1 Chapter Data-Flow Diagrams Table of Contents


The different kinds (and levels) of data-flow



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The different kinds (and levels) of data-flow
diagrams
Although all data-flow diagrams are composed of the same types of symbols, and the validation rules
are the same for all DFDs, there are three main types of data-flow diagram:


Data-Flow Diagrams
4
• Context diagrams — context diagram DFDs are diagrams that present an overview of the system
and its interaction with the rest of the “world”.
• Level 1 data-flow diagrams — Level 1 DFDs present a more detailed view of the system than
context diagrams, by showing the main sub-processes and stores of data that make up the system
as a whole.
• Level 2 (and lower) data-flow diagrams — a major advantage of the data-flow modelling
technique is that, through a technique called “levelling”, the detailed complexity of real world
systems can be managed and modeled in a hierarchy of abstractions. Certain elements of any data-
flow diagram can be decomposed (“exploded”) into a more detailed model a level lower in the
hierarchy.
During this unit we shall investigate each of the three types of diagram in the sequence they are
described above. This is both a sequence of increasing complexity and sophistication, and also the
sequence of DFDs that is usually followed when modeling systems.
For each type of diagram we shall first investigate what the features of the diagram are, then we shall
investigate how to create that type of diagram. However, before looking at particular kinds of data-
flow diagrams, we shall briefly examine each of the symbols from which DFDs are composed.
Elements of data-flow diagrams
Four basic elements are used to construct data-flow diagrams:
• processes
• data-flows
• data stores
• external entities
The rest of this section describes each of the four elements of DFDs, in terms of their purpose, how
the element is notated and the rules associated with how the element relates to others in a diagram.

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