Developing Data Flow Diagrams(DFD).

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Developing Data Flow Diagrams(DFD)

DATA FLOW DIAGRAMS

DFDs models the system by depicting external entities from which the data flows and where results terminate, processes which transform data flows, data stores from which the data are read or into which data are written by the processes.

SYMBOLS USED IN DFD

PROCESS

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• A circle represents a process

• Straight lines with incoming arrows are input data flows

• Straight lines with outgoing arrows are output data flows

• Processes are given serial numbers for easy reference

• Labels are assigned to Data flow. These aid documentation

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A Data Store is a repository of data Data can be written into the data store and this is depicted by an incoming arrow.

Data can be read from a data store and this is depicted by an outgoing arrow External entity cannot read or write to the data store. Two data stores cannot be connected by a data flow

RULES OF DATA FLOW

•Data can flow from

-external entity to process

-process to external entity

-process to store and back

-process to process

• Data cannot flow from

-external entity to external entity

-external entity to store

-store to external entity

-store to store

GOOD STYLE IN DRAWING DFD

• Use meaningful names for data flows, processes and data stores.

• Use top down development starting from context diagram and successively levelling DFD

• Only previously stored data can be read

• A process can only transfer input to output.It cannot create new data

• Data stores cannot create new data

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