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5.  Sample Data Service Defining a Fault Monitor Probe Program  Previous   Contents   Next 
   
 

Probe Exit Status

The sample data service's PROBE program exits with failure if attempts to restart locally have failed and the attempt to fail over to a different node has failed as well. It logs the message, "Failover attempt failed".

Monitor_start Method

The RGM calls the Monitor_start method to launch the dns_probe method after the sample data service is brought online.

This section describes the major pieces of the Monitor_start method for the sample application. This section does not describe functionality common to all callback methods, such as the parse_args() function and obtaining the syslog facility, which are described in Providing Common Functionality to All Methods.

For the complete listing of the Monitor_start method, see Monitor_start Method.

Monitor_start Overview

This method uses the process monitor facility (pmfadm) to launch the probe.

Starting the Probe

The Monitor_start method obtains the value of the Rt_basedir property to construct the full path name for the PROBE program. This method launches the probe using the infinite retries option of pmfadm (-n -1, -t -1), which means if the probe fails to start, PMF tries to start it an infinite number of times over an infinite period of time. 
# Find where the probe program resides by obtaining the value of the
# RT_BASEDIR property of the resource.
RT_BASEDIR=`scha_resource_get -O RT_BASEDIR -R $RESOURCE_NAME -G \
$RESOURCEGROUP_NAME`

# Start the probe for the data service under PMF. Use the infinite retries 
# option to start the probe. Pass the resource name, type, and group to the 
# probe program. 
pmfadm -c $RESOURCE_NAME.monitor -n -1 -t -1 \
   $RT_BASEDIR/dns_probe -R $RESOURCE_NAME -G $RESOURCEGROUP_NAME \
   -T $RESOURCETYPE_NAME

Monitor_stop Method

The RGM calls the Monitor_stop method to stop execution of dns_probe when the sample data service is brought offline.

This section describes the major pieces of the Monitor_stop method for the sample application. This section does not describe functionality common to all callback methods, such as the parse_args() function and obtaining the syslog facility, which are described in Providing Common Functionality to All Methods.

For the complete listing of the Monitor_stop method, see Monitor_stop Method.

Monitor_stop Overview

This method uses the process monitor facility (pmfadm) to see if the probe is running, and if so, to stop it.

Stopping the Monitor

The Monitor_stop method uses pmfadm -q to see if the probe is running, and if so, uses pmfadm -s to stop it. If the probe is already stopped, the method exits successfully anyway, which guarantees the idempotency of the method. 
# See if the monitor is running, and if so, kill it. 
if pmfadm -q $PMF_TAG; then 
   pmfadm -s $PMF_TAG KILL
   if [ $? -ne 0 ]; then 
         logger -p ${SYSLOG_FACILITY}.err \
            -t [$SYSLOG_TAG] \
            "${ARGV0} Could not stop monitor for resource " \
            $RESOURCE_NAME
           exit 1
   else
         # could successfully stop the monitor. Log a message.
         logger -p ${SYSLOG_FACILITY}.err \
            -t [$SYSLOG_TAG] \
            "${ARGV0} Monitor for resource " $RESOURCE_NAME \
            " successfully stopped"
   fi
fi
exit 0

Caution! Caution - Be certain to use the KILL signal with pmfadm to stop the probe and not a maskable signal such as TERM. Otherwise the Monitor_stop method can hang indefinitely and eventually time out. The reason for this problem is that the PROBE method calls scha_control() when it is necessary to restart or fail over the data service. When scha_control() calls Monitor_stop as part of the process of bringing the data service offline, if Monitor_stop uses a maskable signal, it hangs waiting for scha_control() to complete and scha_control() hangs waiting for Monitor_stop to complete.

Monitor_stop Exit Status

The Monitor_stop method logs an error message if it cannot stop the PROBE method. The RGM puts the sample data service into MONITOR_FAILED state on the primary node, which can panic the node.

Monitor_stop should not exit before the probe has been stopped.

Monitor_check Method

The RGM calls the Monitor_check method whenever the PROBE method attempts to fail the resource group containing the data service over to a new node.

This section describes the major pieces of the Monitor_check method for the sample application. This section does not describe functionality common to all callback methods, such as the parse_args() function and obtaining the syslog facility, which are described in Providing Common Functionality to All Methods.

For the complete listing of the Monitor_check method, see Monitor_check Method.

The Monitor_check method must be implemented so that it does not conflict with other methods running concurrently.

The Monitor_check method calls the Validate method to verify that the DNS configuration directory is available on the new node. The Confdir extension property points to the DNS configuration directory. Therefore Monitor_check obtains the path and name for the Validate method and the value of Confdir. It passes this value to Validate, as shown in the following listing. 
# Obtain the full path for the Validate method from
# the RT_BASEDIR property of the resource type.
RT_BASEDIR=`scha_resource_get -O RT_BASEDIR -R $RESOURCE_NAME \
   -G $RESOURCEGROUP_NAMÈ

# Obtain the name of the Validate method for this resource.
VALIDATE_METHOD=`scha_resource_get -O VALIDATE \
   -R $RESOURCE_NAME -G $RESOURCEGROUP_NAMÈ

# Obtain the value of the Confdir property in order to start the
# data service. Use the resource name and the resource group entered to
# obtain the Confdir value set at the time of adding the resource.
config_info=`scha_resource_get -O Extension -R $RESOURCE_NAME \
 -G $RESOURCEGROUP_NAME Confdir`

# scha_resource_get returns the type as well as the value for extension
# properties. Use awk to get only the value of the extension property.
CONFIG_DIR=`echo $config_info | awk `{print $2}'`

# Call the validate method so that the dataservice can be failed over 
# successfully to the new node.
$RT_BASEDIR/$VALIDATE_METHOD -R $RESOURCE_NAME -G $RESOURCEGROUP_NAME \
   -T $RESOURCETYPE_NAME -x Confdir=$CONFIG_DIR

See the Validate Method to see how the sample application verifies the suitability of a node for hosting the data service.

Handling Property Updates

The sample data service implements Validate and Update methods to handle updating of properties by a cluster administrator.

Validate Method

The RGM calls the Validate method when a resource is created and when administrative action updates the properties of the resource or its containing group. The RGM calls Validate before the creation or update is applied, and a failure exit code from the method on any node causes the creation or update to be canceled.

The RGM calls Validate only when resource or group properties are changed through administrative action, not when the RGM sets properties, or when a monitor sets the resource properties Status and Status_msg.

Note - The Monitor_check method also explicitly calls the Validate method whenever the PROBE method attempts to fail the data service over to a new node.

 		 
 
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