When using HTTP Sessions with WebLogic Cluster, you can ensure high availability for your session by configuring HTTP Session Replication. For the replication strategy, you have many options to select from like In-memory, file, JDBC, Cookie etc. When you are using in-memory replication with a web server to front WebLogic Server Cluster, the proxy plug-in must be configured on the web servers. The proxy plug-ins are intelligent enough to stick the request from a client to the same server where the session is hosted. A secondary copy of the session will be sent to a backup server by the primary server. This information will be written to a cookie or encoded in the URL depending on the client's configuration to support cookies.
In the past, during failure of the primary server, the proxy plug-in on the web server will redirect the request to the secondary/backup server. This resulted in increased traffic to the secondary/backup server. So somewhere during the Weblogic Server 9.x the plug-ins are updated to do a random routing incase of failures. The proxy server will randomly distribute all the requests from the failed server to different servers that are still running in the cluster. When the request from the existing clients (with valid session id) gets redirected to servers that are not secondary server then that server will connect to the secondary server for that client and will pull the session to become primary server. This ensures that one server will not get overloaded when a server in a cluster fails.
The same exact strategy was used when you front a WebLogic cluster with a load balancing hardware as not all the load balancing hardware understand the WebLogic cluster configuration. Now the same strategy is applied to the WebLogic Cluster front by a proxy plug-in hosted on a web server. This make the failover strategy consistent whether you have hardware or a software based proxy in front a WebLogic Cluster.
WebLogic Server documentation is still not updated to reflect this change under the "Proxy Failover Procedure" section. If you are running WebLogic Cluster with 9.x and you are not seeing this behavior then you can contact Oracle Support to find out whether there is a patch that can you can apply for your current WebLogic implementation.
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