Can I attach an EC2 instance to an existing Load Balancer using CloudFormation

Question

The way we usually attach instances to a Load Balancer is by using the instances key while defining the load balancer in my aws template. Is there anyway that I can attach an EC2 instance to a Load Balancer that already exists?

Answer 1

If you don't mind creating the EC2 instance indirectly through an AWS::AutoScaling::AutoScalingGroup (e.g., with a DesiredCapacity of 1), you could automatically attach the EC2 instance to an existing ELB using the LoadBalancerNames (or TargetGroupARNs if using an Application Load Balancer) property.

Otherwise, you can call the RegisterInstancesWithLoadBalancer / DeregisterInstancesFromLoadBalancer APIs manually, either directly from the EC2 instance (e.g., from a UserData script using the AWS CLI), or using a Custom Resource (e.g., using the AWS SDK for JavaScript from a Lambda function).