1.5.1 Cluster Framework

Figure 1-17 shows the approximate layering of the various components that constitute the framework required to support HA data services in Sun Cluster. This diagram does not illustrate the relationship between the various components of Sun Cluster. The innermost core consists of the Cluster Membership Monitor (CMM), which keeps track of the current cluster membership. Whenever nodes leave or rejoin the cluster, the CMM on the cluster nodes go through a distributed membership protocol to agree on the new cluster membership. Once the new membership is established, the CMM orchestrates the reconfiguration of the other cluster components through the Sun Cluster framework.

Figure 1-17 Sun Cluster Software Components

In an HA configuration, the membership monitor, fault monitor, and associated programs allow one Sun Cluster server to take over processing of all data services from the other Sun Cluster server when hardware or software fails. This is accomplished by causing a Sun Cluster server without the failure to take over mastery of the logical host associated with the failed Sun Cluster server. Some types of failures do not cause failover. Disk drive failure does not typically result in a failover--mirroring handles this. Similarly, software failures detected by the fault monitors might cause a data service to be restarted on the same physical node rather than failing over to another node.