Terraform - Get a value from parameter store and pass to resource

Question

We store our latest approved AMIs in AWS parameter store. When creating new instances with Terraform I would like to programatically get this AMI ID. I have a command to pull the AMI ID but I'm not sure how to use it with Terraform.

Here is the command I use to pull the AMI ID:

$(aws ssm get-parameter --name /path/to/ami --query 'Parameter.Value' --output text)

And here is my Terraform script:

resource "aws_instance" "nginx" {
  ami           = "ami-c58c1dd3" # pull value from parameter store
  instance_type = "t2.micro"
  #key_name        = "${var.key_name}"

  provisioner "remote-exec" {
    inline = [
      "sudo yum install nginx -y",
      "sudo service nginx start"
    ]
  }
}

How can I use the command to pull the AMI ID in the Terraform script?

Answer 1

You can use the aws_ssm_parameter data source to fetch the value of a parameter at runtime:

data "aws_ssm_parameter" "ami" {
  name = "/path/to/ami"
}

resource "aws_instance" "nginx" {
  ami           = data.aws_ssm_parameter.ami.value # pull value from parameter store
  instance_type = "t2.micro"

  provisioner "remote-exec" {
    inline = [
      "sudo yum install nginx -y",
      "sudo service nginx start"
    ]
  }
}

However, a better approach might be to use the aws_ami data source to filter for the AMI you want more directly instead of pushing the AMI ID to SSM parameter store and then looking it up later. You can filter on a number of criteria including name, account owner and tags. Here's the example from the aws_instance resource documentation that is looking for the latest Ubuntu 20.04 AMI:

data "aws_ami" "ubuntu" {
  most_recent = true

  filter {
    name   = "name"
    values = ["ubuntu/images/hvm-ssd/ubuntu-focal-20.04-amd64-server-*"]
  }

  filter {
    name   = "virtualization-type"
    values = ["hvm"]
  }

  owners = ["099720109477"] # Canonical
}

resource "aws_instance" "web" {
  ami           = data.aws_ami.ubuntu.id
  instance_type = "t2.micro"

  tags = {
    Name = "HelloWorld"
  }
}