Monday, April 13, 2009

Constructing A DFD

Several rules of thumb are used in drawing DFDs:

1. Processes should be named and numbered for easy reference. Each name should be representative of the process.

2. The direction of flow is from top to bottom and from left to right. Data traditionally flow from the source (upper left corner) to the destination (lower right corned, although they may flow back to a source. One way to indicate this is to draw a long flow line back to the source. An alternative way is to repeat the source symbol as a destination. Since it is used more than once in the DFD, it is marked with a short diagonal in the lower right.

3. When a process is exploded into 10weI'-level details, they are numbered.

4. The names of data stores, sources, and destinations are written in capital letters. Process and data flow names have the first letter of each word capitalized.

How detailed should a DFD be? As mentioned earlier, the DFD is designed to aid communication. If it contains dozens of processes and data stores, it gets too unwieldy. The rule of thumb is to explode the DFD to a functional level, so that the next sublevel does not exceed 10 processes. Beyond that, it is best to take each function separately and expand it to show the explosion of the single process. If a user wants to know what happens within a given process, then the detailed explosion of that process may be shown.

A DFD typically shows the minimum contents of data stores. Each data store should contain all the data elements that flow in and out questionnaires can be used to provide information for a first cut. All discrepancies, missing interfaces, redundancies, and the like are then accounted for often through interviews.

The DFD methodology is quite effective, especially when the required design is unclear and the user and the analyst need a notational language for communication. The DFD is easy to understand after a brief orientation.

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