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Terraform
Associate Tutorials

Manage resource drift

  • 12min
  • |
  • Terraform

The Terraform state file is a record of all resources Terraform manages. You should not make manual changes to resources controlled by Terraform, because the state file will be out of sync, or "drift," from the real infrastructure. If your state and configuration do not match your infrastructure, Terraform will attempt to reconcile your infrastructure, which may unintentionally destroy or recreate resources.

In this tutorial, you will detect and fix differences between your state file and real infrastructure. First, you will create an EC2 instance and security group with Terraform. Then, you will manually edit them via the AWS CLI. Next, you will identify and resolve the discrepancies between Terraform state and your infrastructure.

Note

Drift detection is available in HCP Terraform Plus Edition. Refer to HCP Terraform pricing for details.

Prerequisites

This tutorial assumes you are familiar with the standard Terraform workflow. If you are unfamiliar with Terraform, complete the Get Started tutorials first.

For this tutorial, you will need:

Create infrastructure

Start by cloning the example repository. This configuration builds an EC2 instance, an SSH key pair, and a security group rule to allow SSH access to the instance.

$ git clone https://github.com/hashicorp/learn-terraform-drift-management.git

Change into the repository directory.

$ cd learn-terraform-drift-management

Create an SSH key pair in your current directory, replacing your_email@example.com with your email address. Use an empty passphrase.

$ ssh-keygen -t rsa -C "your_email@example.com" -f ./key
Generating public/private rsa key pair.

Enter passphrase (empty for no passphrase):

Confirm your AWS CLI region.

$ aws configure get region
us-east-2

Open the terraform.tfvars file and edit the region to match your AWS CLI configuration.

region = "us-east-2"

Open the main.tf file and review your configuration. The main resources are your EC2 instance, your key pair, and the SSH security group.

##...
resource "aws_key_pair" "deployer" {
  key_name   = "deployer-key"
  public_key = file("${path.module}/key.pub")
}

resource "aws_instance" "example" {
  ami                    = data.aws_ami.ubuntu.id
  key_name               = aws_key_pair.deployer.key_name
  instance_type          = "t2.micro"
  vpc_security_group_ids = [aws_security_group.sg_ssh.id]
  user_data              = <<-EOF
              #!/bin/bash
              apt-get update
              apt-get install -y apache2
              sed -i -e 's/80/8080/' /etc/apache2/ports.conf
              echo "Hello World" > /var/www/html/index.html
              systemctl restart apache2
              EOF
  tags = {
    Name          = "terraform-learn-state-ec2"
    drift_example = "v1"
  }
}


resource "aws_security_group" "sg_ssh" {
  name = "sg_ssh"
  ingress {
    from_port   = "22"
    to_port     = "22"
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }
  // connectivity to ubuntu mirrors is required to run `apt-get update` and `apt-get install apache2`
  egress {
    from_port   = 0
    to_port     = 0
    protocol    = "-1"
    cidr_blocks = ["0.0.0.0/0"]
  }
}

Initialize your configuration.

$ terraform init

Initializing the backend...

Initializing provider plugins...
- Reusing previous version of hashicorp/aws from the dependency lock file
- Installing hashicorp/aws v3.26.0...
- Installed hashicorp/aws v3.26.0 (signed by HashiCorp)

Terraform has been successfully initialized!

You may now begin working with Terraform. Try running "terraform plan" to see
any changes that are required for your infrastructure. All Terraform commands
should now work.

If you ever set or change modules or backend configuration for Terraform,
rerun this command to reinitialize your working directory. If you forget, other
commands will detect it and remind you to do so if necessary.

Apply your configuration. Enter yes when prompted to accept your changes.

$ terraform apply


## ...

Apply complete! Resources: 3 added, 0 changed, 0 destroyed.

Outputs:

instance_id = "i-0250e3c625858c3ee"
public_ip = "18.224.17.153"
security_groups = [
  toset([
    "sg-0bf5fe7b3b54df9c3",
  ]),
]

When your apply operation completes, run terraform state list to review the resources managed by Terraform in your state file.

$ terraform state list
data.aws_ami.ubuntu
aws_instance.example
aws_key_pair.deployer
aws_security_group.sg_ssh

Introduce drift

To introduce a change to your configuration outside the Terraform workflow, create a new security group with the AWS CLI and export that value as an environment variable.

$  export SG_ID=$(aws ec2 create-security-group --group-name "sg_web" --description "allow 8080" --output text)

Confirm you created the environment variable. This will return the security group you just created.

$ echo $SG_ID
sg-04c74100cc8b9fc8c

Next, create a new rule for your group to provide TCP access to the instance on port 8080.

$ aws ec2 authorize-security-group-ingress --group-name "sg_web" --protocol tcp --port 8080 --cidr 0.0.0.0/0

Associate the security group you created manually with the EC2 instance provisioned by Terraform.

$ aws ec2 modify-instance-attribute --instance-id $(terraform output -raw instance_id) --groups $SG_ID

Now, you have replaced your instance's SSH security group with a new security group that is not tracked in the Terraform state file.

