The MDA process has not been completely described if we do not reference the generation of the communication bridges between the relational model and the EJB model and between the Web model and the EJB model.
In Figure 4-1 on page 46, the communication bridges are shown by arrows. This means that there is a direction in the relation between the two models. The Web model uses (and knows) the EJB model, and the EJB model uses (and knows ) the relational model.
Both bridges are simple and have been explained. The data storage for the EJB components is provided by a relational database, structured according to the generated relational model in Figure 5-1. The EJB-Relational bridge is constituted by the relation between the table generated for a UML class and the EJB data class that is generated from the same UML class. The relationship between the generated relational model and the generated EJB component is shown in Figure 5-5. The figure depicts the EJB component model for Rosa's Breakfast Service without showing the EJB data schemas, but with all dependencies on the tables of the relational model of Figure 5-1.
Figure 5-5. Communication bridge between EJB and relational models
The bridge between the Web model and the EJB model is constituted by the links to the EJB key classes and EJB components as shown in Figure 5-4. Note that both communication bridges are relationships between the PSMs. This relationship will need to be preserved when the PSMs are transformed into code.