Data Flow Diagrams Continued
Checking Your Diagram
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Label flows uniquely.
Put boundaries onto each diagram.
Data flows are data only – not orders.
Each process should have a unique reference number.
Each datastore should have a unique number and name.
All diagrams should be labelled.
Check levelling on diagrams.
A data flow should have a process as a source or sink.
What to leave out
• Enquiry processes
– These processes neither add nor take away data.
• They do not show the flow of data through the
system.
• They are not included in a data flow diagram.
• Process triggers
– Show only data flow, not control flow
Stages of DFDs
Current
System
Required
System
Physical System
CPDFD
Current Physical
Logical System
CLDFD
Current Logical
How the existing
system works
RPDFD
Required Physical
What the existing
system does
RLDLD
Required Logical
How the new system is
to be implemented
What the new
system has to do
8
Physical DFDs
• Symbols on the diagram represent physical objects
the user can recognise.
• Data flows or stores.
– Bits of paper, telephone call, ledger book, filing
cabinet.
• Material flows.
– Physical stock changing hands carries implicit data with
it.
– It may be easier for a user to understand the physical
flow.
Physical DFDs
• Show how the current system is implemented.
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Who does a particular job.
Where it is done.
When it is done.
How.
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A flow is communicated – e.g. By telephone.
Data is stored (box, list, etc).
Sequence of data stored.
Nature of the storing document.
CPDFD
• Describes how the system works now.
• Includes manual data stores.
– E.g. The price list on the wall.
• Can include material flows.
– E.g. The stock going out of Joe’s Yard.
DFD Logicalisation
• Remove physical aspects.
– Remove references to ‘who’, ‘when’, ‘where’,
‘how’ a job is done or data is stored.
– Remove material flows, retaining implicit data
flow.
– Remove current processes that are merely
mechanisms to support the current physical
realisation of the system.
DFD Logicalisation
• Improve logic of stored data.
– If several data stores contain the same data,
combine / rearrange them.
– If a single data store contains everything, split it
into logical entities.
– Remove transient data stores (i.E. Datastores
that are used temporarily during the running of
the system).
CLDFD in summary
• The physical aspects of the system are
removed.
• The data flows and stores are abstracted to
give logical data flow.
• The data stores may be split, merged or
renamed.
Presentation of Current System
• The current physical and logical systems are
presented to the user.
• The user describes further requirements.
• The user gives a list of problems with the
current system.
• The designer lists the future requirements
and current problems.
Metamorphosis
• The designer designs a solution to what has
to be done.
• Ideally, this is independent of how it is to be
done.
– The only point that needs to be ascertained here
is that the proposed solution is feasible.
• The designer formulates a required logical
Data flow diagram.
RLDFD
• Describes what the new system has to do.
– includes all that is in the CLDFD.
– includes new requirements for the new system.
– solves any current problems.
• This is a Logical diagram.
– There are no manual data stores.
– There are no physical flows.
RPDFD
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Describes how the new system will work.
Which parts are automated or manual.
What physical devices will be used.
What input and output forms used.
Not all stages may be needed or produced.
Required physical data flow.
– Which parts remain manual.
– Which physical devices will be used.
– Input / output forms used.
Current
Physical DFD
Current Physical DFD
1
Price
Pick
and
pay
choice
Customer
Shop
choice
Price
list
M4
4
Duplicate
docket
M3
required stock
Order
book
3
docket
Yard
Supply invoice
Accept
supplies
Docket
*
docket
financial
report
5
Supply receipts
Joe's Office
Produce
financial
report
Supply order
Supply needs
Supply payment
*
Shareholders
*
supplies
Yard
Issue
supplies
M1
Joe
Supply needs
stock
required stock
2
Supply needs
Check
and
supply
Payment
Duplicate docket (receipt)
Yard
Supply receipts
*
M2 Supply receipts
supplies
Supplier