Terraform applying huge index value for instance EBS block store

Question

I am using Terraform (called via Terragrunt, if that's relevant) to create an instance from an AMI and mount an existing volume:

resource "aws_instance" "jenkins_master_with_snap" {
    count                   = "${var.master_with_snapshot}"
    ami                     = "${var.jenkins_ami}"
    instance_type           = "${var.jenkins_instance_type}"
    iam_instance_profile    = "${data.terraform_remote_state.global.jenkins_profile_name}"
    subnet_id               = "${data.aws_subnet.jenkins_subnet_with_snap.id}"
    key_name                = "${var.key_name}"
    vpc_security_group_ids  = [
        "${aws_security_group.jenkins_master_target_sg.id}",
        "${data.terraform_remote_state.cicd.cicd_sg_ipa}"
    ]

    ebs_block_device {
        snapshot_id = "${var.master_snapshot_id}"
        device_name = "${var.jenkins_volume_device}"
        volume_type = "gp2"
    }
}

It's worth noting that the AMI used to create this resource already has a snapshot mapped to it from the build process, so this resource basically just replaces it with a different snapshot. I'm not sure if this is why I'm having the problem or not.

I'm using the resulting resource attributes to populate a Python template that will be zipped and uploaded as a lambda function. The Python script requires the volume-id from this instance's EBS block device.

data "template_file" "ebs_backup_lambda_with_snapshot_template" {
    count       = "${var.master_with_snapshot}"
    template    = "${file("${path.module}/jenkins_lambda_ebs_backup.py.tpl")}"

    vars {
        volume_id = "${aws_instance.jenkins_master_with_snap.ebs_block_device.???.volume_id}"
    }
}

Onto the actual problem: I do not know how to properly reference the volume ID in the vars section of the template_file resource above. Here is the resulting state:

ebs_block_device.#                                = 1
ebs_block_device.1440725774.delete_on_termination = true
ebs_block_device.1440725774.device_name           = /dev/xvdf
ebs_block_device.1440725774.encrypted             = true
ebs_block_device.1440725774.iops                  = 900
ebs_block_device.1440725774.snapshot_id           = snap-1111111111111
ebs_block_device.1440725774.volume_id             = vol-1111111111111
ebs_block_device.1440725774.volume_size           = 300
ebs_block_device.1440725774.volume_type           = gp2
ebs_optimized                                     = false
root_block_device.#                               = 1
root_block_device.0.delete_on_termination         = false
root_block_device.0.iops                          = 0
root_block_device.0.volume_id                     = vol-1111111111111
root_block_device.0.volume_size                   = 8
root_block_device.0.volume_type                   = standard

The problem is that the index for the EBS volume is that insane integer 1440725774. I have no idea why that is occuring. In the console, there's only a single map in the list I'm interested in:

> aws_instance.jenkins_master_with_snap.ebs_block_device
[
  {    delete_on_termination = 1  device_name = /dev/xvdf  encrypted = 1  iops = 900  snapshot_id = snap-1111111111111  volume_id = vol-1111111111111  volume_size = 300  volume_type = gp2}
]

And it appears the only way to reference any of those keys is to use that index value directly:

> aws_instance.jenkins_master_with_snap.ebs_block_device.1440725774.volume_id
vol-1111111111111

Is there any way to reliably reference a single element in a list like this when I have no idea what the index is going to be? I can't just hardcode that integer into the template_file resource above and assume it's going to be the same every time. Does anyone have any clues as to why this is occurring in the first place?

Answer 1

Perhaps instead of inlining ebs_block_device block, create a separate aws_ebs_volume resource, then attach it with an aws_volume_attachment. Then reference the aws_ebs_volume.name.id attribute to get the ID you need.

Example (extended from the example code in aws_volume_attachment):

resource "aws_volume_attachment" "ebs_att" {
  device_name = "/dev/sdh"
  volume_id   = "${aws_ebs_volume.example.id}"
  instance_id = "${aws_instance.web.id}"
}

resource "aws_instance" "web" {
  ami               = "ami-21f78e11"
  availability_zone = "us-west-2a"
  instance_type     = "t1.micro"
  tags {
    Name = "HelloWorld"
  }

  subnet_id = "<REDACTED>"
}

resource "aws_ebs_volume" "example" {
  availability_zone = "us-west-2a"
  size              = 1
}

data "template_file" "example" {
    template    = "Your volume ID is $${volume_id}"

    vars {
     volume_id = "${aws_ebs_volume.example.id}"
    }
}

output "custom_template" {
  value = "${data.template_file.example.rendered}"
}

The resultant output:

Outputs:

custom_template = Your volume ID is vol-0b1064d4ca6f89a15

You can then use ${aws_ebs_volume.example.id} in your template vars to populate your lambda.