DFD part 2

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DATA FLOW

DIAGRAMPART 2

Course 5

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Creating Data Flow Diagrams

Creating DFDs is a highly iterative process of gradual refinement.

General steps:

1. Create a preliminary Context Diagram2. Identify Use Cases, i.e. the ways in which users

most 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

customer¶s balance

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DFD Rules²Context 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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Context

Diagram of

Order System

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Level-0 DFD Shows the system¶s 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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Context

Diagram of

Order System

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Level-0

DFD of

Order System

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Lower-Level Diagrams Functional Decomposition

Balancing

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Decomposition of DFDs

Functional decomposition An iterative process of breaking a system description

down into finer and finer detail

Uses a series of increasingly detailed DFDs to

describe an IS Act of going from one single system to many

component processes

Repetitive procedure

Lowest level is called a primitive DFD

Level-N Diagrams A 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 Burger¶s 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 Burger¶s 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, B

Level-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

Disaggregateddata flow

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Advanced Rule for DFD

A composite DF on 1 level can be split into componentDF as the next level, but no new data can be added n alldata in the composite must be accounted for in 1 or more subfollows

The inputs to a process must b sufficient to produce the

outputs from the process At the lowest level of DFD, new data flows may be

added to represent data that r transmitted under exceptional condition, these data flows typicallyrepresent error message

To avoid having data flow lines cross each other, u mayrepeat 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 company¶s

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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Context Diagram of Order System

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Level-0 of

Order

System

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HOMEWORK (group) Based on the use cases, draw your project

data flow in:

Context diagram Level 0-DFD

If neccesary, decompose the DFD to the nextlevel DFDs

Put them on your SRS¶s document as theattachment

Documents

DFD part 2