1 Chapter Data-Flow Diagrams Table of Contents

Data store
data-flow IN FROM
data-flow OUT TO
property file
property details FROM record
new client
properties TO match properties
buyer details
buyer details FROM record new
buyer
desired property TO match
properties
invoices
invoice FROM invoice client
reminder TO invoice client
These data-flows can be added to the diagram:

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• Check diagram. We now can check the diagram for correctness, and find a process that has
no output data-flow 'archive sale'. An appropriate data-flow, into data store 'invoices' would be
something like 'record of payment'. The consistent and balanced Level 1 DFD now looks as follows:
However, there is another problem with the diagram — what causes the process 'invoice client' to
send an invoice or reminder to the client? The only input to the process 'invoice client' is a 'reminder'
from the 'invoices' data store. The answer is that there are two things that trigger this process to
send a data-flow to the client:
• knowledge that sale has been completed
• knowledge that a payment on an issued invoice is overdue
The second is a time-based event, and not modelled explicitly in Data-Flow Diagrams. However,
the first indicates there should be a data-flow from an external entity to the system indicating that
contracts have been exchanged. If we look carefully at the case study again, we find that:
Note
When a sale is completed, the buyer confirms that the contracts have been exchanged,
client details are removed from the property file, and an invoice is sent to the client.
This must mean that the buyer informs the system that the sale is complete, so we must create a new
data-flow from 'buyer' to 'invoice client' called something like 'confirmation of sale'. (NOTE: Since
we are adding a new data-flow between the system and the external entities, we shall have to update
the parent diagram — if we forget we will be reminded by any CASE tool consistency checker).
We also notice there should be a data-flow of 'client to delete' from process 'invoice client' to the
data store 'property file'.
Our Level 1 DFD now looks as follows:

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