AWSBeanstalkLeaderManager

A configuration file for your amazon elastic beanstalk that makes sure that one instance in your pool always is tagged as "leader". The ruby part of this configuration is inspired by whenever-elasticbeanstalk (what was too komplex for our purpose).

Installation

Just download the source code and copy the leader-manager.config into your .ebextensions folder. The final path may be your-app-root/.ebextensions/leader-manager.config. leader-manager.rb and leader-manager-crontab.txt are only included for a better overview.

Everything should work out of the box.

EC2 Instance IAM Role Permissions

As described in whenever-elasticbeanstalk/README.md you have to add at least the following policy once:

{
  "Version": "2015-03-29",
  "Statement": [
    {
      "Action": [
        "ec2:DescribeInstanceAttribute",
        "ec2:DescribeInstanceStatus",
        "ec2:DescribeInstances",
        "ec2:DescribeTags",
        "ec2:CreateTags"
      ],
      "Resource": [
        "*"
      ],
      "Effect": "Allow"
    }
  ]
}

Usage and benefit

This solution has two benifits for you:

1. Tagged instance

   At least one instance will be tagged as leader by using the tag leader=true. There may be also instances using the tag leader=false since it is theoretically possible that two instances are leader for a small amout of time (1 minute).

   

2. Local detection

   Since it would be a big overhead to check for the leader flag in your application, this script will touch a file called leader-manager-is-leader in /var/app. So its very easy for you to detect whether you are running code on the leader instance.

Examples

We wrote this solution since it is a "known problem" that an Elastic Beanstalk will terminate the leader instance since the "leader" flag is only available while deploying.

The main apporach of this script is to make sure that cronjobs only run on one instance. We do this using this bashscript:

if [ ! -d /var/app/leader -o -f "/var/app/leader/leader-manager-is-leader" ]; then
    # do something cool
	# e.g.: php /var/app/current/app/console some:symfony2:console:command
fi

You can read more about this problem here, here and here.