When stored only in memory, the sessions can be evicted at any time for any number of reasons, from eviction to node failure. This would lead to data loss (in t

When stored only in memory, the sessions can be evicted at any time for any number of reasons, from eviction to node failure. This would lead to data loss (in this case, session data). Using a cache on top of a permanent data store is the traditional way to use it, for accelerating access, but without risking data loss in case the cache is lost.