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CIS 551

Data Flow Diagrams class

file: dataflows.ppt

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CIS 551

Managerial mapping tools

• Budgets, financial reports

• Architectural and geographical blueprints and maps

• Flowcharts

• Timelines, Gantt and Pert charts

• Mission statements

• Organizational charts

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CIS 551

Understanding & documenting processes

• Entity-relationship models– Show logical structure of data needed to

support business process

• Data flow diagrams– Show what is done

• Flowcharts– Show how it is done

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CIS 551

Information System Analysis & Design

2 keys to Information System Development:

* Entity-Relationship Model - analysis anddesign of information structure in a system.

* Data Flow Diagram - analysis and designof the information flow (actually more of thebusiness processes) in a system.

Concept behind this traditional approach:Procedure + Data Structure = Program

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E-R Diagrams

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Data Flow Diagrams (DFDs)

• DFDs are graphic representations of systems

• DFDs map how data flows through interconnected processes

• Although the name implies data, the emphasis is on processes

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DFD symbols

• DFDs consist of only four symbols:» Environmental (external) elements» Processes» Data Flows» Data Storage

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DFD symbols

Environmental (external) elements

Processes

Data Flows

Data Storage

SupplierAccounting dept.

Print invoice Compute net pay

General ledger

invoice

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Traditional Approach in DFD

Hierarchical Decomposition of Functions - decomposing function process into smaller ones which together accomplish the function.

• Top Down Design

• Bottom Up Implementation

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DFD Examples

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Environmental elements

External elements are those that exist outside the boundaries of the system.

They can be persons, organizations, or other systems.

In a way, they serve to define the system’s boundary.

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Data Flows

Data flows are any collection of logically related data elements that travel from one process to another. They can be of any size.

Think of a record, a note, a phone call, a letter, or a file.

Data flows can converge or diverge, they can be one or two way flows.

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Processes

Processes are transformations of inputs into outputs.

It is best to label each process as verb, or verb + object.

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Data Storage

Data stores hold data (while flows are about data-in-motion).

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Leveled Data Flow Diagrams

• Context level diagram

• Level 0 diagram (“top level”)

• Level n figures (diagrams)

• pyramid, granularity.

• How deep?

• Structured English, pseudocode, flowcharts?

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Steps to develop DFD (1)

Conceptualize data flows from top down–

1. Make a list of business activities and use it todetermine various:external entities, data flows, processes,

data stores.

2. Create a Context Diagram: with externalentities and data flows to and from the system- which is the only process, labeled as #0.The Context DFD should not have any datastores, nor any detailed processes.

3. Draw the next level DFD - Diagram 0.

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Steps to develop DFD (2)

3. Draw the next level DFD ΠDiagram 0. Itshould show major business processes andpossible data stores at this level.

4. Create a Child Diagram for each process inDiagram 0, exploding if necessary.

5. Check for errors and designate good names.

6. Use the DFD to reason about improving theoperation by re-organizing business processes.

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Context level diagrams

• Use only a single process model

• Process represents entire system

• No numbers

• All external entities

• Show all in- and outflows of info

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“Figure “n” diagrams

• Each level “explodes” one process on preceeding level

• Each level numbered for process it explodes

• Include up to seven processes on single diagram

• Stopping rule?

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Exploding…

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Diagramming guidelines

• Label each data flow: use unique names

• Keep data flow names constant across levels

• Avoid (diagnose) read-only processes

• Write-only processes are only allowed under one condition...

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Creating Diagram 0

1. Start with input from an external entity: What happens to this data entering the system?

Is it stored?Is it input for several processes?

2. Work backwards from output data flow:Where does this data come from?Is it calculated on demand, or was it stored in a file?

3. Examine data flow to and from a data store:Which processes put data into the store?Which processes use the data?Which processes are responsible for update?

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Whither and Why DFDs?

• (Most?) Natural way to document a system

• Simple, common communication

• Simple, common language

• Supported by- and support the creation of CASE

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Accounts Payable Problem

Consider the following scenario.

Each month you receive bills from various vendors (e.g., phone, auto insurance, credit cards, utilities) and periodically other billings (e.g., magazine subscriptions, university tuition and fees). Consider all of these billings as being examples of invoices from vendors. Like most people you don't pay your bills on the same day that they are received but rather review each invoice and prioritize them in terms of which you will pay first. These reviewed and prioritized invoices are then placed in an old shoe box until you decide you had better start paying some of them.

You remove an invoice from the shoe box and prepare a check to be mailed to the vendor. You check the old balance in your check register and update the register with the check information and compute a new balance. You may do this several times a month.

At the end of the month you receive a statement from your bank that lists the checks that have cleared your account. You (should) take your check register and compare your list of checks and their amounts with the statement that you received from the bank. If everything is in balance then you place an appropriate mark in your check book else you enter a correcting entry (the bank never makes a mistake).

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Accounts Payable (AP) DFD

Vendor

Bank

AP

Invoice Check

Statement

Vendor

Bank

1.0Approve &Prioritize

Bills

Shoe Box

Prioritized Bills

2.0PrepareCheck

Selected Bill

Check

Check Register

Old Balance

Check Infoand NewBalance

3.0CompareRegister

Info

Balanced CheckbookContextDiagram

Invoice

Statement

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Order Entry Problem

SAM's is a mail order catalogue store. A customer fills out the sales order form that is enclosed in the catalogue and mails it to their headquarters in the Bahamas (postage prepaid). Upon receipt, a clerk opens the envelopes and removes the sales order forms. The clerk gives the forms to a data entry operator, who keys the data onto a diskette using a microcomputer. After data entry, the sales orders are filed in a Sales Order History file. The diskette containing all of the sales order data is then read by a minicomputer. For each ordered item, the Inventory Master Record is retrieved from the Inventory master files and the balance on-hand is reduced by the ordered amount. The updated Inventory Master Record is written back to the file. The same program prints picking tickets and writes a Daily Sales Report file onto the system disk. The picking tickets are input to the order filling system where they are used by warehouse personnel in picking the merchandise from the shelves for shipment to the customers. Another minicomputer program reads the Daily Sales Report file and prepares a daily sales report that goes to the sales manager.

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Sales Order (SO) DFD

Customer

OrderFillingStation

SalesManager

ProcessSales Order

Sales Order

DailySalesReport

PickingTickets

Context Diagram

Customer1.0

Open MailSalesOrder

2.0Enter Order Sales Order

History File

Sales Order Form

EnteredSales Orders

3.0Update

InventoryMaster File

InventoryMaster File

Inventory Master Recordsto be updated

UpdatedInventory Master Records

4.0Print Daily

Sales Report SalesManager

OrderFillingStation

EnteredSales

DailySales

Daily Sales Report

PickingTickets

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Prepare Check Flowchart for AP 2.0

Start

Remove next bill

Get balance

BalanceGT Bill

?

Write Check

Update Register

MoreBills?

Stop

```

Yes

No

Yes

Returnbill to box

No

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Open Mail Flowchart for SO 1.0

Start

Open Mail

Remove Sales Order

TakeS.O. to

Operator

MoreMail?

Stop

```

No

Yes

Start

Open Mail

Remove Sales Order

Place in Pile

MoreMail?

```

Takepile to

Operator

Stop

Yes