Terraform State

One advantage, Terraform has over other configuration management tools like Ansible and Chef is State management. Terraform stores/tracks all our infrastructure information(that we have created using Terraform) in a file with the name terraform.tfstate.

In the last chapter, we have ran terraform apply. This command would create the above mentioned JSON flavoured statefile. Let us look at its the content.

terraform.tfstate

{
    "version": 3,
    "terraform_version": "0.11.7",
    "serial": 1,
    "lineage": "138bb0c6-ca69-0d39-aa90-5e08f29c8efb",
    "modules": [
        {
            "path": [
                "root"
            ],
            "outputs": {},
            "resources": {
                "aws_instance.webserver": {
                    "type": "aws_instance",
                    "depends_on": [],
                    "primary": {
                        "id": "i-02489cdf638f05bbd",
                        "attributes": {
                            "ami": "ami-408c7f28",
                            "arn": "arn:aws:ec2:us-east-1:822941572458:instance/i-02489cdf638f05bbd",
                            "associate_public_ip_address": "true",
                            "availability_zone": "us-east-1d",
                            "cpu_core_count": "1",
                            "cpu_threads_per_core": "1",
                            "credit_specification.#": "1",
                            "credit_specification.0.cpu_credits": "standard",
                            "disable_api_termination": "false",
                            "ebs_block_device.#": "0",
                            "ebs_optimized": "false",
                            "ephemeral_block_device.#": "0",
                            "get_password_data": "false",
                            "iam_instance_profile": "",
                            "id": "i-02489cdf638f05bbd",
                            "instance_state": "running",
                            "instance_type": "t1.micro",
                            "ipv6_addresses.#": "0",
                            "key_name": "",
                            "monitoring": "false",
                            "network_interface.#": "0",
                            "network_interface_id": "eni-00fbc08209349bb38",
                            "password_data": "",
                            "placement_group": "",
                            "primary_network_interface_id": "eni-00fbc08209349bb38",
                            "private_dns": "ip-172-31-24-184.ec2.internal",
                            "private_ip": "172.31.24.184",
                            "public_dns": "ec2-34-230-78-138.compute-1.amazonaws.com",
                            "public_ip": "34.230.78.138",
                            "root_block_device.#": "1",
                            "root_block_device.0.delete_on_termination": "true",
                            "root_block_device.0.iops": "0",
                            "root_block_device.0.volume_id": "vol-0cc495c189e5c8dd2",
                            "root_block_device.0.volume_size": "8",
                            "root_block_device.0.volume_type": "standard",
                            "security_groups.#": "1",
                            "security_groups.3814588639": "default",
                            "source_dest_check": "true",
                            "subnet_id": "subnet-e8b3e6a2",
                            "tags.%": "0",
                            "tenancy": "default",
                            "volume_tags.%": "0",
                            "vpc_security_group_ids.#": "1",
                            "vpc_security_group_ids.3314100969": "sg-a42b7ae9"
                        },
                        "meta": {
                            "e2bfb730-ecaa-11e6-8f88-34363bc7c4c0": {
                                "create": 600000000000,
                                "delete": 1200000000000,
                                "update": 600000000000
                            },
                            "schema_version": "1"
                        },
                        "tainted": false
                    },
                    "deposed": [],
                    "provider": "provider.aws"
                }
            },
            "depends_on": []
        }
    ]
}

Usecase 1

This file contains the metadata of our ec2 instance that we have created in the last chapter. This information will be critical when we have to create other resources which references this ec2 instance. As we add more resources to the terrform template, this file will get populated with more metadata about each resource.

Usecase 2

What happens when someone manually changes(which is not desirable) the properties of the instance created by Terraform? We can undo the manual changes just by running terraform apply again. One point to keep in mind though is, some property changes forces resource recreation which might result in data loss.

Changes in Terrfaform state

Let us see what happens to tfstate file when we destroy the ec2 instance

terraform destroy

[output]
aws_instance.webserver: Refreshing state... (ID: i-02489cdf638f05bbd)

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

Terraform will perform the following actions:

  - aws_instance.webserver


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

Do you really want to destroy?
  Terraform will destroy all your managed infrastructure, as shown above.
  There is no undo. Only 'yes' will be accepted to confirm.

  Enter a value: yes
  [...]
aws_instance.webserver: Still destroying... (ID: i-02489cdf638f05bbd, 1m0s elapsed)
aws_instance.webserver: Destruction complete after 1m0s

Destroy complete! Resources: 1 destroyed.

file: terraform.tfstate

{
    "version": 3,
    "terraform_version": "0.11.7",
    "serial": 2,
    "lineage": "138bb0c6-ca69-0d39-aa90-5e08f29c8efb",
    "modules": [
        {
            "path": [
                "root"
            ],
            "outputs": {},
            "resources": {},
            "depends_on": []
        }
    ]
}

Once the machine is destroyed, all metadata about that machine is also gone.

References

For more information about state management in Terraform, please visit this link.