terraform: How to destroy oldest instance when lowering aws_instance count - terraform

Question

Given a pair of aws instances deployed with
provider "aws" {
region = "us-east-1"
}
resource "aws_instance" "example" {
count = 2
ami = "ami-2757f631"
instance_type = "t2.micro"
tags = {
Name = "Test${count.index}"
}
}
Lowering count = 1 will destroy the last instance deployed:
Terraform will perform the following actions:
- aws_instance.example[1]
Is it possible to get terraform to destroy the first instance. ie.
Terraform will perform the following actions:
- aws_instance.example[0]

Answer 1

Terraform is tracking which instance is which via its state. When you reduce your count on the aws_instance resource Terraform will simply remove the later instances. While this shouldn't really be much of an issue because I would only really recommend that you are deploying groups of homogenous instances that can handle the load being interrupted (and would sit behind some form of load balancer mechanism) if you really needed to you could edit the state file to reorder the instances before reducing the number of instances.
The state file is serialised as JSON so you can just edit it directly (making sure it's uploaded to whatever you're using for remote state if you are using remote state) or better yet you can use the first class tools for editing remote state that the Terraform CLI provides with terraform state mv.
As an example you can do this:
# Example from question has been applied already
# `count` is edited from 2 to 1
$ terraform plan
...
aws_instance.example[1]: Refreshing state... (ID: i-0c227dfbfc72fb0cd)
aws_instance.example: Refreshing state... (ID: i-095fd3fdf86ce8254)
------------------------------------------------------------------------
An execution plan has been generated and is shown below.
Resource actions are indicated with the following symbols:
- destroy
Terraform will perform the following actions:
- aws_instance.example[1]
Plan: 0 to add, 0 to change, 1 to destroy.
...
$
$
$
$ terraform state list
aws_instance.example[0]
aws_instance.example[1]
$
$
$
$ terraform state mv aws_instance.example[1] aws_instance.example[2]
Moved aws_instance.example[1] to aws_instance.example[2]
$ terraform state mv aws_instance.example[0] aws_instance.example[1]
Moved aws_instance.example[0] to aws_instance.example[1]
$ terraform state mv aws_instance.example[2] aws_instance.example[0]
Moved aws_instance.example[2] to aws_instance.example[0]
$
$
$
$ terraform plan
...
aws_instance.example[1]: Refreshing state... (ID: i-095fd3fdf86ce8254)
aws_instance.example: Refreshing state... (ID: i-0c227dfbfc72fb0cd)
------------------------------------------------------------------------
An execution plan has been generated and is shown below.
Resource actions are indicated with the following symbols:
~ update in-place
- destroy
Terraform will perform the following actions:
~ aws_instance.example
tags.Name: "Test1" => "Test0"
- aws_instance.example[1]
Plan: 0 to add, 1 to change, 1 to destroy.
...