The LocalCache
class is used for size-limited caches. It is used both for caching on-heap objects (as in a local cache or the front portion of a near cache) and as the backing map for a partitioned cache. Applications can provide custom eviction policies for use with a LocalCache
.
Note that Coherence's default eviction policy is very effective for most workloads; the majority of applications will not need to provide a custom policy. Generally, it is best to restrict the use of eviction policies to scenarios where the evicted data is present in a backing system (that is, the back portion of a near cache or a database). Eviction should be treated as a physical operation (freeing memory) and not a logical operation (deleting an entity).
Example 11-1 shows the implementation of a simple custom eviction policy:
Example 11-1 Implementing a Custom Eviction Policy
import com.tangosol.net.cache.CacheEvent; import com.tangosol.net.cache.LocalCache; import com.tangosol.net.cache.OldCache; import com.tangosol.util.AbstractMapListener; import com.tangosol.util.MapEvent; import java.util.Iterator; public class MyEvictionPolicy extends AbstractMapListener implements OldCache.EvictionPolicy { LocalCache m_cache = null; public void entryInserted(MapEvent evt) { System.out.println("entryInserted:" + isSynthetic(evt) + evt); if (m_cache == null) { m_cache = (LocalCache) evt.getMap(); } } public void entryUpdated(MapEvent evt) { System.out.println("entryUpdated:" + isSynthetic(evt) + evt); } public void entryDeleted(MapEvent evt) { System.out.println("entryDeleted:" + isSynthetic(evt) + evt); } String isSynthetic(MapEvent evt) { // synthetic events are caused by internal processing - eviction or loading return ((CacheEvent) evt).isSynthetic() ? " SYNTHETIC " : " "; } public void entryTouched(OldCache.Entry entry) { System.out.println("entryTouched:" + entry.getKey()); } public void requestEviction(int cMaximum) { int cCurrent = m_cache.getUnits(); System.out.println("requestEviction: current:" + cCurrent + " to:" + cMaximum); // // ... eviction policy calculations ... // for (Iterator iter = m_cache.entrySet().iterator(); iter.hasNext();) { OldCache.Entry entry = (OldCache.Entry) iter.next(); if (m_cache.getUnits() > cMaximum) { m_cache.evict(entry.getKey()); } else { break; } } } public MyEvictionPolicy() { } }
Example 11-2 illustrates a Coherence cache configuration file (coherence-cache-config.xml
) with an eviction policy:
Example 11-2 Custom Eviction Policy in a coherence-cache-config.xml File
<?xml version="1.0"?>
<!DOCTYPE cache-config SYSTEM "cache-config.dtd">
<cache-config>
<caching-scheme-mapping>
<cache-mapping>
<cache-name>test</cache-name>
<scheme-name>example-near</scheme-name>
</cache-mapping>
</caching-scheme-mapping>
<caching-schemes>
<distributed-scheme>
<scheme-name>example-distributed</scheme-name>
<service-name>DistributedCache</service-name>
<backing-map-scheme>
<local-scheme>
<scheme-ref>example-backing-map</scheme-ref>
</local-scheme>
</backing-map-scheme>
<autostart>true</autostart>
</distributed-scheme>
<near-scheme>
<scheme-name>example-near</scheme-name>
<front-scheme>
<local-scheme>
<eviction-policy>
<class-scheme>
<class-name>MyEvictionPolicy</class-name>
</class-scheme>
</eviction-policy>
<high-units>10</high-units>
</local-scheme>
</front-scheme>
<back-scheme>
<distributed-scheme>
<scheme-ref>example-distributed</scheme-ref>
</distributed-scheme>
</back-scheme>
<invalidation-strategy>all</invalidation-strategy>
<autostart>true</autostart>
</near-scheme>
<local-scheme>
<scheme-name>example-backing-map</scheme-name>
<eviction-policy>HYBRID</eviction-policy>
<high-units>{back-size-limit 0}</high-units>
<expiry-delay>{back-expiry 1h}</expiry-delay>
<flush-delay>1m</flush-delay>
<cachestore-scheme></cachestore-scheme>
</local-scheme>
</caching-schemes>
</cache-config>