Most large scale business systems follow a three layer architecture. They provide a user interface layer on top of a business object layer. The business objects need to be made persistent somehow in a persistence layer.

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Figure 1: Structure of Layered Architecture for Business Systems.

If you want to use pure object-orientation to implement your business system you have to decide which database paradigm to use for your system. Today you have a choice of using:

The functionality covered by the first three of the above options is depicted in Figure 2.

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Figure 2: Three Different Kinds of Database Access Layers

These web-pages contain links to patterns that will help you design or understand object/relational access layers (the item in the middle). If you deal with matter a little longer you will find two interesting things:

But the following discussion will be focused on object/relational access layers

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