DFD – Rules, Tips, Narratives
13th February 2012
Rules for drawing DFD
• All data flows must flow to or from a process.
• External entities cannot flow directly to each other.
• A data flow cannot link a data store to an external
entity.
• Data cannot move between data stores without first
being processed.
Rules for Drawing DFD
• A process must have at least one input flow
and one output flow.
• A black hole arises when a process has input flows but
no output flows.
• A miracle is a situation which occurs when a process
has outputs but no input flows.
Rules for drawing DFD
• The inputs to a process must be sufficient to
produce output flows.
• If a process has outputs that are greater than the sum
of its inputs, a grey hole is said to occur.
Rules for Drawing DFD
• Processes on Data Flow Diagrams must
transform data.
• When naming data flows, adjectives should be used which
show how processing has changed the data flow.
• Data Flows cannot cross each other.
• To overcome this problem, data stores and entities can be
duplicated in MS-Visio.
• However, processes must never be duplicated!
• Data flows must be unidirectional.
• Data flows cannot be recursive.
Steps in Constructing DFD
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Read and re-read this text
State your assumptions
Identify the entities
Highlight the process for the context-level
DFD.
• Identify data flows
• Construct the Context Level DFD
XYZ Ltd. is a nationwide retail chain. MANAGEMENT is considering
conducting Systems Analysis and Design with a view to computerizing the
payroll system.
Hours worked per employee per month are forwarded by the Personnel
Department. This is used to create an employee time record for each
employee and this is held in the Employee Time File. This is used to
calculate Gross Pay, and Gross Pay is used to determine net pay. Pay slips
are issued on the last Friday of every month.
The Employee Master File must be updated on a regular basis. One
instance which necessitates updating the system is when a new employee
joins the firm. The PERSONNEL DEPARTMENT gives the payroll system all
new staff details. When an EMPLOYEE gets married or moves house,
details of the name/address change are given to the payroll system. Pay
cheques then prepared.
Management is in charge of salary changes – these are input to the
Employee Master File when the time arises. Every month, management is
sent a cash list with details of salary payments for each employee for the
month previous.