Applying tags to instances created with for each Terraform

Question

I have multiple EC2 instances created using for each. Each instance is being deployed into a different subnet. I am getting an error when trying to apply tags to each instance being deployed. Any advice would be helpful. Below is the code for my tags and instances:

resource "aws_instance" "private" {
  for_each      = aws_subnet.private
  ami           = var.ec2_amis[var.region]
  instance_type = var.tableau_instance
  key_name      = aws_key_pair.tableau.key_name
  subnet_id     = each.value.id

  tags = {
    Name = var.ec2_tags[each.key]
  }
}

variable "ec2_tags" {
  type = list(string)
  default = [
    "PrimaryEC2",
    "EC2Worker1",
    "EC2Worker2"
  ]
}

Error

Error: Invalid index

  on vpc.tf line 21, in resource "aws_instance" "private":
  21:     Name = var.ec2_tags[each.key]
    |----------------
    | each.key is "3"
    | var.ec2_tags is list of string with 3 elements

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

I had this code working earlier, not sure what happened. I made a change to the AMI it spins up, but I don't see why that could have an effect on tags. Any advice would be helpful.

UPDATE I have updated the resource with the following locals block and dynamic block within my "aws_instance" "private" code:

locals {
  private_instance = [{
    name = "PrimaryEC2"
    },
    {
      name = "EC2Worker1"
    },
    {
      name = "EC2Worker2"
  }]
}


    dynamic "tags" {
    for_each = local.private_instance

    content {
      Name = tags.value.name
    }
  }

Error

Error: Unsupported block type

  on vpc.tf line 28, in resource "aws_instance" "private":
  28:   dynamic "tags" {

Blocks of type "tags" are not expected here.

Any advice how to fix would help. Thanks!

Answer 1

If you want to make your tags dynamic, you could create them as follows:

  tags = {
    Name = each.key == "0" ? "PrimaryEC2" : "EC2Worker${each.key}" 
  }

You would use it as follows (assuming everything else is OK):

resource "aws_instance" "private" {
  for_each      = aws_subnet.private
  ami           = var.ec2_amis[var.region]
  instance_type = var.tableau_instance
  key_name      = aws_key_pair.tableau.key_name
  subnet_id     = each.value.id

  tags = {
    Name = each.key == "0" ? "PrimaryEC2" : "EC2Worker${each.key}" 
  }
}

The code uses conditional expression . It works as follows.

If each.key is equal to "0" (ie, first instance being created) then its tag will be "PrimaryEC2". All remaining instances will be tagged: "EC2Worker1", "EC2Worker2", "EC2Worker3" and so on for as many subnets there are.

Answer 2

One possible cause of this errors is that the aws_subnet.private variable is longer then the list of ec2 tags which would result in an error when the index 3 is used on your ec2_tags list looking for the 4th (nonexistent element).