In the blog I will discuss some of the commonly made mistakes in regards of Data Flow Diagrams.
Illegal Data Flows
- These are mistakes that are easy to check for and easily corrected.
- One of the patterns of data flow analysis is that all flows must begin with or end at a processing step because obviously 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. Diagram
- Diagram below to explain:
A second class of DFD’s mistakes result from when the outputs from one processing step do not match its inputs. For example:
- 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.
DFDs are not flow charts
This mistake however is more difficult to identify then the two previous.
- Flow chart diagrams can be useful for describing programming logic or understanding a single sequence of process activities.
- Flow charts often show both processing steps and data “transfer” steps, DFDs only show “essential” processing steps.
- Flow charts might include arrows without labels: DFDs never show an unnamed data flow.
- Flow charts show conditional logic, DFDs don’t.
- Flow charts show different steps for handling each item of data, DFDs might include several data items on a single flow arrow.