Chapter 17: Clustering In-memory replication (SimpleTcpCluster) Tomcat 6
Chapter 17: Clustering In-memory replication (SimpleTcpCluster) Tomcat 6 instance In-memory replication (SimpleTcpCluster) Tomcat 6 instance Local area network In-memory replication (SimpleTcpCluster) Tomcat 6 instance Replicated session information Guaranteed messaging (Apache Tribes) Communication groups membership (Apache Tribes) Figure 17-7: Tomcat 6 in-memory session replication In Figure 17-7 , the Tomcat 6 server instances running in the cluster are implemented as a communications group. At the Tomcat instance level, the cluster implementation is an instance of the Simple TcpClusterclass. Depending on your needs, and the replication pattern you want, you can configure SimpleTcpCluster with one of two managers. The two available managers with Tomcat 6 are: . DeltaManager to replicate sessions across all Tomcat instances . BackupManager to replicate sessions from a master to a backup instance (see the master-backup topological discussion earlier in this chapter) Under the hood, SimpleTcpCluster uses Apache Tribes to maintain and communicate with the communications group. Apache Tribes is a reusable API library for implementation of communications groups over TCP networks. Group membership is established and maintained by Apache Tribes. If a server crashes, it is automatically removed from the group. A recovered server is automatically made a member of the group again. Apache Tribes also offer several levels of guaranteed message delivery between group members. This capability is utilized by the SimpleTcpCluster to provide a configurable level of service. Any session-creation and modification activities on Tomcat 6 instances are sent to one or all instances within the communications group (depending on the manager implementation configured). The receiving group member(s) then update its (their) own session image in memory to reflect the change. In this way, sessions and changes are replicated immediately in memory, and between members of the group. The main benefits of this approach are as follows: . Full round-robin load balancing enables an even distribution of requests (subject to prefigured lbfactorload factor on mod_jk). Any server can handle any request. . With full HA support, any server can crash and fail-over with no session loss.
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