Configuring Mediators

Figure 9-1 shows a Sun Cluster system configured with two strings and mediators on two Sun Cluster nodes.

Regardless of the number of nodes, there are still only two mediator hosts in the cluster. The mediator hosts are the same for all disksets using mediators in a given cluster, even when a mediator host is not a member of the server set capable of mastering the diskset.

To simplify the presentation, the configurations shown here use only one diskset and a symmetric configuration. The number of disksets is not significant in these sample scenarios. In the stable state, the diskset is mastered by phys-hahost1.

Figure 9-1 Sun Cluster System in Steady State With Mediators

Normally, if half + 1 of the database replicas are accessible, then mediators are not used. When exactly half of the replicas are accessible, the mediator's commit count can be used to determine whether the accessible half is the most up to date. To guarantee that the correct mediator commit count is being used, both of the mediators must be accessible, or the mediator must be golden. Half + 1 of the mediators constitutes a mediator quorum. The mediator quorum is independent of the replica quorum.