Terraform Tutorial - State (terraform.tfstate) and import

Terraform must store state about our managed infrastructure and configuration. This state is used by Terraform to map real world resources to our configuration, keep track of metadata, and to improve performance for large infrastructures.

This state is stored by default in a local file named terraform.tfstate

Terraform uses this local state to create plans and make changes to our infrastructure. Before any terraform operation, Terraform does a refresh to update the state with the real infrastructure.

Ref - State

Import - https://www.terraform.io/docs/import/

Terraform is able to import existing infrastructure. This allows us take resources we've created by some other means (i.e. via console) and bring it under Terraform management.

This is a great way to slowly transition infrastructure to Terraform.

The terraform import command is used to import existing infrastructure.

To import a resource, first write a resource block for it in our configuration, establishing the name by which it will be known to Terraform. For example:

resource "aws_instance" "import_example" {
  # ...instance configuration...
}

Now terraform import can be run to attach an existing instance to this resource configuration:

$ terraform import aws_instance.import_example i-03efafa258104165f
aws_instance.import_example: Importing from ID "i-03efafa258104165f"...
aws_instance.import_example: Import complete!
  Imported aws_instance (ID: i-03efafa258104165f)
aws_instance.import_example: Refreshing state... (ID: i-03efafa258104165f)

Import successful!

The resources that were imported are shown above. These resources are now in
your Terraform state and will henceforth be managed by Terraform.

This command locates the AWS instance with ID i-03efafa258104165f (which has been created outside Terraform) and attaches its existing settings, as described by the EC2 API, to the name aws_instance.import_example in the Terraform state.

As a result of the above command, the resource is recorded in the state file. We can now run terraform plan to see how the configuration compares to the imported resource, and make any adjustments to the configuration to align with the current (or desired) state of the imported object.

$ terraform plan
Refreshing Terraform state in-memory prior to plan...
The refreshed state will be used to calculate this plan, but will not be
persisted to local or remote state storage.

aws_key_pair.terraform-demo: Refreshing state... (ID: terraform-demo)
aws_instance.my-instance: Refreshing state... (ID: i-0aab39eec884aa165)
aws_instance.import_example: Refreshing state... (ID: i-03efafa258104165f)

------------------------------------------------------------------------

An execution plan has been generated and is shown below.
Resource actions are indicated with the following symbols:
-/+ destroy and then create replacement

Terraform will perform the following actions:

-/+ aws_instance.import_example (new resource required)
      id:                           "i-03efafa258104165f" => <computed> (forces new resource)
      ami:                          "ami-0922553b7b0369273" => "ami-04169656fea786776" (forces new resource)
   
      ...
      instance_type:                "t2.nano" => "t2.nano"
      ...
      key_name:                     "einsteinish" => "terraform-demo" (forces new resource)
      ...


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

------------------------------------------------------------------------

Note: You didn't specify an "-out" parameter to save this plan, so Terraform
can't guarantee that exactly these actions will be performed if
"terraform apply" is subsequently run.

Here are the files needed:

provider.tf:

provider "aws" {
  region = "${var.aws_region}"
}

ec2-instance.tf:

resource "aws_key_pair" "terraform-demo" {
  key_name   = "terraform-demo"
  public_key = "${file("terraform-demo.pub")}"
}

resource "aws_instance" "import_example" {
  ami           = "${lookup(var.ami,var.aws_region)}"
  instance_type = "${var.instance_type}"
  key_name      = "${aws_key_pair.terraform-demo.key_name}"
}

resource "aws_instance" "my-instance" {
  count         = "${var.instance_count}"
  ami           = "${lookup(var.ami,var.aws_region)}"
  instance_type = "${var.instance_type}"
  key_name      = "${aws_key_pair.terraform-demo.key_name}"
  user_data     = "${file("install_apache.sh")}"

  tags = {
    Name  = "${element(var.instance_tags, count.index)}"
    Batch = "5AM"
  }
}

vars.tf:

variable "ami" {
  type = "map"

  default = {
    "us-east-1" = "ami-04169656fea786776"
    "us-west-1" = "ami-006fce2a9625b177f"
  }
}

variable "instance_count" {
  default = "1"
}

variable "instance_tags" {
  type = "list"
  default = ["Terraform-1", "Terraform-2"]
}

variable "instance_type" {
  default = "t2.nano"
}

variable "aws_region" {
  default = "us-east-1"
}

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