January 2011 Edition
Computer Science Department, University of Cape Town
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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:
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.