReceiptsEntry

Context

Diagram of

OrderSystem

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1.0

Fill

Order

2.0

Create

Invoice

3.0

Apply

Payment

SALES

REP BANK ACCOUNTING

CUSTOMER WAREHOUSE

Order

Order

Reject

Notice

Picking List

Accounts

ReceivableD1

Invoice

Invoice

Invoice

DetailPayment

Detail

Payment

Commission Bank Deposit Cash Receipts Entry

Completed

Order

Level-0

DFD of

Order

System

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Lower-Level Diagrams

Functional Decomposition

Balancing

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Decomposition of DFDs

Functional decompositionAn iterative process of breaking a system description

down into finer and finer detail

Uses a series of increasingly detailed DFDs to

describe an ISAct of going from one single system to many

component processes

Repetitive procedure

Lowest level is called a primitive DFD

Level-N DiagramsA DFD that is the result of n nested decompositions of

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

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Balancing DFDs

When decomposing a DFD, you must conserveinputs to and outputs from a process at the nextlevel of decomposition

Ensures that the input and output data flows of

the parent DFD are maintained on the child DFD This is called balancing

Example: Hoosier Burgers In Figure 1, notice that there is one input to the

system, the customer order Three outputs:

Customer receipt

Food order

Management reports

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Figure 1

Context diagram of Hoosier Burgers Food ordering

system

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Balancing DFDs Example

(Continued)

Notice Figure 2. We have the same inputs

and outputs

No new inputs or outputs have been

introduced

We can say that the context diagram and

level-0 DFD are balanced

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Figure 2

Level-0 DFD of Hoosier Burgers food ordering system

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Balancing DFDs:

An unbalanced example Figure 3:

In context diagram, we have one input to the

system, A and one output, BLevel-0 diagram has one additional data flow,

C

These DFDs are not balanced

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Figure 3: An unbalanced set of data

flow diagrams

SOURCE

SINK

(a) Context diagram

(b) Level-0 diagram

SOURCE 1

SINK

0

SOURCE 2

1.0

2.0

A B

A

C

B

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Balancing DFDs

We can split a data flow into separate data

flows on a lower level diagram (see Figure

4) Balancing leads to four additional

advanced rules

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Example of data flow splitting

x.0Payment & coupon

x.1Payment

x.2Coupon

Composite data

flow

Disaggregated

data flow

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Advanced Rule for DFD A composite DF on 1 level can be split into component

DF as the next level, but no new data can be added n alldata in the composite must be accounted for in 1 ormore subfollows

The inputs to a process must b sufficient to produce theoutputs from the process

At the lowest level of DFD, new data flows may beadded to represent data that r transmitted underexceptional condition, these data flows typicallyrepresent error message

To avoid having data flow lines cross each other, u may

repeat data strores or sources/sinks on a DFD. Use anadditional symbol, like a double line on the middlevertical line of data strore symbol, or a diagonal line in acorner of a sink/source square to indicate a repeatedsymbol

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Strategies for Developing DFDs

Top-down strategy

Create the high-level diagrams (Context

Diagram), then low-level diagrams (Level-0diagram), and so on

Bottom-up strategy

Create the low-level diagrams, then higher-level diagrams

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Exercise:

Precision Tools sells a line of high-quality woodworkingtools. When customers place orders on the companys

Web site, the system checks to see if the items are in

stock, issues a status message to the customer, and

generates a shipping order to the warehouse, which fills the

order. When the order is shipped, the customer is billed.The system also produces various reports, such as

inventory reports for Accounting.

Draw a context diagram for the order system

Draw DFD diagram 0 for the order system

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Identify Entities,Process,Data Stores & Data Flow

Entities

Customer

Warehouse

Accounting

Processes

1.0 Check Status 2.0 Issue Status Messages

3.0 Generate Shipping Order

4.0 Manage Accounts

Receivable

5.0 Produce Reports

Data Stores

D1 Pending Orders

D2 Accounts Receivable

Data Flows Order

In-Stock Request

Order Data

Status Data

Status Message Shipping Order

Order Data

Invoice

Shipping Confirmation

Payment

Accounting Data

Accounts Receivable Data

Order Data

Inventory Reports

1.0

2.0

3.0

4.0

5.0

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ACCOUNTING

WAREHOUSECUSTOMER

0

Order

System

Order

Payment

In-Stock

Request

StatusMessage

Invoice Shipping Confirmation

ShippingOrder

InventoryReports

Context Diagram of Order System

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1.0

CheckStatus

2.0

IssueStatus

Messages

3.0

GenerateShippingOrder

ACCOUNTING

CUSTOMER WAREHOUSE

4.0

ManageAccountsReceivable

5.0

ProduceReports

Order In-Stock Request

Status Data

Status

Message

PendingOrdersD1

OrderData

Order Data

ShippingOrder

ShippingConfirmation

Invoice

Payment

AccountsReceivableD2

Accounting Data Accounts Receivable Data

Order Data

InventoryReports

Level-0 of

Order

System

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