Define: Terraform - AWS - aws_instance - user_data

Question

I'm working with Terraform, launching an ECS cluster.

I'm using a script that was written by someone else. I understand most of the launch configuration script, other than this one point:

I'm trying to find the link between the autoscaling group and the ECS cluster.

I have an aws_autoscaling_group which uses an aws_launch_configuration (see below). The aws_launch_configuration has a user_data parameter. This is the only link between the autoscaling group and the cluster that I can find.

Looking at the links (bottom) to the documentation, it's not giving a very good explanation of 'what' user_data is. Can someone please help me understand "how" the user_data links the autoscaling group with the cluster, and if at all possible give some example or link to what it's normally used for and how it's normally used.

...
resource "aws_launch_configuration" "ecs_host" {
  ...

  user_data = <<EOF
#!/bin/bash
echo ECS_CLUSTER=${var.cluster_name} >> /etc/ecs/ecs.config
EOF
}
...

terraform apply outputs:

  ...
  + aws_launch_configuration.ecs_host
      ...
      user_data: "4e27e8feea0896af43ada0c647877da3766f5dcb"
  ...

https://www.terraform.io/docs/providers/aws/r/instance.html#user_data https://www.terraform.io/docs/providers/aws/r/launch_configuration.html#user_data

Answer 1

official explanation by terraform

user_data - (Optional) The user data to provide when launching the instance. Do not pass gzip-compressed data via this argument; see user_data_base64 instead.

Offical aws document to explain what's user_data

https://docs.aws.amazon.com/AWSEC2/latest/WindowsGuide/ec2-instance-metadata.html

So in general, after you define the launch configuration and autoscaling group, the autoscaling group will make sure how many ec2 instances need be started (desired capacity).

When start a new instance, it will reference the launch configuration, which define which AWS AMI for the instance and what init command will be run. the init commands are saved in user_data script.

In your case, the user data script registers itself to nominated ECS cluster. So ecs cluster can discover these ec2 instances easily

echo ECS_CLUSTER=${var.cluster_name} >> /etc/ecs/ecs.config

In background, there is a docker container called amazon/amazon-ecs-agent running to help these registions and manage other containers (ecs services)

If you want to know the details, you can go through this document: https://docs.aws.amazon.com/AmazonECS/latest/developerguide/ecs-agent-install.html