Will resources definition moving to a different module make 'terraform apply' to delete and recreate these resources?

Question

I have created some VMs with a main.tf, and terraform generates a cluster.tfstate file.

Now because of refactoring, I move the VM resource definitions into a module, and refer to this module in main.tf. When I run terraform apply --state=./cluster.tfstate, will terraform destroy and recreate these VMs?

I would expect it will not. Is my understanding correct?

Answer 1

Let's try this using the example given in the aws_instance documentation:

# Create a new instance of the latest Ubuntu 14.04 on an
# t2.micro node with an AWS Tag naming it "HelloWorld"
provider "aws" {
  region = "us-west-2"
}

data "aws_ami" "ubuntu" {
  most_recent = true

  filter {
    name   = "name"
    values = ["ubuntu/images/hvm-ssd/ubuntu-trusty-14.04-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"
  }
}

If we terraform apply this, we get an instance that is referenced within Terraform as aws_instance.web:

Apply complete! Resources: 1 added, 0 changed, 0 destroyed.

If we move this definition to a module ubuntu_instance, the directory structure might look like this with the above code in instance.tf:

.
├── main.tf
└── ubuntu_instance
    └── instance.tf

Now you intend to create the same instance as before, but internally Terraform now names this resource module.ubuntu_instance.aws_instance.web

If you attempt to apply this, you would get the following:

Plan: 1 to add, 0 to change, 1 to destroy.

The reason this happens is that Terraform has no idea that the old and new code reference the same instance. When you refactor in a module, you are removing a resource, and thus Terraform deletes that resource.

Terraform maps your code to real resources in the state file. When you create an instance, you can only know that instance maps to your aws_instance because of the state file. So the proper way (as mentioned by Jun) is to refactor your code, then tell Terraform to move the mapping to the real instance from aws_instance.web to module.ubuntu_instance.aws_instance.web Then when you apply, Terraform will leave the instance alone because it matches what your code says. The article Jun linked to is a good discussion of this.