Data Flow Diagramming Mistakes
DFDs look easy on the surface – after all, what’s hard about writing down a few
bubbles and arrows? In practice the techniques proves to be somewhat more
difficult than one might first think.
This final blog post considers some of the common mistakes that occur when one first
tries to build DFDs. There are several common types of mistakes. One that is
easy to check for and correct involves using so-called illegal data flows.
Illegal Data Flows
One of the patterns of data flow analysis is that all flows must begin with or
end at a processing step. This makes sense, since presumably data cannot simply
metastasize on its own without being processed. This simple rule means that the following mistakes can be fairly easily identified and corrected in a DFD.
Black Holes, Grey Holes and Miracles
A second class of DFD mistakes arise when the outputs from one processing step do not match its inputs. It is not hard to list situations in which this might occur:
- A processing step may have input flows but no output flows. This situation is sometimes called a black hole.
- A processing step may have output flows but now input flows. This situation is sometimes called a miracle.
- A processing step may have outputs that are greater than the sum of its inputs – e.g., its inputs could not produce the output shown. This situation is sometimes referred to as a grey hole.
I hope my blog posts have helped you understand the difference between Flow charts and DFD’s, the advantages of choosing each one, and also the disadvantages that come with them!
Thanks for reading!