Check DNS Failover, and exec promote read replica.

failover.rb

#!/usr/bin/env ruby 

require 'rubygems'
require 'aws-sdk'
require 'resolv'
require 'pp'

$target_fqdn       = 'target-fqen.example.com'
$target_elb        = 'target-elb-XXXXXX.ap-northeast-1.elb.amazonaws.com'
$access_key_id     = 'YOUR ACCESS KEY ID'
$secret_access_key = 'YOUR SECRET ACCESS KEY'
$failover_region   = 'ap-southeast-1'
$failover_replica  = 'drtest-apse'

fqdn_ip = Resolv.getaddress($target_fqdn)
elb_ip  = Resolv.getaddress($target_elb)

if fqdn_ip == elb_ip
  puts "[#{Time.now.strftime("%Y/%m/%d %H:%M:%S")}] Status OK."
  exit
end

rds = AWS::RDS.new(
  :access_key_id     => $access_key_id,
  :secret_access_key => $secret_access_key,
  :region            => $failover_region
).client
 
rds.promote_read_replica({:db_instance_identifier => $failover_replica})
warn 'Failover occurd.'