DFD Basics

DFD Basics

1. Graphical representation of the flow of data throughout a system

2. shows what kinds of information will be input to and output from the system

   • Also shows where data will come from and go to, and where the data will be stored

3. does not show information about the timing, or about whether processes will operate in sequence or in parallel

4. common practice to draw the context-level data flow diagram first

   • shows the interaction between the system and external agents which act as data sources
     • context diagram shows the entire system as a single process, and gives no clues as to its internal organization
   • Context Level DFD
     • no inside workings of the process are detailed

5. Level 1 DFD that shows some of the detail of the system being modeled

   • shows how the system is divided into sub-systems (processes)
     • Every process detailed on the level 0 diagram has a level 1 diagram
       • Must be balanced - i.e. show same input/output flows & data stores as detailed in level 0 DFD

6. Data flow diagrams can be used in both Analysis and Design phase of SDLC.

7. Gane and Sarson Elements

   • Process - has a number, name, description, at least one output flow, at least one input flow
   • Data flow arrow - has a name, one or more connections to a process
   • Data store - has a number, name, description, 1 or more output flows, 1 or more input flows
   • External entity - has a name and description

8. Level 0 DFD

   • Shows all processes, data stores,external entities, and data flows amongst them
   • Purpose is to show major high level processes that are interrelated
   • Overview of the system

9. Level 2 DFD

   • Shows a level 1 DFD in more detail
   • Again has to be balanced by showing the same input/output flows & data stores as the level 1 equivalent
   • Numbering at this level has two decimal points e.g. 1.2.4