Apache Commons logo

Commons JCS™

Remote Auxiliary Cache Client / Server

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.

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.

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 tries 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.

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.

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.

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.

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.

Client Configuration

The configuration is fairly straightforward and is done in the auxiliary cache section of the cache.ccf configuration file. In the example below, I created a Remote Auxiliary Cache Client referenced by RFailover .

This auxiliary cache will use localhost:1102 as its primary remote cache server and will attempt to failover to localhost:1103 if the primary is down.

Setting RemoveUponRemotePut to false would cause remote puts to be translated into put requests to the client region. By default it is true , 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.

Setting GetOnly to true would cause the remote cache client to stop propagating updates to the remote server, while continuing to get items from the remote store.

# Remote RMI Cache set up to failover

This cache region is setup to use a disk cache and the remote cache configured above:

#Regions preconfirgured for caching

Server Configuration

The remote cache configuration is growing. For now, the configuration is done at the top of the remote.cache.ccf file. The startRemoteCache 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 1102 and performs call backs on the remote.cache.service.port , also specified as port 1102 .

# Registry used to register and provide the
# IRemoteCacheService service.
# call back port to local caches.
# rmi socket factory timeout
# cluster setting

Remote servers can be chained (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.

The LocalClusterConsistency setting tells the remote cache server if it should broadcast updates received from other cluster servers to registered local caches.

The AllowClusterGet setting tells the remote cache server whether it should allow the cache to look in non-local auxiliaries for items if they are not present. Basically, if the get request is not from a cluster server, the cache will treat it as if it originated locally. If the get request originated from a cluster client, then the get will be restricted to local (i.e. memory and disk) auxiliaries. Hence, cluster gets can only go one server deep. They cannot be chained. By default this setting is true.

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 RCluster1 .

# sets the default aux value for any non configured caches


RCluster1 is configured to talk to a remote server at localhost:1103 . Additional servers can be added in a comma separated list.

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.

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.

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.

	# Remote RMI Cache set up to failover

Server Startup / Shutdown

It is highly recommended that you embed the Remote Cache Server in a Servlet container such as Tomcat. Running inside Tomcat allows you to use the JCSAdmin.jsp page. It also takes care of the complexity of creating working startup and shutdown scripts.

JCS provides a convenient startup servlet for this purpose. It will start the registry and bind the JCS server to the registry. To use the startup servlet, add the following to the web.xml file and make sure you have the remote.cache.ccf file in the WEB-INF/classes directly of your war file.