net.sf.ehcache.distribution

Class RMIAsynchronousCacheReplicator

public class RMIAsynchronousCacheReplicator extends RMISynchronousCacheReplicator

Listens to CacheManager and Cache events and propagates those to CachePeer peers of the Cache asynchronously.

Updates are guaranteed to be replicated in the order in which they are received.

While much faster in operation than RMISynchronousCacheReplicator, it does suffer from a number of problems. Elements, which may be being spooled to DiskStore may stay around in memory because references are being held to them from EventMessages which are queued up. The replication thread runs once per second, limiting the build up. However a lot of elements can be put into a cache in that time. We do not want to get an OutOfMemoryError using distribution in circumstances when it would not happen if we were just using the DiskStore.

Accordingly, the Element values in EventMessages are held by java.lang.ref.SoftReference in the queue, so that they can be discarded if required by the GC to avoid an OutOfMemoryError. A log message will be issued on each flush of the queue if there were any forced discards. One problem with GC collection of SoftReferences is that the VM (JDK1.5 anyway) will do that rather than grow the heap size to the maximum. The workaround is to either set minimum heap size to the maximum heap size to force heap allocation at start up, or put up with a few lost messages while the heap grows.

Version: $Id: RMIAsynchronousCacheReplicator.java 191 2006-09-03 22:41:48Z gregluck $

Author: Greg Luck

Field Summary
protected intasynchronousReplicationInterval
The amount of time the replication thread sleeps after it detects the replicationQueue is empty before checking again.
protected ListreplicationQueue
A queue of updates.
protected ThreadreplicationThread
A thread which handles replication, so that replication can take place asynchronously and not hold up the cache
Constructor Summary
RMIAsynchronousCacheReplicator(boolean replicatePuts, boolean replicateUpdates, boolean replicateUpdatesViaCopy, boolean replicateRemovals, int asynchronousReplicationInterval)
Constructor for internal and subclass use
Method Summary
protected voidaddToReplicationQueue(RMIAsynchronousCacheReplicator.CacheEventMessage cacheEventMessage)
Adds a message to the queue.
Objectclone()
Creates a clone of this listener.
voiddispose()
Give the replicator a chance to cleanup and free resources when no longer needed
voidnotifyElementPut(Ehcache cache, Element element)
{@inheritDoc }

This implementation queues the put notification for in-order replication to peers.

voidnotifyElementRemoved(Ehcache cache, Element element)
Called immediately after an attempt to remove an element.
voidnotifyElementUpdated(Ehcache cache, Element element)
Called immediately after an element has been put into the cache and the element already existed in the cache.
voidnotifyRemoveAll(Ehcache cache)
Called during removeAll to indicate that the all elements have been removed from the cache in a bulk operation.

Field Detail

asynchronousReplicationInterval

protected int asynchronousReplicationInterval
The amount of time the replication thread sleeps after it detects the replicationQueue is empty before checking again.

replicationQueue

protected final List replicationQueue
A queue of updates.

replicationThread

protected Thread replicationThread
A thread which handles replication, so that replication can take place asynchronously and not hold up the cache

Constructor Detail

RMIAsynchronousCacheReplicator

public RMIAsynchronousCacheReplicator(boolean replicatePuts, boolean replicateUpdates, boolean replicateUpdatesViaCopy, boolean replicateRemovals, int asynchronousReplicationInterval)
Constructor for internal and subclass use

Parameters: replicatePuts replicateUpdates replicateUpdatesViaCopy replicateRemovals asynchronousReplicationInterval

Method Detail

addToReplicationQueue

protected void addToReplicationQueue(RMIAsynchronousCacheReplicator.CacheEventMessage cacheEventMessage)
Adds a message to the queue.

This method checks the state of the replication thread and warns if it has stopped and then discards the message.

Parameters: cacheEventMessage

clone

public Object clone()
Creates a clone of this listener. This method will only be called by ehcache before a cache is initialized.

This may not be possible for listeners after they have been initialized. Implementations should throw CloneNotSupportedException if they do not support clone.

Returns: a clone

Throws: CloneNotSupportedException if the listener could not be cloned.

dispose

public final void dispose()
Give the replicator a chance to cleanup and free resources when no longer needed

notifyElementPut

public final void notifyElementPut(Ehcache cache, Element element)
{@inheritDoc }

This implementation queues the put notification for in-order replication to peers.

Parameters: cache the cache emitting the notification element the element which was just put into the cache.

notifyElementRemoved

public final void notifyElementRemoved(Ehcache cache, Element element)
Called immediately after an attempt to remove an element. The remove method will block until this method returns.

This notification is received regardless of whether the cache had an element matching the removal key or not. If an element was removed, the element is passed to this method, otherwise a synthetic element, with only the key set is passed in.

Parameters: cache the cache emitting the notification element the element just deleted, or a synthetic element with just the key set if no element was removed.

notifyElementUpdated

public final void notifyElementUpdated(Ehcache cache, Element element)
Called immediately after an element has been put into the cache and the element already existed in the cache. This is thus an update.

The put method will block until this method returns.

Implementers may wish to have access to the Element's fields, including value, so the element is provided. Implementers should be careful not to modify the element. The effect of any modifications is undefined.

Parameters: cache the cache emitting the notification element the element which was just put into the cache.

notifyRemoveAll

public void notifyRemoveAll(Ehcache cache)
Called during removeAll to indicate that the all elements have been removed from the cache in a bulk operation. The usual notifyElementRemoved is not called.

This notification exists because clearing a cache is a special case. It is often not practical to serially process notifications where potentially millions of elements have been bulk deleted.

Parameters: cache the cache emitting the notification