Friday, September 21, 2012

Identify the Level 0 processes

  • Identify the Level 0 processes that are  within the circle of the context diagram, without defining any data stores.
  • The difficulty of this activity varies with your understanding of the problem domain and the  scope of the project.
  • After the processes are  identified, next define file locations  on  the  Level  0  data flow  diagram.  
  • You could  leave  files  for  a  lower  level  of analysis  as many texts and companies do by convention. In that case, you are ready to  draw the diagram.  
  • To  identify data stores, first consider each process.  
  • Can the process be completed without reading or writing to a data store? 
  • If your answer is yes, then you do  not need a file  at this level. 
  • If the answer is no,  you need one data store for every required read action and every required write action.  
  • Many times, the reads and writes are to the same data store. Then, you have one data flow per input/output action. 
  • As these required reads and writes are identified, you add to the DFD to  include the data store name and data flow(s).  
  • When you do  this part of the drawing, make sure that each flow  and store has a name. 

Level 0 Syntax Rules:
  1. All processes are connected to something else. 
  2. All process have both inputs and outputs. 
  3. No processes have only outputs or only inputs. 
  4. Processes may connect to  anything:  other processes, data stores, or entities. 
  5. All processes have a unique name and number. 
  6. Each process number is used once in the diagram set. 
  7. Only subprocesses of a process shall follow the numbering scheme of the parent process. 
  8. Entities and data stores may connect only to processes. Another way to state this is that each data flow must have at least one end connected to  a process. 
  9. Data flows  are the only legal type of connection between entities, processes, and data stores. 
  10. Make sure there are no  dangling arrows. 
  11. The net data flows  to  and from context diagram external entities must balance, that is, be present, in each level of DFDs. 
  12. Trivial errors and exceptions are  not handled until L1 or lower in the DFD set. 
  13. Trivial data stores show up in the diagram set the first time they are referenced by a process. 
Sumber: Conger, 2008

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