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