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| Oracle Solaris Cluster Data Services Developer's Guide Oracle Solaris Cluster 4.1

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## Overview of the Sample Data Service

The sample data service starts, stops, restarts, and switches the DNS application among the nodes of the cluster in response to cluster events, such as administrative action, application failure, or node failure.

Application restart is managed by the Process Monitor Facility (PMF). If the number of applications that die exceeds the failure count within the failure time window, the fault monitor fails over the resource group that contains the application resource to another node.

The sample data service provides fault monitoring in the form of a `PROBE` method that uses the `nslookup` command to ensure that the application is healthy. If the probe detects a hung DNS service, the probe tries to correct the situation by restarting the DNS application locally. If restarting the DNS application locally does not improve the situation and the probe repeatedly detects problems with the service, the probe attempts to fail over the service to another node in the cluster.

Specifically, the sample data service includes the following elements:

- A resource type registration file that defines the static properties of the data service.

- A `Start` callback method that is run by the RGM to start the `in.named` daemon when the resource group that contains the HA-DNS data service is brought online.

- A `Stop` callback method that is run by the RGM to stop the `in.named` daemon when the resource group that contains HA-DNS goes offline.

- A fault monitor to check the availability of the service by verifying that the DNS server is running. The fault monitor is implemented by a user-defined `PROBE` method, and is started and stopped by the `Monitor_start` and `Monitor_stop` callback methods.

- A `Validate` callback method that is run by the RGM to validate that the configuration directory for the service is accessible.

- An `Update` callback method that is run by the RGM to restart the fault monitor when the cluster administrator changes the value of a resource property.

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