Run a refresh-only plan

By default, Terraform compares your state file to real infrastructure whenever you invoke terraform plan or terraform apply. The refresh updates your state file in-memory to reflect the actual configuration of your infrastructure. This ensures that Terraform determines the correct changes to make to your resources.

If you suspect that your infrastructure configuration changed outside of the Terraform workflow, you can use a -refresh-only flag to inspect what the changes to your state file would be. This is safer than the refresh subcommand, which automatically overwrites your state file without displaying the updates.

Tip

The -refresh-only flag was introduced in Terraform 0.15.4, and is preferred over the terraform refresh subcommand.

Run terraform plan -refresh-only to determine the drift between your current state file and actual configuration.

$ terraform plan -refresh-only
aws_key_pair.deployer: Refreshing state... [id=deployer-key]
aws_security_group.sg_ssh: Refreshing state... [id=sg-0b318a348a4a4e391]
aws_instance.example: Refreshing state... [id=i-008bef01721ee7f7c]

Note: Objects have changed outside of Terraform

Terraform detected the following changes made outside of Terraform since the last "terraform apply":

  # aws_instance.example has been changed
  ~ resource "aws_instance" "example" {
        id                           = "i-008bef01721ee7f7c"
        tags                         = {
            "Name"          = "terraform-learn-state-ec2"
            "drift_example" = "v1"
        }
      ~ vpc_security_group_ids       = [
          + "sg-0226a51361bf1497a",
          - "sg-0b318a348a4a4e391",
        ]
        # (27 unchanged attributes hidden)




        # (4 unchanged blocks hidden)
    }

This is a refresh-only plan, so Terraform will not take any actions to undo
these. If you were expecting these changes then you can apply this plan to
record the updated values in the Terraform state without changing any remote
objects.

────────────────────────────────────────────────────────────────────────────────────

Changes to Outputs:
  ~ security_groups = [
      - [
          - "sg-0b318a348a4a4e391",
        ],
      + [
          + "sg-0226a51361bf1497a",
        ],
    ]

You can apply this plan to save these new output values to the Terraform state,
without changing any real infrastructure.

─────────────────────────────────────────────────────────────────────────────────────

Note: You didn't use the -out option to save this plan, so Terraform can't
guarantee to take exactly these actions if you run "terraform apply" now.

As shown in the output, Terraform has detected differences between the infrastructure and the current state, and sees that your original security group allowing access on port 22 is no longer attached to your EC2 instance. The refresh-only plan output indicates that Terraform will update your state file to modify the configuration of your EC2 instance to reflect the new security group with access on port 8080.

Apply these changes to make your state file match your real infrastructure, but not your Terraform configuration. Respond to the prompt with a yes.

$ terraform apply -refresh-only
aws_key_pair.deployer: Refreshing state... [id=deployer-key-rita]
aws_security_group.sg_ssh: Refreshing state... [id=sg-0b318a348a4a4e391]
aws_instance.example: Refreshing state... [id=i-008bef01721ee7f7c]

Note: Objects have changed outside of Terraform

Terraform detected the following changes made outside of Terraform since the last "terraform apply":

  # aws_instance.example has been changed
##...
Would you like to update the Terraform state to reflect these detected changes?
  Terraform will write these changes to the state without modifying any real infrastructure.
  There is no undo. Only 'yes' will be accepted to confirm.

  Enter a value: yes


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

Outputs:

instance_id = "i-008bef01721ee7f7c"
public_ip = "35.163.80.243"
security_groups = [
  toset([
    "sg-0226a51361bf1497a",
  ]),
]

A refresh-only operation does not attempt to modify your infrastructure to match your Terraform configuration -- it only gives you the option to review and track the drift in your state file.

If you ran terraform plan or terraform apply without the -refresh-only flag now, Terraform would attempt to revert your manual changes. Instead, you will update your configuration to associate your EC2 instance with both security groups.

Add the security group to configuration

Import the sg_web security group resource to your state file to bring it under Terraform management.

First, add the resource definition to your configuration by adding a new security group resource and rule resource to your main.tf file.

resource "aws_security_group" "sg_web" {
  name        = "sg_web"
  description = "allow 8080"
}

resource "aws_security_group_rule" "sg_web" {
  type      = "ingress"
  to_port   = "8080"
  from_port = "8080"
  protocol  = "tcp"
  cidr_blocks = ["0.0.0.0/0"]
  security_group_id = aws_security_group.sg_web.id
}

Add the security group ID to your instance resource.

resource "aws_instance" "example" {
  ami                    = data.aws_ami.ubuntu.id
  key_name               = aws_key_pair.deployer.key_name
  instance_type          = "t2.micro"
- vpc_security_group_ids = [aws_security_group.sg_ssh.id]
+ vpc_security_group_ids = [aws_security_group.sg_ssh.id, aws_security_group.sg_web.id]
  user_data              = <<-EOF
              #!/bin/bash
              apt-get update
              apt-get install -y apache2
              sed -i -e 's/80/8080/' /etc/apache2/ports.conf
              echo "Hello World" > /var/www/html/index.html
              systemctl restart apache2
              EOF
  tags = {
    Name          = "terraform-learn-state-ec2"
    drift_example = "v1"
  }
}

Import the security group

Tip

This tutorial uses terraform import to bring infrastructure under Terraform management. Terraform 1.5+ supports configuration-driven import, which lets you import multiple resources at once, review the import in your plan-and-apply workflow, and generate configuration for imported resources. Review the import tutorial to learn more.

