Avoid Update of resources in Terraform - digital-ocean

Question

Currently we are using the Blue/Green Deployment Model for our Application using Terraform.
And our TF Files have resources for both Blue & Green as seen below -
resource "aws_instance" "green_node" {
count = "${var.node_count * var.keep_green * var.build}"
lifecycle = {
create_before_destroy = true
}
ami = "${var.green_ami_id}"
instance_type = "${lookup(var.instance_type,lower(var.env))}"
security_groups = "${split(",", lookup(var.security_groups, format("%s-%s", lower(var.env),var.region)))}"
subnet_id = "${element(split(",", lookup(var.subnets, format("%s-%s", lower(var.env),var.region))), count.index)}"
iam_instance_profile = "${var.iam_role}"
key_name = "${var.key_name}"
associate_public_ip_address = "false"
tags {
Name = "node-green-${var.env}-${count.index + 1}"
}
user_data = "${data.template_cloudinit_config.green_node.rendered}"
}
resource "aws_instance" "blue_node" {
count = "${var.node_count * var.keep_blue * var.build}"
lifecycle = {
create_before_destroy = true
}
ami = "${var.blue_ami_id}"
instance_type = "${lookup(var.instance_type,lower(var.env))}"
security_groups = "${split(",", lookup(var.security_groups, format("%s-%s", lower(var.env),var.region)))}"
subnet_id = "${element(split(",", lookup(var.subnets, format("%s-%s", lower(var.env),var.region))), count.index)}"
iam_instance_profile = "${var.iam_role}"
key_name = "${var.key_name}"
associate_public_ip_address = "false"
tags {
Name = "node-blue-${var.env}-${count.index + 1}"
}
user_data = "${data.template_cloudinit_config.blue_node.rendered}"
}
My question - Is there a way to update the Green Resources without updating the Blue Resources and vice versa Without Using Targeted Plan. For eg. If we update the Security Groups(var.security_groups) which is a common variable, the update will occur for both Blue and Green and i will have to do a targeted plan(seen below)to avoid Blue Resources from getting updated with the New Security Group's -
terraform plan -out=green.plan -target=<green_resource_name>

Answer 1

This is a good question.
If you need to make the blue/green stack work as your expect and reduce the complexity of the code, You can use terraform modules, and set a variable to control which color you will update.
So the stack shares the module when you need update blue or green resources. Define a variable, such as TF_VAR_stack_color to blue or green
Add ${var.stack_color} in the name of any resources you try to create/update in modules.
module "nodes" {
source = "modules/nodes"
name = "${var.name}-${var.stack_color}-${var.others}"
...
}
So you can deploy the blue resource with below command without impact the running green resources.
TF_VAR_stack_color=blue terraform plan
or
terraform plan -var stack_color=blue
With terraform modules, you needn't write resource aws_instance two times for blue and green nodes.
I will recommend splitting the resources into different state files by terraform init, so they will be the totally separate stacks.