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Chapter 6. Data-Flow Diagrams Table of Contents Objectives .................................................................................................................... 1
Introduction to data-flow diagrams .................................................................................. 2
What are data-flow diagrams? ................................................................................ 2
An example data-flow diagram ............................................................................... 2
The benefits of data-flow diagrams .......................................................................... 3
Case study ................................................................................................................... 3
The different kinds (and levels) of data-flow diagrams ......................................................... 3
Elements of data-flow diagrams ....................................................................................... 4
Processes ............................................................................................................ 4
Data-flows ........................................................................................................... 5
Data stores ........................................................................................................... 6
External entities ................................................................................................... 7
Multiple copies of entities and data stores on the same diagram ................................... 8
Context diagrams ......................................................................................................... 8
What is a context diagram? ................................................................................... 8
Constructing a context diagram ............................................................................... 9
Level 1 data-flow diagrams ........................................................................................... 9
What is a level 1 DFD? ........................................................................................ 9
Constructing level 1 DFDs ................................................................................... 10
Decomposing diagrams into level 2 and lower hierarchical levels ....................................... 10
What is a level 2 (or lower) DFD? ........................................................................ 11
Constructing level 2 (and lower) DFDs — functional decomposition ........................... 11
Making levels ............................................................................................................. 12
Balancing ................................................................................................................... 13
Numbering ................................................................................................................. 13
Process descriptions ..................................................................................................... 13
Validation .................................................................................................................. 14
An example in constructing a data-flow diagram ............................................................... 14
Identify the system boundaries ............................................................................... 15
Follow inputs .................................................................................................... 15
Follow events .................................................................................................... 16
Fill in gaps ........................................................................................................ 17
Repeat .............................................................................................................. 17
Review ...................................................................................................................... 18
Questions ........................................................................................................... 18
Answers ............................................................................................................ 21
