活动是一组注册的操作(比如数据库更新操作)。
An activity is a set of registered actions (such as database update operations).
程序本身的其他方面，比如数据库操作已经进行了优化。
Aspects of the program itself, such as database operations, were already tuned.
最后，数据访问层是持久化的数据存储，比如数据库或文件存储。
Finally, the data access layer is a persistent data store, such as a database or file storage.
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