RML™ Requirements Model 5 – The Data Flow Diagram (DFD)

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The Data Flow Diagram (DFD) is a very useful part of the Requirements Modeling Language (RML™). The Structured Analysis Wiki contains a great explanation of how to create a DFD, so I’m not going to cover that information here. Instead, I’m going to provide one answer to the question “How do I know when to use a DFD?” This answer comes from my own (unique) view of the world, so some of you probably won’t relate to it, but others will–I’m sure there is at least one more out there…


Sometimes I have what I think of as an “equation” in my head. In vague terms, I may be thinking “Customer Data + Product Data + Input from Sales Rep + Taxes = Order”. But that’s not really right, nor is the equation a good representation of the information.



And, the “equation” also misses a few other particulars about creating an order that I’d like to convey: Sales Reps update customer data, the Finance staff maintains the rules for the tax calculations, and orders flow to the Order Fulfillment system after creation.



The data flow diagram is great for representing all of this. Here’s my “equation,” expressed as a data flow diagram:




I have found that business users, as well as developers, react well to this model—it provides a “big picture” with which to begin a conversation about creating an order. It paints exactly the picture I want to convey and validate when I’m thinking “Customer Data + Product Data + Input from Sales Rep + Taxes = Order”.




Display this picture, and you’ll get some interesting questions and comments:


    • How is customer data populated initially?


    • Does the order fulfillment system update the order store with information about the fulfillment of the orders?


    • Where does the product data come from?


    • Are all of the tax rules manually entered, or is there also an electronic source for them?


  • And, maybe, “No, that’s wrong, updating customer data isn’t a separate process. Customer updates need to automatically flow out of changes made when creating the order.”


One note: it doesn’t have to be technically perfect to be useful. I often provide “conceptual” DFDs, in that I intentionally provide conceptual, but not technical information. For example, conceptually, there is a “products” data store. Technically, there may be multiple stores: product list, product descriptions, etc. The important thing is that they work together to provide product data. Developers and architects are very receptive to this; they understand I’m illustrating the behaviors of the system without defining the implementation (which, after all, is their job). Oh, and how does the audience know it is conceptual? I put the word “conceptual” in the title!


You may notice I didn’t number my processes. That’s because I rarely decompose them and many people tend to take the numbering as ordering. So, for my usage, numbering adds confusion rather than clarity.


Happy diagramming!


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