Terraform - cannot launch AWS EC2 instance with Launch Template

Question

My EC2 instance resource code

resource "aws_instance" "my-sample-webapp-ec2" {
  availability_zone                    = var.availability_zone
  subnet_id                            = var.subnet_id
  key_name                             = var.ec2_instance_name
  instance_initiated_shutdown_behavior = "stop"
  disable_api_termination              = false
  #  vpc_security_group_ids               = var.vpc_security_group_ids

  launch_template {
    id      = var.launch_template_id
    version = "$Latest"
  }

  tags = {
    "Name" = var.ec2_instance_name
  }

  root_block_device {
    delete_on_termination = true
  }

}

My launch template already exists in AWS region - checked

My Module import for the above EC2 resource

module "aws_ec2_machines" {
  source              = "./modules/ec2_machines"
  count               = length(local.availability_zones)
  launch_template_id  = var.launch_template_id
  launch_template_ver = var.launch_template_ver
  ec2_instance_name   = "${var.ec2_instance_name}-${count.index + 1}"
  availability_zone   = local.availability_zones[count.index]
}

what I want to do is below

1. Specify my launch template and launch EC2 instance(s)

2. Subnet association should happen based on availability_zone

Currently, I have only 3 subnets (1 per availability zone), but they are not default. Also, the VPC under which the subnets are created is also not the default VPC.

The error I am getting

│ Error: Error launching source instance: InvalidParameterValue: Value (us-east-2b) for parameter availabilityZone is invalid. Subnet 'subnet-xxxxxx' is in the availability zone us-east-2a
│       status code: 400, request id: 75a126cb-59eb-40fe-9fa5-579ed908edbd
│
│   with module.aws_ec2_machines[1].aws_instance.my-sample-webapp-ec2,
│   on modules\ec2_machines\main.tf line 7, in resource "aws_instance" "my-sample-webapp-ec2":
│    7: resource "aws_instance" "my-sample-webapp-ec2" {
│
╵

What am I doing wrong?

Answer 1

OK - I figured out to problem (pen and pencil writing)

1. Launch Template ID does not need to have any subnet defined

2. But any VPC should have subnets created and (good practice) assigned to a different AZ

   a. e.g. subnet-2a should be with AZ 1, subnet-2b should be with AZ 2 etc.

3. Now, with the below resource/module invocation - it all worked well

   resource "aws_instance" "my-sample-webapp-ec2" {
     subnet_id                            = var.subnet_id
     instance_initiated_shutdown_behavior = "stop"
     disable_api_termination              = false
   
     launch_template {
       id      = var.launch_template_id
       version = "$Latest"
     }
   
     tags = {
       "Name" = var.ec2_instance_name
     }
   
   root_block_device {
   delete_on_termination = true
   }
   
   }
   

And the subsequent module call in main.tf

  module "aws_ec2_machines" {
  source              = "./modules/ec2_machines"
  count               = length(local.availability_zones)
  launch_template_id  = var.launch_template_id
  launch_template_ver = var.launch_template_ver
  ec2_instance_name   = "${var.ec2_instance_name}-${count.index + 1}"
  subnet_id           = local.subnets[count.index % local.available_subnet_count]
}