» Change Infrastructure

In the previous page, you created your first infrastructure with Terraform: a single EC2 instance. In this page, we're going to modify that resource, and see how Terraform handles change.

Infrastructure is continuously evolving, and Terraform was built to help manage and enact that change. As you change Terraform configurations, Terraform builds an execution plan that only modifies what is necessary to reach your desired state.

By using Terraform to change infrastructure, you can version control not only your configurations but also your state so you can see how the infrastructure evolved over time.

» Configuration

Let's modify the ami of our instance. Edit the aws_instance.example resource in your configuration and change it to the following:

resource "aws_instance" "example" {
  ami           = "ami-b374d5a5"
  instance_type = "t2.micro"
}

We've changed the AMI from being an Ubuntu 16.04 LTS AMI to being an Ubuntu 16.10 AMI. Terraform configurations are meant to be changed like this. You can also completely remove resources and Terraform will know to destroy the old one.

» Apply Changes

After changing the configuration, run terraform apply again to see how Terraform will apply this change to the existing resources.

$ terraform apply
# ...

-/+ aws_instance.example
    ami:                      "ami-2757f631" => "ami-b374d5a5" (forces new resource)
    availability_zone:        "us-east-1a" => "<computed>"
    ebs_block_device.#:       "0" => "<computed>"
    ephemeral_block_device.#: "0" => "<computed>"
    instance_state:           "running" => "<computed>"
    instance_type:            "t2.micro" => "t2.micro"
    private_dns:              "ip-172-31-17-94.ec2.internal" => "<computed>"
    private_ip:               "172.31.17.94" => "<computed>"
    public_dns:               "ec2-54-82-183-4.compute-1.amazonaws.com" => "<computed>"
    public_ip:                "54.82.183.4" => "<computed>"
    subnet_id:                "subnet-1497024d" => "<computed>"
    vpc_security_group_ids.#: "1" => "<computed>"

The prefix -/+ means that Terraform will destroy and recreate the resource, rather than updating it in-place. While some attributes can be updated in-place (which are shown with the ~ prefix), changing the AMI for an EC2 instance requires recreating it. Terraform handles these details for you, and the execution plan makes it clear what Terraform will do.

Additionally, the execution plan shows that the AMI change is what required the resource to be replaced. Using this information, you can adjust your changes to possibly avoid destroy/create updates if they are not acceptable in some situations.

Once again, Terraform prompts for approval of the execution plan before proceeding. Answer yes to execute the planned steps:

# ...
aws_instance.example: Refreshing state... (ID: i-64c268fe)
aws_instance.example: Destroying...
aws_instance.example: Destruction complete
aws_instance.example: Creating...
  ami:                      "" => "ami-b374d5a5"
  availability_zone:        "" => "<computed>"
  ebs_block_device.#:       "" => "<computed>"
  ephemeral_block_device.#: "" => "<computed>"
  instance_state:           "" => "<computed>"
  instance_type:            "" => "t2.micro"
  key_name:                 "" => "<computed>"
  placement_group:          "" => "<computed>"
  private_dns:              "" => "<computed>"
  private_ip:               "" => "<computed>"
  public_dns:               "" => "<computed>"
  public_ip:                "" => "<computed>"
  root_block_device.#:      "" => "<computed>"
  security_groups.#:        "" => "<computed>"
  source_dest_check:        "" => "true"
  subnet_id:                "" => "<computed>"
  tenancy:                  "" => "<computed>"
  vpc_security_group_ids.#: "" => "<computed>"
aws_instance.example: Still creating... (10s elapsed)
aws_instance.example: Still creating... (20s elapsed)
aws_instance.example: Creation complete

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

# ...

As indicated by the execution plan, Terraform first destroyed the existing instance and then created a new one in its place. You can use terraform show again to see the new values associated with this instance.

» Next

You've now seen how easy it is to modify infrastructure with Terraform. Feel free to play around with this more before continuing. In the next section we're going to destroy our infrastructure.