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Hierarchy - Vertical Composition
Related to the system, another important part of the context is the hierarchy in which a component is used. In figure 2.10, a composite component (BillingManager) is depicted which has been designed to create bills and store each one in a single PDF (Portable Document Format) file. The component is additionally supposed to write a summary of all the created bills as PDF file. Hence, the component PDFCreator is used in two different places. Notice however, that this kind of usage is usually unknown to the creators of the outer composite component. For them, the inner component (BillCreator) is a black box. They do not know the internal details and, hence, the usage of the inner PDFCreator is hidden.
Figure 2.10:
Component hierarchy.
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In this case, the PDFCreator component is used in different contexts on different hierarchy levels. Note, that this only makes sense if the underlying component model supports hierarchical components at all. Considering parametric contracts, both components might offer different characteristics (QoS, functions offered, etc.). Additionally, they are used differently in their contexts. The PDFCreator of the inner component produces bills with less pages than the summary PDF file created by the outer PDFCreator.
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Snowball
2007-03-16