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I am looking for a way to show duplicate data-stores in data-flow diagrams (DFD). With "duplicate" I mean multiple occurrences of the same symbol to avoid too many crossing flows.

In Gane & Sarson's DFD notation, duplicate data-stores are indicated by adding an extra vertical line to the left of the open ended data-store rectangle. Another website suggested adding an asterisk next to the data store name.

But I was unable to find any guidance for Yourdon/DeMarco's DFD notation, which uses different symbols. Is there a generally accepted way to represent duplicate data-stores with Yourdon/DeMarco? Or should I adopt one of the methods stated above?

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  • Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.
    – Community Bot
    Sep 6, 2021 at 12:33
  • @Community should be clearer now :-)
    – Christophe
    Sep 6, 2021 at 17:49

1 Answer 1

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Yourdon/Demarco avoids duplication where possible. For example, processes are unique, and data-flow names are supposed to be unique.

They do not mention uniqueness for data-stores nor external entities, mostly because the same stores and entities may be used in different levelled diagrams.

And they don't mention anything about duplicates in the same diagram. They carefully avoid such situation because duplicates symbols make it difficult to visually follow the flow. I found only a single case of duplicate store in their whole book (fig. 9): there is no graphical distinction between the two occurrences.

Conclusion: try to avoid duplicates in the same diagram. If you can't avoid it, and want to draw attention of the reader on the special situation, use and asterisk exponent behind the store name, and add a legend for the asterisk somewhere.

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