Data Flow Diagram Summary


Data Flow Diagrams : 
  • graphical system model that shows all main requirements for an IS in one diagram. Inputs/outputs, processes, data storage.


DFD’s and levels of abstraction:
  • Higher-level diagrams provide general views of system
  • Lower-level diagrams provide detailed views of system
  • Differing views are called levels of abstraction.


Context diagrams:
  • DFD  that summarizes all processing activity for the system or subsystem
  • Highest level (most abstract) view of system
  • Shows system boundaries
  • System scope is represented by a single process, external agents, and all data flows into and out of the system.


DFD fragments:
  • Created for each use case in the event table
  • Represent system response to one event within a single process symbol
  • Self-contained models
  • Focus attention on single part of system
  • Show only data stores required in the use case
  • Event-partitioned system model:
  • DFD to model system requirements using single process for each use case/activity in system or subsystem
  • Combines all DFD fragments together to show decomposition of the context-level diagram
  • Sometimes called “diagram 0”
  • Used primarily as a presentation tool
  • Decomposed into more detailed DFD fragments


Decomposing DFD Fragments:
  • Most DFD fragments can be further described using structured English
  • Sometimes DFD fragments need to be diagrammed in more detail
  • Decomposed into subprocesses in a detailed DFD
  • Hierarchical decomposition
  • DFD fragment 2 is decomposed into diagram 2
  • Diagram 2 has processes 2.1, 2.2…

Physical and logical DFDs:
Logical model 
  •  Assumes implementation in perfect technology
  • Does not tell how system is implemented


Physical model 
  • Describes assumptions about implementation technology
  • Developed in last stages of analysis or in early design

Evaluating DFD quality:
-          Readable
-          Internally consistent and balanced
-          Accurately represents system requirements
-          Reduces information overload – single DFD should not have more than 7+/-2 processes. No more than 7+/-2 data flows should enter or leave a process or data store in a single DFD
-          Minimizes required number of interfaces

Data flow consistency problems:
  • Differences in data flow content between a process and its process decomposition
  • Data outflows without corresponding inflows
  • Data inflows without corresponding outflows
  • Results in unbalanced DFDs
  • Consistency rules:
  • All data that flows into a process must – flow out of the process, or be used to generate data that flows out of the process
  • All data that flows out of a process must – have flowed into the process, or have been generated form the data that flowed into the process


Documentation of DFD components:
  • Lowest-level processes need to be described in detail
  • Data flow contents need to be described
  • Data stores need to be described in terms of data elements
  • Each data element needs to be described
  • Various options for process definition exist

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