Enabling Web applications to share the same session
By default, Web applications do not share the same session. If you would like Web applications to share the same session, you can configure the session descriptor at the application level in the weblogic-application.xml deployment descriptor. To enable Web applications to share the same session, set the sharing-enabled attribute in the session descriptor to true in the weblogic-application.xml deployment descriptor. See "sharing-enabled" in session-descriptor.
The session descriptor configuration that you specify at the application level overrides any session descriptor configuration that you specify at the Web application level for all of the Web applications in the application. If you set the sharing-enabled attribute to true at the Web application level, it will be ignored.
All Web applications in an application are automatically started using the same session instance if you specify the session descriptor in the weblogic-application.xml deployment descriptor and set the sharing-enabled attribute to true as in the following example: *"sharing-enabled"*
It is quite possible to replicate session across the Cluster : http://download-llnw.oracle.com/docs/cd/E12840_01/wls/docs103/cluster/failover.html#wp1040345
WebLogic server provides the ability to replicate HTTP session state across multiple clusters. This improves high-availability and fault tolerance by allowing clusters to be spread across multiple geographic regions, power grids, and internet service providers
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Oracle Coherence has functionality to enable sharing sessions across WebLogic and other types of java application servers, it's called Coherence*Web.