GTM Question

Possible if you use an iRule.

Something like this:

when DNS_REQUEST {
  if { [active_members pool_test] < 1 } {
    pool POOL_TEST_Fallback
  } else {
    pool pool_test
}

I have found an alternate solution to do this.

Configure below script as external monitor. Create new monitor using this external script and append it in the VIP along with your existing health monitor. Change the 'Availability Requirement ' to 'All'.

Above script monitor the availability of the pool members. If it is less than 3, then it will change the wide-ip lb method.

!/bin/sh
pidfile="/var/run/$MONITOR_NAME.$1..$2.pid"
 Send signal to the process group to kill our former self and any children
 as external monitors are run with SIGHUP blocked
if [ -f $pidfile ]
then
    kill -9 -`cat $pidfile` > /dev/null 2>&1
fi
echo "$$" > $pidfile

pool=Enter Pool Name
minup=3
virtual=Enter VIP name
wideip=Enter Wide IP Name

upmembers=`tmsh show /ltm pool $pool members |grep "Current Active Members" |awk -F" : " '{ print $2 }'`

if [ $upmembers -ge $minup ]
then
    rm -f $pidfile
    state=`tmsh show /gtm wideip $wideip | grep "Availability" |awk -F" : " '{ print $2 }'`
    if [ $state == "available" ]
    then
        logger -p local0.info -t MONITOR-ALERT "Pool $pool Monitor UP - changing the lb method to global-availability"
        tmsh modify /gtm wideip $wideip pool-lb-mode global-availability
    fi
    echo "up"
else
    rm -f $pidfile
    state=`tmsh show /gtm wideip $wideip | grep "Availability" |awk -F" : " '{ print $2 }'`
    if [ $state == "available" ]
    then
        logger -p local0.info -t MONITOR-ALERT "Pool $pool Monitor DOWN - changing the lb method to ratio"
        tmsh modify /gtm wideip $wideip pool-lb-mode ratio
    fi    
    echo "up"
fi

-Jinshu