1 Chapter Data-Flow Diagrams Table of Contents

Data-Flow
Sender
Customer
video loan
system - loan of video
customer
customer details
customer
system - create new customer
membership card
customer
system - loan of video
membership card
system - create new customer
customer
request for video
customer
system - loan of video
payment
customer
system - loan of video
return of video
customer
system - loan of video
overdue reminder
system - loan of video
customer

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Data-Flow Diagrams
27
Data-Flow
Sender
Customer
available titles
supplier
system - stock control
order
system - stock control
supplier
payment
system - stock control
supplier
requested videos
supplier
system - stock control
• Draw the data-flows between the external entities and processes. After creating process boxes
and drawing the data-flows the diagram looks as follows:
• Identify data stores by establishing where documents / data needs to be held within the system.
Add the data stores to the diagram, labelling them with their local name or description.
There seem to be 2 main data stores required: a store of customer details 'customer file' and a store
of which videos are in stock 'stock file'.
After adding these to the diagram looks as follows:
• Add data-flows flowing between processes and data stores within the system. Each data store
must have at least one input data-flow and one output data-flow.
We can create a table to indicate which processes send and receive data from each data store:
Data store
data-flow IN FROM
data-flow OUT TO
customer file
customer details FROM create
new customer
customer details TO loan of
video
stock file
new video details FROM stock
control
overdue items TO loan of video

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Data-Flow Diagrams
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After adding these data-flows the diagram looks as follows:
• check diagram No record seems to be made of when a video is lent to a customer — there ought to
be a data-flow from 'loan of video' to 'stock file' called something like 'item on loan'. Likewise when
an item is returned the details should be recorded in a data-flow called something like 'item returned'.
Apart from these extra two data-flows the diagram appears to be correct.
So our Level 1 DFD for the Video Rental case study is now: