Session Replication across WLS Cluster
【早期文章，迁移过来】
Weblogic即可以实现一个Cluster内的instance的session复制，也可以实现多个Cluster之间的session复制。通常用的就是一个Custer内的负载均衡，实现高可用性。Weblogic提供两个方式的Http Session复制：
要实现Weblogic Cluster负载均衡，在网络上必须要有一个负载均衡器，比如F5、Apache等，如果是跨机器的Cluster节点，网络必须通
3.1 Session必须序列化
为了支持in-memory http session复制，所有的servlet和jsp会话数据必须序列化，实现java.io.Serializable接口，而且注意下面内容，来自官方文档：
Serialization is the process of converting a complex data structure, such as a parallel arrangement of data (in which a number of bits are transmitted at a time along parallel channels) into a serial form (in which one bit at a time is transmitted); a serial interface provides this conversion to enable data transmission.
3.2 用setAttribute 修改 Session 状态
在http servlet实现了javax.servlet.http.HttpSession，用HttpSession.setAttribute代替putValue方法去修改session 对象。putValue方法是不建议用，同样用removeAttribute代替removeValue去删除session对象。
3.3 避免大的session对象
因为往session中存放的数据比较大时，系统的响应速度明显变慢，有时会出现内存溢出的情况。
3.4 框架的使用
在特定的框架集（frameset）中，确保只有一个框架（frame）创建和修改会话数据；必须确保只在第一个框架集的一个框架中创建会话,其他框架集访问该session。
If you are designing a Web application that utilizes multiple frames, keep in mind that there is no synchronization of requests made by frames in a given frameset. For example, it is possible for multiple frames in a frameset to create multiple sessions on behalf of the client application, even though the client should logically create only a single session.
In a clustered environment, poor coordination of frame requests can cause unexpected application behavior. For example, multiple frame requests can “reset” the application’s association with a clustered instance, because the proxy plug-in treats each request independently. It is also possible for an application to corrupt session data by modifying the same session attribute via multiple frames in a frameset.
4.1 在Cluster中需要配置复制组
4.2 Weblogic.xml配置
domain_directory/applications/application_directory/Web-Inf/weblogic.xml
<session-descriptor>
<session-param>
<param-name>PersistentStoreType</param-name>
<param-value>replicated</param-value>
</session-param>
</session-descriptor>