Run terraform import to associate your resource definition with the security group created in the AWS CLI.

$ terraform import aws_security_group.sg_web $SG_ID

aws_security_group.sg_web: Importing from ID "sg-04c74100cc8b9fc8c"...
aws_security_group.sg_web: Import prepared!
  Prepared aws_security_group for import
aws_security_group.sg_web: Refreshing state... [id=sg-04c74100cc8b9fc8c]

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.

Import your security group rule.

$ terraform import aws_security_group_rule.sg_web "$SG_ID"_ingress_tcp_8080_8080_0.0.0.0/0

aws_security_group_rule.sg_web: Importing from ID "sg-04c74100cc8b9fc8c_ingress_tcp_8080_8080_0.0.0.0/0"...
aws_security_group_rule.sg_web: Import prepared!
  Prepared aws_security_group_rule for import
aws_security_group_rule.sg_web: Refreshing state... [id=sg-04c74100cc8b9fc8c_ingress_tcp_8080_8080_0.0.0.0/0]

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.

Run terraform state list to return the list of resources Terraform is managing, which now includes the imported resources.

$ terraform state list

data.aws_ami.ubuntu
aws_instance.example
aws_key_pair.deployer
aws_security_group.sg_web
aws_security_group.sg_ssh
aws_security_group_rule.sg_web

Terraform successfully associated both security groups with the instance in state. However, your instance still only allows port 8080 access because the modify-instance-attribute AWS CLI command detached the SSH security group.

Update your resources

Now that the sg_web security group is represented in state, re-run terraform apply to associate the SSH security group with your EC2 instance.

Notice how this updates your EC2 instance's security group to include both the security groups allowing SSH and 8080. Enter yes when prompted to confirm your changes.

$ terraform apply
aws_security_group.sg_ssh: Refreshing state... [id=sg-09d0b575577f258d5]
aws_key_pair.deployer: Refreshing state... [id=deployer-key]
aws_security_group.sg_web: Refreshing state... [id=sg-0acc6237c67c07e4b]
aws_security_group_rule.sg_web: Refreshing state... [id=sgrule-4278118923]
aws_instance.example: Refreshing state... [id=i-092c09eed28bdb2f7]

Terraform used the selected providers to generate the following execution plan. Resource actions
are indicated with the following symbols:
  ~ update in-place

Terraform will perform the following actions:

  # aws_instance.example will be updated in-place
  ~ resource "aws_instance" "example" {
        id                           = "i-092c09eed28bdb2f7"
        tags                         = {
            "Name"          = "terraform-learn-state-ec2"
            "drift_example" = "v1"
        }
      ~ vpc_security_group_ids       = [
          + "sg-09d0b575577f258d5",
            # (1 unchanged element hidden)
        ]
        # (27 unchanged attributes hidden)




        # (4 unchanged blocks hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.
Do you want to perform these actions?
  Terraform will perform the actions described above.
  Only 'yes' will be accepted to approve.

  Enter a value: yes
##...

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

Outputs:

instance_id = "i-092c09eed28bdb2f7"
public_ip = "3.142.238.150"
security_groups = [
  toset([
    "sg-09d0b575577f258d5",
    "sg-0acc6237c67c07e4b",
  ]),
]

Access the instance

Confirm your instance allows SSH. Enter yes when prompted to connect to the instance.

$ ssh ubuntu@$(terraform output -raw public_ip) -i key
The authenticity of host '3.142.238.150 (3.142.238.150)' can't be established.
ECDSA key fingerprint is SHA256:7PCkol+dVFps8YkOPMVZ7zG9sKXq0tnzqRENB7FTodM.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added '3.142.238.150' (ECDSA) to the list of known hosts.
Welcome to Ubuntu 20.04.2 LTS (GNU/Linux 5.4.0-1041-aws x86_64)
##...
ubuntu@ip-172-31-20-193:~$

Exit the SSH connection by typing exit in the SSH prompt.

Confirm your instance allows port 8080 access.

$ curl $(terraform output -raw public_ip):8080
Hello, World

Clean up your resources

When you are finished with this tutorial, destroy the resources you created. Enter yes when prompted to confirm your changes.

$ terraform destroy
##...
Destroy complete! Resources: 5 destroyed.

Next steps

In this tutorial, you created an EC2 instance and security group deployment with Terraform. Then, you introduced drift by editing your security groups outside the Terraform workflow. Finally, you learned how to detect your drifted configuration with -refresh-only operations and how to reconcile your state file and configuration with the Terraform CLI refresh and import commands.

For more information about Terraform state and drift management, review the resources below:

Previous

Target resources

 Next

Refresh state

This tutorial also appears in:

  • 11 tutorials
    Manage Terraform State
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    • Terraform