We now examine our emphasis-the DOC architecture-through its different layers. As mentioned in the previous section, our study approach is top-down which is good for understanding. In practice, most specific implementations in each layer have been developed priori to the awareness of the DOC architecture. Figure 2 shows the basic layer structure of the DOC architecture concluded in [9].
Figure 2: Layers of DOC middleware and surrounding context.
The DOC architecture consists of four layers, and tries to build a bridge between the underlying operating system and the distributed applications. The layer decomposition of the whole architecture is a typical model-driven approach to simplify the design tasks, and classify the functionalities. We describe each of the four middleware layers in the follows. The layer contents have been introduced in [9]. We put more effort on describing the distribution middleware layer, which includes our main focus CORBA.