A cluster manager is an extension to Tomcat's session manager interface, org.apache.catalina.Manager A cluster manager must implement the org.apache.catalina.ha.ClusterManager and is solely responsible for how the session is replicated.
There are currently two different managers, the org.apache.catalina.ha.session.DeltaManager replicates deltas of session data to all members in the cluster. This implementation is proven and works very well, but has a limitation as it requires the cluster members to be homogeneous, all nodes must deploy the same applications and be exact replicas. The org.apache.catalina.ha.session.BackupManager also replicates deltas but only to one backup node. The location of the backup node is known to all nodes in the cluster. It also supports heterogeneous deployments, so the manager knows at what locations the webapp is deployed.
We are planning to add more managers with even more sophisticated backup mechanism to support even larger clusters. Check back soon!

The <Manager> element defined inside the <Cluster> element is the template defined for all web applications that are marked <distributable/> in their web.xml file. However, you can still override the manager implementation on a per web application basis, by putting the <Manager> inside the <Context> element either in the context.xml file or the server.xml file.

Common Attributes

Attribute Description className name The name of this cluster manager, the name is used to identify a session manager on a node. The name might get modified by the Cluster element to make it unique in the container. defaultMode Deprecated since 6.0.0 notifyListenersOnReplication Set to true if you wish to have session listeners notified when session attributes are being replicated or removed across Tomcat nodes in the cluster. expireSessionsOnShutdown When a webapplication is being shutdown, Tomcat issues an expire call to each session to notify all the listeners. If you wish for all sessions to expire on all nodes when a shutdown occurs on one node, set this value to true. Default value is false.

org.apache.catalina.ha.session.DeltaManager Attributes

Attribute Description domainReplication Set to true if you wish sessions to be replicated only to members that have the same logical domain set. If set to false, session replication will ignore the domain setting the <Membership> element. expireSessionsOnShutdown When a webapplication is being shutdown, Tomcat issues an expire call to each session to notify all the listeners. If you wish for all sessions to expire on all nodes when a shutdown occurs on one node, set this value to true. Default value is false. stateTransferTimeout The time in seconds to wait for a session state transfer to complete from another node when a node is starting up. Default value is 60 seconds. sessionAttributeValueClassNameFilter A regular expression used to filter which session attributes will be replicated. An attribute will only be replicated if the implementation class name of the value matches this pattern. If the pattern is zero length or null, all attributes are eligible for replication. The pattern is anchored so the fully qualified class name must fully match the pattern. If not specified, the default value of null will be used unless a SecurityManager is enabled in which case the default will be java\\.lang\\.(?:Boolean|Integer|Long|Number|String). warnOnSessionAttributeFilterFailure If sessionAttributeNameFilter or sessionAttributeValueClassNameFilter blocks an attribute, should this be logged at WARN level? If WARN level logging is disabled then it will be logged at DEBUG. The default value of this attribute is false unless a SecurityManager is enabled in which case the default will be true.

org.apache.catalina.ha.session.BackupManager Attributes

Attribute Description mapSendOptions The backup manager uses a replicated map, this map is sending and receiving messages. You can setup the flag for how this map is sending messages, the default value is 6(asynchronous). sessionAttributeValueClassNameFilter A regular expression used to filter which session attributes will be replicated. An attribute will only be replicated if the implementation class name of the value matches this pattern. If the pattern is zero length or null, all attributes are eligible for replication. The pattern is anchored so the fully qualified class name must fully match the pattern. If not specified, the default value of null will be used unless a SecurityManager is enabled in which case the default will be java\\.lang\\.(?:Boolean|Integer|Long|Number|String). warnOnSessionAttributeFilterFailure If sessionAttributeNameFilter or sessionAttributeValueClassNameFilter blocks an attribute, should this be logged at WARN level? If WARN level logging is disabled then it will be logged at DEBUG. The default value of this attribute is false unless a SecurityManager is enabled in which case the default will be true.