Data Flow Diagram

DFD

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ICT Level VCOT - Jaffna

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A Data Flow Diagram (DFD) is a graphical representation of the "flow" of data through an information system, modeling its process aspects.

Data Flow Diagram (DFD)

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Examples of LCs

DFDs are attractive technique because they provide what users do rather than what computers do.

Data Flow Diagrams can be used in both Analysis and Design phase of the SDLC.

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DFDs only involve four symbols.

• Entity

• Process

• Data store

• Data flow

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There are two types of symbols:

Symbol Gane & Sarson Symbol

DeMarco & Yourdan Symbol

Entity

Process

Data store

Data flow

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Descriptions of Symbols: external entity - people or organisations

that send data into the system or receive data from the system.

process - models what happens to the data i.e. transforms incoming data into outgoing data.

data store - represents permanent data that is used by the system.

data flow - models the actual flow of the data between the other elements.

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External Entity Noun

Data Flow Names of data

Process verb phrase

Data Store Noun

Symbol naming

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The Levels

Levels Description Explanation

Level 0

Context diagramContains only one process

Level 1

Overview diagramUtilizes all four elements

Level 2

Detailed diagramA breakdown of a level 2 process

There is no rule as to how many levels of DFD that can be used.

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A Context Diagram (Level 0)

The major information flows between the entities and the system.

A Context Diagram addresses only one process.

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Rules for Level 0 Diagram : 1 process represents the entire system.

Data arrows show input and output.

Data Stores NOT shown. They are within the system.

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Rules for Level 1 Diagram : Level 1 DFD, must balance with the context

diagram it describes.

Input going into a process are different from

outputs leaving the process.

Data stores are first shown at this level.

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Rules for Level 2 Diagram : Level 2 DFD must balance with the Level 1 it

describes.

Input going into a process are different from

outputs leaving the process.

Continue to show data stores.

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Numbering On level 1 processes are numbered 1,2,3…

On level 2 processes are numbered x.1, x.2, x.3… where x is the number of the parent level 1 process.

Number is used to uniquely identify process not to represent any order of processing

Data store numbers usually D1, D2, D3...

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DFD Example

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Common Rules :1. All processes must have at least one data flow

in and one data flow out.

2. All processes should modify the incoming data, producing new forms of outgoing data.

3. Each data store must be involved with at least one data flow.

4. Each external entity must be involved with at least one data flow.

5. A data flow must be attached to at least one process.

6. In DFDs, all arrows must be labeled. S.Sakthybaalan

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Common errors in DFD

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DFD Example: Bus Garage Repairs

Buses come to a garage for repairs.

A mechanic and helper perform the repair, record the reason for the repair and record the total cost of all parts used on a Shop Repair Order.

Information on labor, parts and repair outcome is used for billing by the Accounting Department, parts monitoring by the inventory management computer system and a performance review by the supervisor.

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Bus Garage Repairs (cont’d) External Entities:

Bus, Mechanic, Helper, Supervisor, Inventory Management System, Accounting Department

Key process (“the system”): performing repairs and storing information related to repairs.

Processes: Record Bus ID and reason for repair, Determine parts needed, Perform repair, Calculate parts extended and total cost, Record labor hours, cost

Data stores: Personnel file, Repairs file, Bus master list, Parts list

Data flows:Repair order, Bus record, Parts record, Employee timecard, Invoices

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Any Questions ?

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