Terraform: Create new GCE instance using same terraform code - google-cloud-platform

Question

I have created a new gcp vm instance successfully using terraform modules. The contents in my module folder are as follows
#main.tf
# google_compute_instance.default:
resource "google_compute_instance" "default" {
machine_type = "${var.machinetype}"
name = "${var.name}"
project = "demo"
tags = []
zone = "${var.zone}"
boot_disk {
initialize_params {
image = "https://www.googleapis.com/compute/v1/projects/centos-cloud/global/images/centos-7-v20210701"
size = 20
type = "pd-balanced"
}
}
network_interface {
network = "default"
subnetwork = "default"
subnetwork_project = "gcp-infrastructure-319318"
}
service_account {
email = "971558418058-compute#developer.gserviceaccount.com"
scopes = [
"https://www.googleapis.com/auth/devstorage.read_only",
"https://www.googleapis.com/auth/logging.write",
"https://www.googleapis.com/auth/logging.write",
"https://www.googleapis.com/auth/monitoring.write",
"https://www.googleapis.com/auth/service.management.readonly",
"https://www.googleapis.com/auth/servicecontrol",
"https://www.googleapis.com/auth/trace.append",
]
}
}
-----------
#variables.tf
variable "zone" {
default="us-east1-b"
}
variable "machinetype" {
default="f1-micro"
}
------------------
#terraform.tfvars
machinetype="g1-small"
zone="europe-west1-b"
My main code block is follows
$ cat providers.tf
provider "google" {
}
$ cat main.tf
module "gce" {
source = "../../../modules/services/gce"
name = "new-vm-tf"
}
With this code I am able create a new vm instance successfully
$ gcloud compute instances list
NAME ZONE MACHINE_TYPE PREEMPTIBLE INTERNAL_IP EXTERNAL_IP STATUS
new-vm-tf us-east1-b f1-micro 10.142.0.3 RUNNING
Now I have a requirement to create a new VM instance of machine type e2-standard. how can I handle that scenario?
If I edit my existing main.tf as shown below, it will destroy the existing vm instance to create the new vm instance .
$ cat main.tf
module "gce" {
source = "../../../modules/services/gce"
name = "new-vm-tf1"
}
terraform plan confirms as below
~ name = "new-vm-tf" -> "new-vm-tf1" # forces replacement
Plan: 1 to add, 0 to change, 1 to destroy.
I need pointers to resuse the same code to create a new vm existing without any changes to existing one . Please suggest

Answer 1

I recommend you to deep dive into terraform mechanism and best practices. I have 2 keywords to start: tfstate and variables.
The tfstate is the deployment context. If you change the deployment and the context isn't consistent, Terraform delete what is not consistent and create the missing pieces.
The variables are useful to reuse piece of generic code by customizing the values in entry.