Systems Analysis & DesignSystems Analysis & Design

Data Flow DiagramsData Flow Diagrams

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Data Flow Diagrams – Data Flow Diagrams – DefinitionDefinition

A A data flow diagramdata flow diagram is a pictorial model that is a pictorial model that shows the flow of data between the various shows the flow of data between the various subsystems or processes of a systemsubsystems or processes of a system

The purpose of the data flow diagram is to The purpose of the data flow diagram is to highlight…highlight…

the functions of a system and to show how the system stores information and the how information is transferred between data stores

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Data Flow Diagrams – Data Flow Diagrams – Definition(2)Definition(2)

A data flow diagram A data flow diagram shows how data flows shows how data flows around a systemaround a system

An external source of data

Some Process

A System

Abstracted to DFD

Data Storage

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Data Flow Diagrams – Data Flow Diagrams – Logical/PhysicalLogical/Physical

Logical DFDsLogical DFDso Show how the business operateso Show processes independently of the

implemented system

Physical DFDsPhysical DFDso Show how the system operates or how

the system will be implementedo Processes are not independent of the

implemented system

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Data Flow DiagramsData Flow Diagrams – – GeneralGeneral

A data flow diagram is an important analysis A data flow diagram is an important analysis tools for analysing data-oriented systemstools for analysing data-oriented systems

In a DFD, data is organized in several ways. In a DFD, data is organized in several ways. We have…We have…

• data storesdata stores

• • data flow data flow

• • processesprocesses

• • datadata transfer to or from the external environmenttransfer to or from the external environment

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Data Flow Diagrams – Data Flow Diagrams – External EntityExternal Entity

External EntityExternal Entity

The source or destination of the data outside the The source or destination of the data outside the system system

This may be a person or organisation, outside the This may be a person or organisation, outside the system boundaries that supplies data to the system or system boundaries that supplies data to the system or receives data from the systemreceives data from the system

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Data Flow Diagrams – Data Flow Diagrams – Data FlowData Flow

Data FlowData Flow

Data flow is data or information that is Data flow is data or information that is transferred to or from a process (input or output)transferred to or from a process (input or output)

The data flow name must be a nounThe data flow name must be a noun

a data flow must be input to a process or output from a a data flow must be input to a process or output from a processprocess

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Data Flow Diagrams – Data Flow Diagrams – Data ProcessData Process

Data ProcessData Process

A process is an activity of the systemA process is an activity of the system

A process name should be either a subsystem name or A process name should be either a subsystem name or a kind of verb-adjective-noun for describing the a kind of verb-adjective-noun for describing the processprocess

System e.g. Southwark Library System

Subsystem e.g. Student Loan SubsystemVerb-adjective-noun e.g. Update Loan Info

every process must have at least one input and one every process must have at least one input and one output data flowoutput data flow

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Data Flow Diagrams – Data Flow Diagrams – Data StoresData Stores

Data StoreData Store

A data store is an inventory of data (i.e. data that A data store is an inventory of data (i.e. data that is stored for later use)is stored for later use)

The data store name must be a nounThe data store name must be a noun

in physical DFD’s, the format of storage may also be in physical DFD’s, the format of storage may also be includedincluded

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Data Flow SymbolsData Flow Symbols

External source or destination dataExternal source or destination data

Process that transforms data

File or other data storage

Flow of data

A data flow diagram can consist of four symbolsA data flow diagram can consist of four symbols

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Data Flow Symbols – Data Flow Symbols – External EntityExternal Entity

A square rectangle indicates an external entity A square rectangle indicates an external entity that sends data into the system, or receives data that sends data into the system, or receives data from the system.from the system.The entity may be a person or organisation that is not part of the system.

E1

Customer

IdentifierIdentifier

NameName

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Data Flow Symbols – Data Flow Symbols – Data FlowData Flow

A line represents a route by which data A line represents a route by which data flows from one element to anotherflows from one element to another

Customer Customer DetailsDetails

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Data Flow Symbols – Data Flow Symbols – Data ProcessData Process

A rounded rectangle represents a process that the A rounded rectangle represents a process that the system performs on the data.system performs on the data.

Processes are the changing of data by performing calculations, updating or sorting, either manually or by computerisation.

LocationLocationIdentifierIdentifier

Process Process DescriptionDescription

P1 Accounts

Verify customer details

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Data Flow Symbols – Data Flow Symbols – Data StoreData Store

The open rectangle indicates stored data. It is The open rectangle indicates stored data. It is referred to as a referred to as a data repositorydata repository or or data storedata store. .

Examples of data stores are filing cabinets, floppy disks, Examples of data stores are filing cabinets, floppy disks, magnetic tape, paper records.magnetic tape, paper records.

S1 Inventory

NameNameIdentifierIdentifier

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Valid Data FlowValid Data Flow

External entity to processExternal entity to process E.G. Data flowing into the systemE.G. Data flowing into the system

and vice versaand vice versa

Process to external entityProcess to external entity E.G. Data flowing out of the systemE.G. Data flowing out of the system

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Valid Data FlowValid Data Flow

Process to data storeProcess to data store E.G. Writing data to a diskE.G. Writing data to a disk

and vice versaand vice versa

Data store to processData store to process E.G. Reading data from a diskE.G. Reading data from a disk

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Valid Data FlowValid Data Flow

Process to ProcessProcess to Process E.G. Passing data from one process to anotherE.G. Passing data from one process to another

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Data Flow Diagrams – Data Flow Diagrams – RulesRules

All data flows to stores must All data flows to stores must come from a processcome from a process

External Entity Data StoreData Store

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Data Flow Diagrams – Data Flow Diagrams – RulesRules

No MiraclesNo Miracles

ProcessProcess

A miracle is a A miracle is a process with no process with no inputinput

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Data Flow Diagrams – Data Flow Diagrams – RulesRules

No Black HolesNo Black Holes

ProcessProcess

Data Storage A black hole A black hole is a process with is a process with no outputno output

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Data Flow Diagrams – Data Flow Diagrams – Common ErrorsCommon Errors

Miracle:Miracle: Process with no input Process with no input

Black Hole:Black Hole: Process with no output Process with no output

Data flows not Data flows not namednamed

Processes that leave the data Processes that leave the data unchangedunchanged

Data flows to processes that are neither Data flows to processes that are neither inputsinputs or or outputsoutputs..

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Data Flow Diagrams – Data Flow Diagrams – GuidelinesGuidelines

Place external entities on the outside of the diagramPlace external entities on the outside of the diagram

Place data stores and processes in the middle of the Place data stores and processes in the middle of the diagramdiagram

Try not to cross flow linesTry not to cross flow lines

Do not duplicate processes, external entities or data Do not duplicate processes, external entities or data storesstores

For each individual data flow fragment, try to fit the For each individual data flow fragment, try to fit the diagram on one page.diagram on one page.

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Data Flow Diagrams – Data Flow Diagrams – ExampleExample

Library SystemLibrary SystemData Flow Diagram Data Flow Diagram

FragmentFragment

P1

Joining

Library Customer

E1E1 Confirmation Confirmation DetailsDetails

Library Joining Library Joining DetailsDetails

Update DatabaseUpdate DatabaseBorrowers Database

S1S1