This is a work-in-progress script I whipped up over the weekend. This assumes the following architecture:

• For a 3-tier architecture
• Autoscaling Group may or may not run on multi-AZ
• Each AZ will have a software loadbalancer
• SNS and SQS are utilized for notification of scaling activities (Starting and Terminating of instances)
• Script runs through through cron and most be installed in between the Web and App servers
• Uses the string "# Begin" to mark where new HAProxy configs will go. So do an "echo '# Begin' >> /etc/haproxy/haproxy.cnf" before running this script
• Step-by-step procedure on how to setup autoscaling is documented here.

#!/usr/bin/perl
# EC2 Autoscaling dynamic registration script for HAProxy
# Requirements: SNS topic, SQS subscription
# Notes: Supposed to be run as a cronjob
# John Homer H Alvero
# April 29, 2013
#
# Install pre-reqs by
# yum install perl-Amazon-SQS-Simple perl-Net-Amazon-EC2 --enablerepo=epel
use Amazon::SQS::Simple;
use Net::Amazon::EC2;
 
my $access_key   = '';
my $secret_key   = '';
my $queue_endpoint  = 'https://sqs.us-east-1.amazonaws.com/123456789012/yourqueue';
my $haproxy_file  = '/etc/haproxy/haproxy.cfg';
my $my_az  = `wget -qO- http://169.254.169.254/latest/meta-data/placement/availability-zone`;
 
# Create an SQS object
my $sqs = new Amazon::SQS::Simple($access_key, $secret_key);
 
# Connect to an existing queue
my $q = $sqs->GetQueue($queue_endpoint);
 
my $ec2 = Net::Amazon::EC2->new(AWSAccessKeyId => $access_key, SecretAccessKey => $secret_key);
 
# Retrieve a message
while (my $msg = $q->ReceiveMessage()) {
 $sqs_msg = $msg->MessageBody();
 
 # parse message, get instance id
 (my $action = $1, $instance_id = $2) if $sqs_msg =~ /(Terminating|Launching).+EC2InstanceId\\\"\:\\\"(i-.{8})/;
 
 # do action
 my $running_instances = $ec2->describe_instances(InstanceId => $instance_id);
 
 foreach my $reservation (@$running_instances) {
  foreach my $instance ($reservation->instances_set) {
   $pdns_name = $instance->private_dns_name;
   $instance_az = $reservation->instances_set->[0]->placement->availability_zone;
  }
 }
 
 if ($my_az eq $instance_az) { 
         if ($action eq "Launching") {
                 print "adding instance id $instance_id $pdns_name\n";
 
   # Get last app number
   $lastapp = `grep '\# Begin' $haproxy_file -A1000 | grep server | sort -k1 | cut -f6 -d' ' | tail -1`;
   chomp($lastapp);
   $lastapp = "app000" if $lastapp eq "";
   $lastapp++;
 
   # Update haproxy config file
   system("/bin/echo \"    server $lastapp $pdns_name:80 check # $instance_id\" >> $haproxy_file | service haproxy reload");
         } elsif ($action eq "Terminating") {
                 print "removing instance id $instance_id\n";
   system("sed -i \"/$instance_id/d\" $haproxy_file" | service haproxy reload);
         } else {
                 die("unhandled exception. exiting.\n");
         }
 
  # delete from queue
         $q->DeleteMessage($msg->ReceiptHandle());
 
 } else {
  print "$instance_id $instance_az does not belong to this AZ.\n";
 }
 
 # unset variables
 $instance_id = "";
 $launch_id = "";
 $action = "";
 $pdns_name = "";
 $instance_az = "";
}