Chapter 2. Eviction and Expiration

Eviction types define the maximum limit for entries in the cache.

Storage types define how JBoss Data Grid stores entries in the cache.

The <memory> element controls how Red Hat JBoss Data Grid stores entries in memory.

For example, as a starting point to configure eviction for a standalone cache, add the <memory> element as follows:

<local-cache ...>
  <memory>
  </memory>
</local-cache>

<local-cache ...>
  <memory>
  </memory>
</local-cache>

Define the storage type as a child element under <memory>, as follows:

Include the eviction attribute with the value set to COUNT or MEMORY.

Include the size attribute with a value set to a number greater than zero.

Include the address-count attribute when using OFFHEAP storage to prevent collisions that might decrease performance. This attribute specifies the number of pointers that are available in the hash map.

Set the value of the address-count attribute to a number that is greater than the number of cache entries. By default address-count is 2^20, or 1048576. The parameter is always rounded up to a power of 2.

<memory>
  <offheap eviction="MEMORY" size="10000000000" address-count="1048576"/>
</memory>

<memory>
  <offheap eviction="MEMORY" size="10000000000" address-count="1048576"/>
</memory>

Eviction strategies control how Red Hat JBoss Data Grid performs eviction. You set eviction strategies with the strategy attribute.

The default strategy is NONE, which disables eviction unless you explicitly configure it. For example, here are two configurations that have the same effect:

<memory/>

<memory/>

<memory>
  <object strategy="NONE"/>
</memory>

<memory>
  <object strategy="NONE"/>
</memory>

When you configure eviction, you implicitly use the REMOVE strategy. For example, the following two configurations have the same effect:

<memory>
  <object/>
</memory>

<memory>
  <object/>
</memory>

<memory>
  <object strategy="REMOVE"/>
</memory>

<memory>
  <object strategy="REMOVE"/>
</memory>

Passivation configures Red Hat JBoss Data Grid to write entries to a persistent cache store when it removes those entries from memory. In this way, passivation ensures that only a single copy of an entry is maintained, either in-memory or in a cache store but not both.

For more information, see Setting Up Passivation.

Expiration parameters configure the amount of time entries can remain in the cache.

Configure Red Hat JBoss Data Grid to perform expiration for a cache as follows:

Red Hat JBoss Data Grid cannot always expire entries immediately when they reach the time limit. Instead, JBoss Data Grid marks entries as expired and removes them when:

This behavior might indicate that JBoss Data Grid is not performing expiration as expected. However it is the case that the entries are marked as expired but not yet removed from either the memory or the cache store.

To ensure that users cannot receive expired entries, JBoss Data Grid returns null values for entries that are marked as expired but not yet removed.