定义角色来划分系统边界，角色位于用户接口层。
Finally we defined roles to describe the boundary of the DAS, and it belongs to user interface tier.
根据进程对中不同进程的功能，模型分为代理传输层和用户接口层。
According to the function of the different process in process Pair, the model is divided into Agent Transfer Layer and User Interface Layer.
数据访问应用程序的其他层不会使用实现类的名称；它们总是引用和使用用户定义接口。
The implementation class name is never used by other layers of the data access application; they always reference and use the user-defined data access interface.
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