Chapter 17: Clustering Sessions Filtered Replication valve (filter) TOMCAT Replication Apache Tribes To other nodes in cluster Figure 17-9: Tomcat components for in-memory session replication This operation of Apache Tribes based implementation is shown in Figure 17-10 . It uses regular multicast (heartbeat packets) to determine membership (and TCP for data transfer and other communications). Any node that is up and running must multicast a heartbeat at regular frequency. Nodes within the same cluster listen to and multicast at the same multicast address and port. Nodes that do not multicast within a required dropTime are considered dead and are removed from the cluster (until they start multicasting again). The membership listens for the multicast and manages the set of current nodes in the cluster dynamically. Session replication requests and session updates are sent between member nodes in the cluster using TCP connections directly, in an end-to-end connection. This means that a node sending replication data makes a direct TCP connection to each and every member node in the cluster when replicating a session. Because of the amount of network traffic generated, the default implementation is practical for use only for clusters with a small membership size. You can achieve some economy by using the BackupManager instead of the default DeltaManagerwith the component. The BackupManagersends data to a backup node (and nodes with the corresponding application deployed) instead of to all members in the cluster. Cluster Session Manager Configuration with the Element The first component is a new component. For this example, the component is nested inside an enclosing element. Including this component in a essentially enables session replication for all applications in the host. The standard manager component used to manage sessions in Tomcat will be replaced with a session replication enabled manager (either DeltaManageror BackupManager).
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