A clustered environment supports load balancing, so the workload is distributed among the application servers that compose the cluster.
If one of the servers in the cluster fails, it is possible for the request to reroute to another server in the cluster. If you enable distributed sessions support, the new server can access session data from the database or another instance of the application server. You can retrieve the session data only if a new server has access to an external location from which it can retrieve the session.
The solution that is provided by IBM WebSphere Application Server is establishing session affinity in a cluster; this solution is available as part of the application server's plug-ins for web servers. It also provides for better performance because the sessions are cached in memory. In clustered environments other than WebSphere Application Server clusters, you must use an affinity mechanism, such as IBM WebSphere Edge Server affinity.