Most of the enterprise applications require scalability, fault tolerance and redundancy
The ability to scale websphere portal server or any other websphere application server is essentially achieved by clustering. Clustering allows requests to be workload managed between a number of cloned copies of the concerned application. In addition clustering also takes care of redundancy and fault tolerance.
Clustering helps in rendundancy and fault tolerance by making sure that there is no single point of failure.
In clustered architecture utilizes the same user customization, community and release data throughout an environment, any user customization made against one portal cluster member by a user would be available to the same user as and when that user accesses any of the other cluster members participating in the same portal cluster.
Under normal conditions, session affinity is maintained against the same portal cluster member until such time that a user terminates his or her session or the portal cluster member becomes unavailable, either through a deliberate or an unscheduled outage.