ContactStore：这是基于HashMap的内存数据存储库，其实现对于本文不重要。
ContactStore: It's a HashMap-based in-memory data store whose implementation is not important for this article.
应用程序状态可以存储在内存、文件或者数据库中。
Application state may be stored in memory, in files, or in a database.
此问题的常用解决方案是，把会话存储在数据库而不是内存中。
The common solution to this problem is to store the session in a database instead of memory.
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