Unable to fetch Subnet ID's | Terraform Plan

Question

I am getting below error while executing Terraform plan . I am following modules concept here.

Below is my resource aws_efs_mount_target and aws_subnet blocks.

efs.tf:-

resource "aws_efs_file_system" "master_efs" {
  creation_token   = "master"
  performance_mode = "generalPurpose"
  kms_key_id       = "${data.terraform_remote_state.kms_ebs.outputs.key_arn}"
  encrypted        = "true"
  tags = {
    Name        = "master"
    Environment = "${var.environment}"
    Terraform   = "true"
  }
}
resource "aws_efs_mount_target" "master_mt" {
  file_system_id  = "${aws_efs_file_system.master_efs.id}"
  count           = "${length(var.availability_zones)}"
  subnet_id      = "${data.aws_subnet.app_subnet_0.*.id[count.index]}"
  security_groups = [ "${aws_security_group.sg.id}" ]
}

data "aws_subnet" "app_subnet_0" {
  vpc_id = "${data.aws_vpc.cng.id}"
  filter {
    name    = "tag:Name"
    values  = ["${var.search_pattern_app}-0"]
  }
}

Error: Invalid index

     on ../../modules/efs.tf line 16, in resource "aws_efs_mount_target" "master_mt":
      16:   subnet_id      = "${data.aws_subnet.app_subnet_0.*.id[count.index]}"
        |----------------
        | count.index is 2
        | data.aws_subnet.app_subnet_0 is object with 15 attributes

    The given key does not identify an element in this collection value.


Error: Invalid index



     on ../../modules/efs.tf line 16, in resource "aws_efs_mount_target" "master_mt":
      16:   subnet_id      = "${data.aws_subnet.app_subnet_0.*.id[count.index]}"
        |----------------
        | count.index is 1
        | data.aws_subnet.app_subnet_0 is object with 15 attributes

    The given key does not identify an element in this collection value.

Error: Invalid index

  on ../../modules/efs.tf line 35, in resource "aws_efs_mount_target" "worker_mt":
  35:   subnet_id      = "${data.aws_subnet.app_subnet_0.*.id[count.index]}"
    |----------------
    | count.index is 1
    | data.aws_subnet.app_subnet_0 is object with 15 attributes

The given key does not identify an element in this collection value.


Error: Invalid index

  on ../../modules/efs.tf line 35, in resource "aws_efs_mount_target" "worker_mt":
  35:   subnet_id      = "${data.aws_subnet.app_subnet_0.*.id[count.index]}"
    |----------------
    | count.index is 2
    | data.aws_subnet.app_subnet_0 is object with 15 attributes

The given key does not identify an element in this collection value.

Answer 1

From what I'm understanding from your code you're are trying to create a mounting point for your EFS for each subnet in VPC (one per availability zone), and to do that you want to Terraform to automatically parse the availability zone and then assign the subnet id for the mount target ? Apparently the code you wrote is trying to match one of the subnet attribute with the value of the counter, if you just want to add aws_subnet.app_subnet_0.1 to aws_subnet.app_subnet_0.x maybe you can put it that way

aws_subnet.app_subnet_0.${count.index}.id

Answer 2

The aws_subnet data source returns a single subnet, not a list and you don't appear to be looping over it with count .

If you're expecting multiple subnets to be returned then you probably want the aws_subnet_ids data source instead.

So if you want to create an EFS mount target in each subnet that matches a subnet filter you could use the following:

resource "aws_efs_file_system" "master_efs" {
  creation_token   = "master"
  performance_mode = "generalPurpose"
  kms_key_id       = "${data.terraform_remote_state.kms_ebs.outputs.key_arn}"
  encrypted        = "true"

  tags = {
    Name        = "master"
    Environment = "${var.environment}"
    Terraform   = "true"
  }
}

data "aws_subnet_ids" "app_subnet" {
  vpc_id = "${data.aws_vpc.cng.id}"

  tags = {
    Name = "${var.search_pattern_app}"
  }
}

resource "aws_efs_mount_target" "master_mt" {
  file_system_id  = "${aws_efs_file_system.master_efs.id}"
  count           = "${length(var.availability_zones)}"
  subnet_id       = "${data.aws_subnet.app_subnet.ids[count.index]}"
  security_groups = ["${aws_security_group.sg.id}"]
}

This finds all the subnets with a name matching the search_pattern_app variable and creates EFS mount targets in each of them.