elbfailover.sh

#! /bin/bash
 
SELF_INSTANCE_ID=`curl -s http://169.254.169.254/latest/meta-data/instance-id`
ELB_NAME=elbfailover
SLEEPTIME=2
 
while :; do
  TIMESTAMP=`date -u +"%Y-%m-%dT%H:%M:%SZ"`
  ELB_STATUS=`aws elb describe-instance-health --load-balancer-name $ELB_NAME`
  HEALTHY_HOST_COUNT=`echo $ELB_STATUS | jq "[ .InstanceStates[] | select(.InstanceId != \"$SELF_INSTANCE_ID\" and .State == \"InService\") ] | length"`
  SORRY_HOST_COUNT=`echo $ELB_STATUS | jq "[ .InstanceStates[] | select(.InstanceId == \"$SELF_INSTANCE_ID\") ] | length"`
   
  if [ $HEALTHY_HOST_COUNT -gt 0 ]; then
    echo $TIMESTAMP : $HEALTHY_HOST_COUNT healty host\(s\) found on $ELB_NAME
    if [ $SORRY_HOST_COUNT -gt 0 ]; then
      echo deregistering sorry host: $SELF_INSTANCE_ID from $ELB_NAME
      RESULT=`aws elb deregister-instances-from-load-balancer \
          --instances "{\"instance_id\": \"$SELF_INSTANCE_ID\"}" --load-balancer-name $ELB_NAME`
      if [ $? -eq 0 ]; then
        echo sorry host was deregistered successfully
      else
        echo "ERROR: fail to deregister sorry host"
        echo $RESULT | jq "."
      fi
    fi
  else
    echo $TIMESTAMP : healthy host not found on $ELB_NAME
    if [ $SORRY_HOST_COUNT -eq 0 ]; then
      echo registering sorry host: $SELF_INSTANCE_ID to $ELB_NAME
      RESULT=`aws elb register-instances-with-load-balancer \
          --instances "{\"instance_id\": \"$SELF_INSTANCE_ID\"}" --load-balancer-name $ELB_NAME`
      if [ $? -eq 0 ]; then
        echo sorry host was registered successfully
      else
        echo "ERROR: fail to register sorry host"
        echo $RESULT | jq "."
      fi
    fi
  fi
   
  sleep $SLEEPTIME
done