DATA FLOW DIAGRAMS - (DFDs)

1. Purpose - logical description of a system, i.e., "what" is being done.
   • DFDs do not represent the physical operation of the system, i.e., who is doing the work or how the work is being done.
   • DFDs are useful for several reasons.
     • Permanent record of what the system is supposed to do.
     • Facilitates future modification.
     • Improves auditability.
   • DFDs can be broken down into many sublevels to show the desired level of detail.

2. Highest level is context diagram. Here is a context diagram based on figure 2.7. Some important points:
   • 1 bubble represents entire system.
   • Squares represent entities that send & receive data.
   • Data flow arrows show inputs and outputs.
   • Data stores NOT shown. They are within the system.

3. Next level is a level 0 DFD. This explodes the context diagram. Here is a level 0 dfd based on figure 2.8. Some important points:
   • Flow is from upper left to lower right.
   • Level 0 DFD must balance with the context diagram it describes.
   • Inputs going into a process are different from outputs leaving the process.
   • Number processes as 1.0, 2.0, 3.0, .......
   • Use verbs to name the processes. Use nouns to name the data flows.
   • Data stores are first shown at this level.

4. Next level is a level 1 DFD. This explodes the level 0 diagram. Here is a level 1dfd based on figure 2.9. Some important points:
   • Flow is from upper left to lower right.
   • Level 1 DFD must balance with the level 0 it describes.
   • Inputs going into a process are different from outputs leaving the process.
   • Numbering depends upon which level 0 process is exploded. When exploding process N, number the level 1 processes as N.1, N.2 N.3, ....
   • Continue to show data stores.
   • Use verbs to name processes. Use nouns to name data flows.

5. Continue to refine and explode the diagram until each process represents a simple function.

6. Steps for drawing a DFD.