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Analysis and Design
Conventions used when drawing DFD's
Some conventions used when drawing DFD's should be explained. Assuming the example
DFD explained earlier all external files such as employee record, company
record and tax rates are shown as a one side open rectangle. The need for multiple
data flow by a process is represented by a * between the data flows. The symbol
represents the AND relationship. For example, if there is a * between the two
input data flow A and B for process, it means that A AND B are needed for the
process. In the DFD, for the process "weekly pay" the data flow "hours"
and "pay rate" both are needed, as shown in the DFD. Similarly, the
OR relationship is represented by "+" between the data flows.
It should be pointed out that a DFD is not a flowchart. A DFD represents that
flow of data, while flow chart shows the flow of control. A DFD does not represent
procedural information. So, while drawing a DFD, one must not get involved in
procedural details, and procedural thinking must be consciously avoided.
Consideration of loops and decisions must be ignored. In drawing the DFD the
designer has to specify major transforms in the path of the data flowing from
the input to output. How those transforms are performed is not an issue while
drawing the data flow graph. There are no detailed procedures that can be used
to draw a DFD for a given problem. Only some directions can be provided. One way
to construct a DFD is to start by identifying the major inputs and outputs. Minor
inputs and outputs (like error messages) should be ignored at first. Then starting
from the inputs, work towards the outputs, identifying the major inputs (remember
that is is important that procedural information like loops and decision not be
shown in DFD, and designer should not worry about such as issues while drawing
Following are some suggestion for constructing a data flow graph
Klork your way consistently from the inputs to the outputs, or vice versa.
If you get stuck, reverse direction. Start with a high level data flow graph with
few major transforms describing the entire transformation from the inputs to outputs
and then refine each transform with more detailed transformation.
Never try to show control logic. If you find yourself thinking in terms of
loops and decisions, it is time to stop and start again.
Label each arrow with proper data elements. Inputs and outputs of each transform
should be carefully identified.
Make use of * and + operation and show sufficient detail in the data flow graph.
Try drawing alternate data flow graphs before setting on one.
Many systems are too large for a single DFD to describe the data processing
clearly. It is necessary that some decomposition and abstraction mechanism be
used for such systems. DFDs can be hierarchically organized, which helps in progressively
partitioning and analyzing large systems. Such DFDs together are called a leveled
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