2.3. Using Eviction

In Red Hat JBoss Data Grid, eviction is disabled by default. If an empty <eviction /> element is used to enable eviction without any strategy or maximum entries settings, the following default values are used:
  • Strategy: If no eviction strategy is specified, EvictionStrategy.NONE is assumed as a default.
  • size: If no value is specified, the size value is set to -1, which allows unlimited entries.
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2.3.1. Initialize Eviction

To initialize eviction, set the eviction element's size attributes value to a number greater than zero. Adjust the value set for size to discover the optimal value for your configuration. It is important to remember that if too large a value is set for size, Red Hat JBoss Data Grid runs out of memory.
The following procedure outlines the steps to initialize eviction in JBoss Data Grid:

Procedure 2.1. Initialize Eviction

  1. Add the Eviction Tag

    Add the <eviction> tag to your project's <cache> tags as follows: 
    <eviction />

  2. Set the Eviction Strategy

    Set the strategy value to set the eviction strategy employed. Possible values are LRU, UNORDERED and LIRS (or NONE if no eviction is required). The following is an example of this step: 
    <eviction strategy="LRU" />

  3. Set the Maximum Size to use for Eviction

    Set the maximum number of entries allowed in memory by defining the size element. The default value is -1 for unlimited entries. The following demonstrates this step: 
    <eviction strategy="LRU" size="200" />
Result

Eviction is configured for the target cache.

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2.3.2. Eviction Configuration Examples

Eviction may be configured in Red Hat JBoss Data Grid programmatically or via the XML file. Eviction configuration is done on a per-cache basis.
A sample XML configuration for is as follows: 
<eviction strategy="LRU" size="2000"/>
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2.3.3. Utilizing Memory Based Eviction

Red Hat JBoss Data Grid 7 introduced memory based eviction, allowing eviction of entries based on memory usage of the entries instead of the number of entries. This can be particularly useful if the entries vary in size.
Key/Value Limitations

Only keys and values that are stored as primitives, primitive wrappers (such as java.lang.Integer), java.lang.String instances, or an Array of these values may be used with memory based eviction.

Due to this limitation if custom classes are used then either store-as-binary must be enabled on the cache, or the data from the custom class may be serialized, storing it in a byte array.
Compatibility mode prevents serialization into byte arrays, and as such these two features are mutually exclusive.
Eviction Strategy Limitations

Memory based eviction is only supported with the LRU eviction strategy.

Enabling Memory Based Eviction

This eviction method may be used by defining MEMORY as the eviction type, as seen in the following example: 

<local-cache name="local">
    <eviction size="10000000000" strategy="LRU" type="MEMORY"/>
</local-cache>
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2.3.4. Eviction and Passivation

To ensure that a single copy of an entry remains, either in memory or in a cache store, use passivation in conjunction with eviction.
The primary reason to use passivation instead of a normal cache store is that updating entries require less resources when passivation is in use. This is because passivation does not require an update to the cache store.
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