Typically, you implement monitors to run periodic fault probes on resources to detect whether the probed resources are functioning correctly. If a fault probe fails, the monitor can attempt to restart locally or request failover of the affected resource group by calling the scha_control RMAPI function or the scds_fm_action DSDL function.
You can also monitor the performance of a resource and tune or report performance. Writing a resource type-specific fault monitor is completely optional. Even if you choose not to write such a fault monitor, the resource type benefits from the basic monitoring of the cluster that Sun Cluster itself does. Sun Cluster detects failures of the host hardware, gross failures of the host's operating system, and failures of a host to be able to communicate on its public networks.
Although the RGM does not call a resource monitor directly, it does provide for automatically starting monitors for resources. When bringing a resource offline, the RGM calls the Monitor_stop method to stop the resource's monitor on the local nodes before stopping the resource itself. When bringing a resource online, the RGM calls the Monitor_start method after the resource itself has been started.
The scha_control RMAPI function and the scds_fm_action DSDL function (which calls scha_control) allow resource monitors to request the failover of a resource group to a different node. As one of its sanity checks, scha_control calls Monitor_check (if defined), to determine if the requested node is reliable enough to master the resource group containing the resource. If Monitor_check reports back that the node is not reliable, or the method times out, the RGM looks for a different node to honor the failover request. If Monitor_check fails on all nodes, the failover is canceled.
The resource monitor can set the Status and Status_msg properties to reflect the monitor's view of the resource state. Use the RMAPI scha_resource_setstatus(1HA), (3HA) command or function, or the DSDL scds_fm_action(3HA) function to set these properties.
Although Status and Status_msg are of particular use to a resource monitor, any program can set these properties.
See Defining a Fault Monitor for an example of a fault monitor implemented with the RMAPI. See The SUNW.xfnts Fault Monitor for an example of a fault monitor implemented with the DSDL. See the Sun Cluster 3.1 Data Services Installation and Configuration Guide for information on fault monitors built into Sun supplied data services.