ASG DNS handler

Purpose

This Terraform module sets up everything necessary for dynamically setting hostnames following a certain pattern on instances spawned by AWS Auto Scaling Groups (ASGs).

Learn more about our motivation to build this module in this blog post.

Requirements

Terraform 0.12+
Terraform AWS provider 2.0+

Usage

Create an ASG and set the asg:hostname_pattern tag for example like this:

asg-test-#instanceid.asg-handler-vpc.testing@Z3QP9GZSRL8IVA

Could be interpolated in Terraform like this:

tag {
  key                 = "asg:hostname_pattern"
  value               = "${var.hostname_prefix}-#instanceid.${var.vpc_name}.testing@${var.internal_zone_id}"
  propagate_at_launch = true
}

Once you have your ASG set up, you can just invoke this module and point to it:

module "clever_name_autoscale_dns" {
  source  = "meltwater/asg-dns-handler/aws"
  version = "x.y.z"
  # use_public_ip = true
  autoscale_handler_unique_identifier = "clever_name"
  autoscale_route53zone_arn           = "ABCDEFGHIJ123"
  vpc_name                            = "my_vpc"
}

How does it work?

The module sets up the following

A SNS topic
A Lambda function
A topic subscription sending SNS events to the Lambda function

The Lambda function then does the following:

Fetch the asg:hostname_pattern tag value from the ASG, and parse out the hostname and Route53 zone ID from it.
If it's a instance being created
- Fetch internal IP from EC2 API
- Create a Route53 record pointing the hostname to the IP
- Set the Name tag of the instance to the initial part of the generated hostname
If it's an instance being deleted
- Fetch the internal IP from the existing record from the Route53 API
- Delete the record

Setup

Add initial_lifecycle_hook definitions to your aws_autoscaling_group resource , like so:

resource "aws_autoscaling_group" "my_asg" {
  name = "myASG"

  vpc_zone_identifier = var.aws_subnets

  min_size                  = var.asg_min_count
  max_size                  = var.asg_max_count
  desired_capacity          = var.asg_desired_count
  health_check_type         = "EC2"
  health_check_grace_period = 300
  force_delete              = false

  launch_configuration = aws_launch_configuration.my_launch_config.name

  lifecycle {
    create_before_destroy = true
  }

  initial_lifecycle_hook {
    name                    = "lifecycle-launching"
    default_result          = "ABANDON"
    heartbeat_timeout       = 60
    lifecycle_transition    = "autoscaling:EC2_INSTANCE_LAUNCHING"
    notification_target_arn = module.autoscale_dns.autoscale_handling_sns_topic_arn
    role_arn                = module.autoscale_dns.agent_lifecycle_iam_role_arn
  }

  initial_lifecycle_hook {
    name                    = "lifecycle-terminating"
    default_result          = "ABANDON"
    heartbeat_timeout       = 60
    lifecycle_transition    = "autoscaling:EC2_INSTANCE_TERMINATING"
    notification_target_arn = module.autoscale_dns.autoscale_handling_sns_topic_arn
    role_arn                = module.autoscale_dns.agent_lifecycle_iam_role_arn
  }

  tag {
    key                 = "asg:hostname_pattern"
    value               = "${var.hostname_prefix}-#instanceid.${var.vpc_name}.testing@${var.internal_zone_id}"
    propagate_at_launch = true
  }
}

module "autoscale_dns" {
  source = "meltwater/asg-dns-handler/aws"
  version = "x.y.z"
  
  autoscale_handler_unique_identifier = "my_asg_handler"
  autoscale_route53zone_arn           = var.internal_zone_id
  vpc_name                            = var.vpc_name
}

Difference between Lifecycle action

Lifecycle_hook can have CONTINUE or ABANDON as default_result. By setting default_result to ABANDON will terminate the instance if the lambda function fails to update the DNS record as required. Complete_lifecycle_action in lambda function returns LifecycleActionResult as CONTINUE on success to Lifecycle_hook. But if lambda function fails, Lifecycle_hook doesn't get any response from Complete_lifecycle_action which results in timeout and terminates the instance.

At the conclusion of a lifecycle hook, the result is either ABANDON or CONTINUE. If the instance is launching, CONTINUE indicates that your actions were successful, and that the instance can be put into service. Otherwise, ABANDON indicates that your custom actions were unsuccessful, and that the instance can be terminated.

If the instance is terminating, both ABANDON and CONTINUE allow the instance to terminate. However, ABANDON stops any remaining actions, such as other lifecycle hooks, while CONTINUE allows any other lifecycle hooks to complete.

TODO

Reverse lookup records?

License and Copyright

This project was built at Meltwater. It is licensed under the Apache License 2.0.

kgizdov/terraform-aws-asg-dns-handler