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Data Flow Diagrams class

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1 Data Flow Diagrams class
file: dataflows.ppt

2 Managerial mapping tools
Budgets, financial reports Architectural and geographical blueprints and maps Flowcharts Timelines, Gantt and Pert charts Mission statements Organizational charts

3 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

4 Information System Analysis & Design
2 keys to Information System Development: * Entity-Relationship Model - analysis and design of information structure in a system. * Data Flow Diagram - analysis and design of the information flow (actually more of the business processes) in a system. Concept behind this traditional approach: Procedure + Data Structure = Program

5 E-R Diagrams

6 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

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

8 DFD symbols Environmental (external) elements Processes Data Flows
Data Storage Supplier Accounting dept. Print invoice Compute net pay invoice General ledger

9 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

10 DFD Examples

11 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.

12 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.

13 Processes Processes are transformations of inputs into outputs.
It is best to label each process as verb, or verb + object.

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

15 Leveled Data Flow Diagrams
Context level diagram Level 0 diagram (“top level”) Level n figures (diagrams) pyramid, granularity. How deep? Structured English, pseudocode, flowcharts?

16 Steps to develop DFD (1) Conceptualize data flows from top down–
1. Make a list of business activities and use it to determine various:external entities, data flows, processes, data stores. 2. Create a Context Diagram: with external entities and data flows to and from the system - which is the only process, labeled as #0. The Context DFD should not have any data stores, nor any detailed processes. 3. Draw the next level DFD - Diagram 0.

17 Steps to develop DFD (2) 3. Draw the next level DFD ΠDiagram 0. It
should show major business processes and possible data stores at this level. 4. Create a Child Diagram for each process in Diagram 0, exploding if necessary. 5. Check for errors and designate good names. 6. Use the DFD to reason about improving the operation by re-organizing business processes.

18 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

19 “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?

20 Exploding…

21 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...

22 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?

23 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

24 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).

25 Accounts Payable (AP) DFD
Invoice Vendor 1.0 Approve & Prioritize Bills Vendor Prioritized Bills Check Invoice Check 2.0 Prepare Check Selected Bill AP Shoe Box Check Info and New Balance Statement Bank Old Balance Check Register Context Diagram Balanced Checkbook 3.0 Compare Register Info Bank Statement

26 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.

27 Sales Order (SO) DFD Process Sales Order Context Diagram Sales Order
1.0 Open Mail Customer Customer Sales Order Sales Order Form 2.0 Enter Order Entered Sales Orders Sales Order History File Process Sales Order Entered Sales Inventory Master Records to be updated 3.0 Update Inventory Master File Daily Sales Report Inventory Master File Picking Tickets Updated Inventory Master Records Order Filling Station Daily Sales Sales Manager Order Filling Station Picking Tickets 4.0 Print Daily Sales Report Context Diagram Sales Manager Daily Sales Report

28 Prepare Check Flowchart for AP 2.0
Start Remove next bill Get balance Balance GT Bill ? No Yes Write Check Return bill to box Update Register ``` Yes More Bills? No Stop

29 Open Mail Flowchart for SO 1.0
Start Start Open Mail Open Mail Remove Sales Order Remove Sales Order Place in Pile Take S.O. to Operator ``` Yes More Mail? ``` Yes More Mail? Take pile to Operator No Stop Stop

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