Often when running a web service, you'll have your instances behind a load balancer. But when deploying new code to these instances, you don't want the load balancer to continue sending customer traffic to an instance while the deployment is in progress. Lifecycle event scripts give you the ability to integrate your AWS CodeDeploy deployments with instances that are behind an Elastic Load Balancer or in an Auto Scaling group. Simply set the name (or names) of the Elastic Load Balancer your instances are a part of, set the scripts in the appropriate lifecycle events, and the scripts will take care of deregistering the instance, waiting for connection draining, and re-registering after the deployment finishes.

The register and deregister scripts have a couple of dependencies in order to properly interact with Elastic Load Balancing and AutoScaling:

1. The AWS CLI. In order to take advantage of AutoScaling's Standby feature, the CLI must be at least version 1.3.25. If you have Python and PIP already installed, the CLI can simply be installed with pip install awscli. Otherwise, follow the installation instructions in the CLI's user guide.
2. An instance profile with a policy that allows, at minimum, the following actions:

    elasticloadbalancing:Describe*
    elasticloadbalancing:DeregisterInstancesFromLoadBalancer
    elasticloadbalancing:RegisterInstancesWithLoadBalancer
    autoscaling:Describe*
    autoscaling:EnterStandby
    autoscaling:ExitStandby
    autoscaling:UpdateAutoScalingGroup

Note: the AWS CodeDeploy Agent requires that an instance profile be attached to all instances that are to participate in AWS CodeDeploy deployments. For more information on creating an instance profile for AWS CodeDeploy, see the Create an IAM Instance Profile for Your Amazon EC2 Instances topic in the documentation.

1. All instances are assumed to already have the AWS CodeDeploy Agent installed.

To use these scripts in your own application:

1. Install the AWS CLI on all your instances.
2. Update the policies on the EC2 instance profile to allow the above actions.
3. Copy the .sh files in this directory into your application source.
4. Edit your application's appspec.yml to run deregister_from_elb.sh on the ApplicationStop event, and register_with_elb.sh on the ApplicationStart event.
5. Edit common_functions.sh to set ELB_LIST to contain the name(s) of the Elastic Load Balancer(s) your deployment group is a part of.
6. Deploy!