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Terraform Cheat Sheet

Terraform has become more mature and can help us in many scenarios in provisioning infrastructure. These are a few scenarios that might be quite common for us in day-to-day jobs.


Take values from another state as a data source

This might be used when we already maintain a base state, and then a few child configurations need to access certain values from the base state.

First, define the data source with attributes for accessing another state

data "terraform_remote_state" "SOME_NAME" {
  backend = "local"

  config = {
    path = "/path/to/another/terraform.tfstate"
  }
}

Then, we can pass the value into any resources. For example, we output a value into another output value.

output "public_ip" {
  value = data.terraform_remote_state.SOME_NAME.outputs.public_ip 
}

Redeploy a resource

This might be useful when we find an error in our resource that requires us to redeploy the resource.

terraform apply -replace="SOME_RESOURCE.SOME_NAME"

Move a resource into another state

After running the following command, we should update the configuration to match the new resource, if not the moved resource will be deleted anytime we apply a configuration.

terraform state mv -state-out=/path/to/target/terraform.tfstate SOME_RESOURCE.SOME_SOURCE_NAME SOME_RESOURCE.SOME_TARGET_NAME

Remove a resource from a state

This process will remove resource declaration in the state, but not in the real infrastructure. It also won't remove output values that are related to the resource. It is useful when we don't want to maintain certain resources initially created using Terraform.

terraform state rm SOME_RESOURCE.SOME_NAME

Import a resource into a state

It is useful to add a real infrastructure that hasn't been in a state. We should update the configuration accordingly after running this process to maintain the resource. The reference typically is the resource ID provided by the cloud providers.

terraform import SOME_NEW_RESOURCE.SOME_NEW_NAME RESOURCE_ID

Another safer way to import is utilising import block in the configuration. The steps are as follows.

1. Create the import details in the configuration file with the ID of the real resource and the target resource that will be output to a configuration.

import {
    id = "REAL_RESOURCE_ID"
    to = TARGET_RESOURCE.NAME
}

2. Generate the resource block into a configuration

terraform plan -generate-config-out=generated.tf

3. Adjust the detail of the generated resource so that it won't destroy the real resource when we perform apply.


Synchronize a state with real infrastructure

When we manually change/delete an instance in real infrastructure, it is necessary to make the existing state reflect the actual condition. This command will only update the state but not the configuration file. We need to update/remove related configurations accordingly.

terraform refresh

However, the command above is discouraged from being used because it instantly updates the state. Another method is using the -refresh-only parameter in plan or apply. It will give us time to review the changes.

terraform plan -refresh-only
terraform apply -refresh-only

The condition that requires us to perform this action can also be caused by changes in credentials or regions that make Terraform detect that the resource is deleted.


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