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Wednesday, September 21, 2011

Data Flow Diagram.




WHAT IS DFD
The Data Flow Diagram is commonly used also for the visualization of structured design data processing. The normal flow is represented graphically. A designer typically draws context level DFD first showing interaction between the system and the outside entities. Then this context level DFD will then be exploded in order to further show the details of system being modeled. A data flow diagram (DFD) is a graphical representation of the "flow" of data through an information system, modelling its process aspects. Often they are a preliminary step used to create an overview of the system which can later be elaborated. DFDs can also be used for the visualization of data processing.A DFD shows what kinds of data will be input to and output from the system, where the data will come from and go to, and where the data will be stored. It does not show information about the timing of processes, or information about whether processes will operate in sequence or in parallel.

RULES OF DFD
  • Each process must have a minimum of one data flow going into it and one data flow leaving it.
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  • Data in data stores must be used (data shown flowing out) otherwise there is no reason to store it. Data must also be updated (new data shown flowing in) otherwise it will be obsolete and will not have been updated by the system.
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  • A data flow out of a process should have some relevance to one or more of the data flows into a process.

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    • Data stored in a system must go through a process.
    • Data cannot be filed directly from an external entity (terminator) or retrieved directly by an external entity. This would be classified as a breach of security! A person or business outside another business should not have direct access to another business's filing system.



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      • Similarly, filing systems within an organisation cannot logically communicate with one another unless there is a process involved.One data store cannot have a direct link to another data store.
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      • All processes in DFDs must be linked to either another process or a data store. A process cannot exist by itself, unconnected to the rest of the system.


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      Sample DFD of cyber cafe management

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