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

Probe Program

The dns_probe program implements a continuously running process that verifies the DNS resource controlled by the sample data service is running. The dns_probe is launched by the dns_monitor_start method, which is automatically invoked by the RGM after the sample data service is brought online. The data service is stopped by the dns_monitor_stop method, which then RGM invokes before bringing the sample data service offline.

This section describes the major pieces of the PROBE method for the sample application. It 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 PROBE method, see PROBE Program.

Probe Overview

The probe runs in an infinite loop. It uses nslookup to verify that the proper DNS resource is running. If DNS is running, the probe sleeps for a prescribed interval (set by the Thorough_probe_interval system-defined property) and then checks again. If DNS is not running, this program attempts to restart it locally, or depending on the number of restart attempts, requests that the RGM relocate the data service to a different node.

Obtaining Property Values

This program needs the values of the following properties:

  • Thorough_probe_interval - To set the period during which the probe sleeps

  • Probe_timeout - to enforce the time-out value of the probe on the nslookup command that does the probing

  • Network_resources_used - To obtain the IP address on which DNS is running

  • Retry_count and Retry_interval - To determine the number of restart attempts and the period over which to count them

  • Rt_basedir - To obtain the directory containing the PROBE program and the gettime utility

The scha_resource_get() function obtains the values of these properties and stores them in shell variables, as follows.
PROBE_INTERVAL=`scha_resource_get -O THOROUGH_PROBE_INTERVAL \
-R $RESOURCE_NAME -G $RESOURCEGROUP_NAME`

probe_timeout_info=`scha_resource_get -O Extension -R $RESOURCE_NAME
\
-G $RESOURCEGROUP_NAME Probe_timeout` 
PROBE_TIMEOUT=`echo $probe_timeout_info | awk '{print $2}'`

DNS_HOST=`scha_resource_get -O NETWORK_RESOURCES_USED -R $RESOURCE_NAME
\
-G $RESOURCEGROUP_NAME`

RETRY_COUNT=`scha_resource_get -O RETRY_COUNT -R $RESOURCE_NAME
-G\
 $RESOURCEGROUP_NAME`

RETRY_INTERVAL=`scha_resource_get -O RETRY_INTERVAL -R $RESOURCE_NAME
-G\
 $RESOURCEGROUP_NAME`

RT_BASEDIR=`scha_resource_get -O RT_BASEDIR -R $RESOURCE_NAME -G\
 $RESOURCEGROUP_NAME`

Note - For system-defined properties, such as Thorough_probe_interval, scha_resource_get() returns the value only. For extension properties, such as Probe_timeout, scha_resource_get() returns the type and value. Use the awk command to obtain the value only.

Checking the Reliability of the Service

The probe itself is an infinite while loop of nslookup commands. Before the while loop, a temporary file is set up to hold the nslookup replies. The probefail and retries variables are initialized to 0. 
# Set up a temporary file for the nslookup replies.
DNSPROBEFILE=/tmp/.$RESOURCE_NAME.probe
probefail=0
retries=0

The while loop itself:

  • Sets the sleep interval for the probe

  • Uses hatimerun to launch nslookup passing the Probe_timeout value and identifying the target host

  • Sets the probefail variable based on the success or failure of the nslookup return code

  • If probefail is set to 1 (failure), verifies that the reply to nslookup came from the sample data service and not some other DNS server

Here is the while loop code.
while :
do
   # The interval at which the probe needs to run is specified in the
   # property THOROUGH_PROBE_INTERVAL. Therefore, set the probe to sleep
   # for a duration of THOROUGH_PROBE_INTERVAL.
   sleep $PROBE_INTERVAL

   # Run an nslookup command of the IP address on which DNS is serving.
   hatimerun -t $PROBE_TIMEOUT /usr/sbin/nslookup $DNS_HOST $DNS_HOST \
          > $DNSPROBEFILE 2>&1

      retcode=$?
      if [ $retcode -ne 0 ]; then
            probefail=1
      fi

   # Make sure that the reply to nslookup comes from the HA-DNS
   # server and not from another nameserver mentioned in the 
   # /etc/resolv.conf file.
   if [ $probefail -eq 0 ]; then
# Get the name of the server that replied to the nslookup query.
   SERVER=` awk ' $1=="Server:" { print $2 }' \
   $DNSPROBEFILE | awk -F. ' { print $1 } ' `
   if [ -z "$SERVER" ]; then
      probefail=1
      else
         if [ $SERVER != $DNS_HOST ]; then
            probefail=1
         fi
   fi
fi

 		 
 
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