By Rosemary Rock-Evans
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Extra info for Analysis Within the Systems Development Life-Cycle. Book 3: Activity Analysis–the Deliverables
These are the Data Flow Diagram (DFD (or 'activity dependency' diagram)) and the activity decomposition diagram. The two techniques are complementary and when the basic conventions have been introduced, they will be discussed together, including the rules for drawing them and a number of hints and guidelines which should help during analysis. One of the most important sub-sections is that dealing with how the two techniques fit together, as it is in this discussion that the processes and purpose of the diagrams, when used together, are explained.
46). The activities select what they require from the database of information. At any moment they will probably be dealing with only one entity, but to achieve their overall tasks they all need to use several entity types. Some may only be looking, some may be making additions, some may be removing and archiving or throwing away. Any type of 'update' will put the data into a new state. When the activities act—whatever they are doing—they will look at data which is in the correct state for them.
As illustrated, all the diagrams can be used to express the same content—the choice is simply one of aesthetics, simplicity of drawing and ease of presentation. I like lists because they are easy to compile, neat to look at and easy to understand. 1 Networks of activities One fundamentally important point has not been mentioned yet. Until now the implication may have been that activity decomposition produces a true 'hierarchy' or tree structure. This is not the case. If we consider what is being represented rather than the mechanics of what we are doing, we realise that in real life there are many actions performed which satisfy more than one overall purpose.