A common use-case is to configure the underlying Core Cache to enable passivation. Passivation is a feature used to reduce cache memory usage by evicting stale data that can later be reloaded. In JBoss Cache, it is done via a combination of an eviction policy and a cache loader. That is, when a node is evicted from the Cache's in-memory store, it will be stored in a persistent store by the cache loader. When the node is requested again, it will be loaded from the persistent store and stored into memory.

There is a restriction, however. Since POJO Cache maps object data into an internal area, there are two places that have object information. One is under the regular String ID that the user specifies, and the other is located under /__JBossInternal__. Therefore, to maintain consistentency, when you specify the eviction region, you can only specify one global (i.e., /_default_) region. This way, when the nodes associated with a POJO are passivated, they will do so across the whole region.

Below is a snippet from a cache configuration xml illustrating how the eviction policy along with cache loader can be configured. Please note that this is simply an aspect of the underlying Cache. That is, PojoCache layer is agnostic to this behavior.

<attribute name="EvictionPolicyConfig">
   <config>
      <attribute name="wakeUpIntervalSeconds">5</attribute>
      <attribute name="policyClass">org.jboss.cache.eviction.LRUPolicy</attribute>
      <!-- Cache wide default -->
      <region name="/_default_">
          <attribute name="maxNodes">5000</attribute>
          <attribute name="timeToLiveSeconds">3</attribute>
      </region>
   </config>
</attribute>

<attribute name="CacheLoaderConfiguration">
   <config>
      <passivation>true</passivation>
      <preload>/</preload>
      <shared>false</shared>

      <!-- we can now have multiple cache loaders, which get chained -->
      <cacheloader>
         <class>org.jboss.cache.loader.FileCacheLoader</class>
         <!-- whether the cache loader writes are asynchronous -->
         <async>false</async>
         <!-- only one cache loader in the chain may set fetchPersistentState to true.
              An exception is thrown if more than one cache loader sets this to true. -->
         <fetchPersistentState>true</fetchPersistentState>
         <!-- determines whether this cache loader ignores writes - defaults to false. -->
         <ignoreModifications>false</ignoreModifications>
      </cacheloader>
   </config>
</attribute>

Another way to support multiple regions in eviction is to use region-based marshalling. See the "Architecture" chapter in the JBoss Cache User Guide for more information on region-based marshalling. When the Cache uses region-based marshalling, POJO Cache will store internal node data on the region that is specified. This allows for a more flexible eviction policy.