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<document>
	<properties>
		<title>Remote Auxiliary Cache Client / Server</title>
		<author email="pete@kazmier.com">Pete Kazmier</author>
		<author email="ASmuts@apache.org">Aaron Smuts</author>
	</properties>

	<body>
		<section name="Remote Auxiliary Cache Client / Server">
			<p>
				The Remote Auxiliary Cache is an optional plug in for
				JCS. It is intended for use in multi-tiered systems to
				maintain cache consistency. It uses a highly reliable
				RMI client server framework that currently allows for
				any number of clients. Using a listener id allows
				multiple clients running on the same machine to connect
				to the remote cache server. All cache regions on one
				client share a listener per auxiliary, but register
				separately. This minimizes the number of connections
				necessary and still avoids unnecessary updates for
				regions that are not configured to use the remote cache.
			</p>
			<p>
				Local remote cache clients connect to the remote cache
				on a configurable port and register a listener to
				receive cache update callbacks at a configurable port.
			</p>
			<p>
				If there is an error connecting to the remote server or
				if an error occurs in transmission, the client will
				retry for a configurable number of tires before moving
				into a failover-recovery mode. If failover servers are
				configured the remote cache clients will try to register
				with other failover servers in a sequential order. If a
				connection is made, the client will broadcast all
				relevant cache updates to the failover server while
				trying periodically to reconnect with the primary
				server. If there are no failovers configured the client
				will move into a zombie mode while it tries to
				re-establish the connection. By default, the cache
				clients run in an optimistic mode and the failure of the
				communication channel is detected by an attempted update
				to the server. A pessimistic mode is configurable so
				that the clients will engage in active status checks.
			</p>
			<p>
				The remote cache server broadcasts updates to listeners
				other than the originating source. If the remote cache
				fails to propagate an update to a client, it will retry
				for a configurable number of tries before de-registering
				the client.
			</p>
			<p>
				The cache hub communicates with a facade that implements
				a zombie pattern (balking facade) to prevent blocking.
				Puts and removals are queued and occur asynchronously in
				the background. Get requests are synchronous and can
				potentially block if there is a communication problem.
			</p>
			<p>
				By default client updates are light weight. The client
				listeners are configured to remove elements form the
				local cache when there is a put order from the remote.
				This allows the client memory store to control the
				memory size algorithm from local usage, rather than
				having the usage patterns dictated by the usage patterns
				in the system at large.
			</p>
			<p>
				When using a remote cache the local cache hub will
				propagate elements in regions configured for the remote
				cache if the element attributes specify that the item to
				be cached can be sent remotely. By default there are no
				remote restrictions on elements and the region will
				dictate the behavior. The order of auxiliary requests is
				dictated by the order in the configuration file. The
				examples are configured to look in memory, then disk,
				then remote caches. Most elements will only be retrieved
				from the remote cache once, when they are not in memory
				or disk and are first requested, or after they have been
				invalidated.
			</p>
			<subsection name="Client Configuration">
				<p>
					The configuration is fairly straightforward and is
					done in the auxiliary cache section of the
					<code>cache.ccf</code>
					configuration file. In the example below, I created
					a Remote Auxiliary Cache Client referenced by
					<code>RFailover</code>
					.
				</p>
				<p>
					This auxiliary cache will use
					<code>localhost:1102</code>
					as its primary remote cache server and will attempt
					to failover to
					<code>localhost:1103</code>
					if the primary is down.
				</p>
				<p>
					Setting
					<code>RemoveUponRemotePut</code>
					to
					<code>false</code>
					would cause remote puts to be translated into put
					requests to the client region. By default it is
					<code>true</code>
					, causing remote put requests to be issued as
					removes at the client level. For groups the put
					request functions slightly differently: the item
					will be removed, since it is no longer valid in its
					current form, but the list of group elements will be
					updated. This way the client can maintain the
					complete list of group elements without the burden
					of storing all of the referenced elements. Session
					distribution works in this half-lazy replication
					mode.
				</p>
				<p>
					Setting
					<code>GetOnly</code>
					to
					<code>true</code>
					would cause the remote cache client to stop
					propagating updates to the remote server, while
					continuing to get items from the remote store.
