terraform/website/source/intro/getting-started/change.html.md

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intro	Change Infrastructure	gettingstarted-change

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.web" resource in your configuration and change it to the following:

resource "aws_instance" "example" {
	ami = "ami-aa7ab6c2"
	instance_type = "t1.micro"
}

We've changed the AMI from being an Ubuntu 14.04 AMI to being an Ubuntu 12.04 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.

Execution Plan

Let's see what Terraform will do with the change we made.

$ terraform plan
...

-/+ aws_instance.example
    ami:               "ami-408c7f28" => "ami-aa7ab6c2" (forces new resource)
    availability_zone: "us-east-1c" => "<computed>"
    key_name:          "" => "<computed>"
    private_dns:       "domU-12-31-39-12-38-AB.compute-1.internal" => "<computed>"
    private_ip:        "10.200.59.89" => "<computed>"
    public_dns:        "ec2-54-81-21-192.compute-1.amazonaws.com" => "<computed>"
    public_ip:         "54.81.21.192" => "<computed>"
    security_groups:   "" => "<computed>"
    subnet_id:         "" => "<computed>"

The prefix "-/+" means that Terraform will destroy and recreate the resource, versus purely updating it in-place. While some attributes can do in-place updates (which are shown with a "~" prefix), AMI changing on EC2 instance requires a new resource. Terraform handles these details for you, and the execution plan makes it clear what Terraform will do.

Additionally, the plan output shows that the AMI change is what necessitated the creation of a new resource. Using this information, you can tweak your changes to possibly avoid destroy/create updates if you didn't want to do them at this time.

Apply

From the plan, we know what will happen. Let's apply and enact the change.

$ terraform apply
aws_instance.example: Destroying...
aws_instance.example: Modifying...
  ami: "ami-408c7f28" => "ami-aa7ab6c2"

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

...

As the plan predicted, Terraform started by destroying our old instance, then creating the new one. You can use terraform show again to see the new properties 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.