用户的首选项不是一成不变的，并且可能会依服务和系统当前的状态不同而发生变化。
User preferences may not be static, and can vary based on the services and current state of the system.
一个有状态操作系统服务的范例。
OperatingSystem.ear: a sample stateful operating system service.
核查系统日志，获得服务实现的状态消息，如图15所示。
Check the system log for the status messages of our service implementations as shown in Figure 15.
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