				</p>
				<source>
					<![CDATA[
# Remote RMI Cache set up to failover
jcs.auxiliary.RFailover=
    org.apache.jcs.auxiliary.remote.RemoteCacheFactory
jcs.auxiliary.RFailover.attributes=
    org.apache.jcs.auxiliary.remote.RemoteCacheAttributes
jcs.auxiliary.RFailover.attributes.FailoverServers=
    localhost:1102,localhost:1103
jcs.auxiliary.RC.attributes.RemoveUponRemotePut=true
jcs.auxiliary.RFailover.attributes.GetOnly=false
        ]]>
				</source>
				<p>
					This cache region is setup to use a disk cache and
					the remote cache configured above:
				</p>
				<source>
					<![CDATA[
#Regions preconfirgured for caching
jcs.region.testCache1=DC,RFailover
jcs.region.testCache1.cacheattributes=
    org.apache.jcs.engine.CompositeCacheAttributes
jcs.region.testCache1.cacheattributes.MaxObjects=1000
jcs.region.testCache1.cacheattributes.MemoryCacheName=
    org.apache.jcs.engine.memory.lru.LRUMemoryCache
        ]]>
				</source>
			</subsection>
			<subsection name="Server Configuration">
				<p>
					The remote cache configuration is growing. For now,
					the configuration is done at the top of the
					<code>remote.cache.ccf</code>
					file. The
					<code>startRemoteCache</code>
					script passes the configuration file name to the
					server when it starts up. The configuration
					parameters below will create a remote cache server
					that listens to port
					<code>1102</code>
					and performs call backs on the
					<code>remote.cache.service.port</code>
					, also specified as port
					<code>1102</code>
					.
				</p>
				<p>
					The tomcat configuration section is evolving. If
					<code>remote.tomcat.on</code>
					is set to
					<code>true</code>
					an embedded tomcat server will run within the remote
					cache, allowing the use of management servlets.
				</p>
				<source>
					<![CDATA[
# Registry used to register and provide the 
# IRemoteCacheService service.
registry.host=localhost
registry.port=1102
# call back port to local caches.
remote.cache.service.port=1102
# cluster setting
remote.cluster.LocalClusterConsistency=true
        ]]>
				</source>
				<p>
					Remote servers can be chainied (or clustered). This
					allows gets from local caches to be distributed
					between multiple remote servers. Since gets are the
					most common operation for caches, remote server
					chaining can help scale a caching solution.
				</p>
				<p>
					The
					<code>LocalClusterConsistency</code>
					setting tells the remote cache server if it should
					broadcast updates received from other cluster
					servers to registered local caches.
				</p>
				<p>
					To use remote server clustering, the remote cache
					will have to be told what regions to cluster. The
					configuration below will cluster all
					non-preconfigured regions with
					<code>RCluster1</code>
					.
				</p>
				<source>
					<![CDATA[
# sets the default aux value for any non configured caches
jcs.default=DC,RCluster1
jcs.default.cacheattributes=
    org.apache.jcs.engine.CompositeCacheAttributes
jcs.default.cacheattributes.MaxObjects=1000

jcs.auxiliary.RCluster1=
    org.apache.jcs.auxiliary.remote.RemoteCacheFactory
jcs.auxiliary.RCluster1.attributes=
    org.apache.jcs.auxiliary.remote.RemoteCacheAttributes
jcs.auxiliary.RCluster1.attributes.RemoteTypeName=CLUSTER
jcs.auxiliary.RCluster1.attributes.RemoveUponRemotePut=false
jcs.auxiliary.RCluster1.attributes.ClusterServers=localhost:1103
jcs.auxiliary.RCluster1.attributes.GetOnly=false
        ]]>
				</source>
				<p>
					RCluster1 is configured to talk to a remote server
					at
					<code>localhost:1103</code>
					. Additional servers can be added in a comma
					separated list.
				</p>
				<p>
					If we startup another remote server listening to
					port 1103, (ServerB) then we can have that server
					talk to the server we have been configuring,
					listening at 1102 (ServerA). This would allow us to
					set some local caches to talk to ServerA and some to
					talk to ServerB. The two remote servers will
					broadcast all puts and removes between themselves,
					and the get requests from local caches could be
					divided. The local caches do not need to know
					anything about the server chaining configuration,
					unless you want to use a standby, or failover
					server.
				</p>
				<p>
					We could also use ServerB as a hot standby. This can
					be done in two ways. You could have all local caches
					point to ServerA as a primary and ServerB as a
					secondary. Alternatively, you can set ServerA as the
					primary for some local caches and ServerB for the
					primary for some others.
				</p>
				<p>
					The local cache configuration below uses ServerA as
					a primary and ServerB as a backup. More than one
					backup can be defined, but only one will be used at
					a time. If the cache is connected to any server
					except the primary, it will try to restore the
					primary connection indefinitely, at 20 second
					intervals.
				</p>
				<source>
					<![CDATA[
	# Remote RMI Cache set up to failover
	jcs.auxiliary.RFailover=
	    org.apache.jcs.auxiliary.remote.RemoteCacheFactory
	jcs.auxiliary.RFailover.attributes=
	    org.apache.jcs.auxiliary.remote.RemoteCacheAttributes
	jcs.auxiliary.RFailover.attributes.FailoverServers=
	    localhost:1102,localhost:1103
	jcs.auxiliary.RC.attributes.RemoveUponRemotePut=true
	jcs.auxiliary.RFailover.attributes.GetOnly=false
	        ]]>
				</source>
			</subsection>
		</section>
	</body>
</document>