Does Terraform expose the instance ID for an EC2 instance used in an ECS launch type?

Question

I'm deploying an app to an ECS cluster with an EC2 launch type. I've had some success here, and have stood up a load balancer in front of the cluster, and the instance receives and responds to traffic fine.

For a separate task, I'm also looking at being able to individually address the instance, which Terraform lets me do if I've got the instance ID, but I don't see the instance ID being one of the data outputs of aws_launch_configuration .

Is there a way to extract the instance ID of an EC2 instance used in an ECS cluster?

Answer 1

You can do this by using the aws_instance data source

In it, you can specify a few filters to retrieve any EC2 instance.

For example, let's say that your launch configuration (by the way, you should use a launch template, it's "launch configuration v2" basically with more features) assigns some tags to your ECS Container Instance, one of them being Name:ECSClusterX . Knowing this you can retrieve the said EC2 instance by:

data "aws_instance" "ecs-instance" {
  filter {
    name   = "tag:Name"
    values = ["ECSClusterX"]
  }
}

then you can retrieve the id of the found instance, by: aws_instance.ecs-instance.id .

That's it, you are all set!

Answer 2

If you are looking for launch a task in a specific instances, a good practice is using attributes.

In your cluster, move to the ECS Instances tab, then select one instance. Open the Actions menu like in the picture and go to View/Edit Attributes :

Add your attribute:

The last step is to configure your task definition in your terraform.

resource "aws_ecs_task_definition" "component1" {
    ...
    placement_constraints {
      type = "memberOf"
      expression =  "attribute:deploy_type == standard"
    }
    ...
}

Now the task only will be executed in the instances that match the attribute.

I use lambda function to keep all the time at least one instance with the each attribute block that I need. But you can integrate instances to the cluster with different launch templates and different attributes.