PojoCache also provides an eviction policy, org.jboss.cache.aop.eviction.AopLRUPolicy , that is a subclass of org.jboss.cache.eviction.LRUPolicy (with the same configuration parameters). The reason we need a distinctive implementation is because eviction in PojoCache is quite different from the regular TreeCache. In the plain cache world, a unit is a FQN node, while in the aop world, the concept of a unit is an object (which can have multiple nodes and children nodes!).

In addition, once a user obtains a POJO reference, everything is supposed to be transparent, e.g., cache retrieval and update operations. But if an object is evicted, that means there is no CacheInterceptor for the POJO, and the contents are not intercepted by the cache. Instead, every operation access will be channeled to the in-memory reference. So all operations will succeed but then a user has no way of knowing that it is merely updating the in-memory reference!

To remedy this problem, we currently requires that eviction policy is used in combination of a cache loader to persist the data. It can either be fully persistency or passivation (e.g., only persist when it is evicted). In this way, when the node is not available it will be retrieved from the persistent store. The downside is that the POJO won't be transient, e.g., it is persistent all time unless a specific user-based removeObject is called.