Terraform: Understanding Desired & Current State

In this post, we will learn in detail what is terraform desired and current state.

Terraform’s responsibility is to create/update/destroy infrastructure resources to match the desired state as described in the configuration.

Desired State:

For example: If our desired state is as below

resource "aws_instance" "myec2" {
  ami           = "ami-0ca285d4c2cda3300"
  instance_type = "t2.medium"
}

This should result in an AWS EC2 t2.medium instance.

The code you saw above is the desired state that we want.

Current State:

The current state is the actual state of a resource that is deployed.

For example: If our desired state is as below

resource "aws_instance" "myec2" {
  ami           = "ami-0ca285d4c2cda3300"
  instance_type = "t2.medium"
}

our desired state is t2.medium instance but let’s say the current instance running is t2.micro. So it means our desired state and the current state is not matching.

Try it out

Let’s first deploy a t2.medium ec2 instance using the below code.

provider "aws" {
    region = "us-west-2"
    access_key = "<access_key>"
    secret_key = "<secret_key>"
}resource "aws_instance" "myec2" {
  ami           = "ami-0ca285d4c2cda3300"
  instance_type = "t2.micro"
}

Learn how to deploy from the link: Terraform — 101

This will deploy a AWS EC2 t2.micro instance.

Now let’s modify the instance_type to t2.medium

provider "aws" {
    region = "us-west-2"
    access_key = "<access_key>"
    secret_key = "<secret_key>"
}resource "aws_instance" "myec2" {
  ami           = "ami-0ca285d4c2cda3300"
  instance_type = "t2.medium"
}

run the terraform plan

Terraform will detect the changes between the desired state (t2.medium) and the current state (t2.micro)

# aws_instance.myec2 will be updated in-place
  ~ resource "aws_instance" "myec2" {
        id                                   = "i-0a84f30f5656d7800"
      ~ instance_type                        = "t2.micro" -> "t2.medium"
        tags                                 = {
            "Name" = "terraform"
        }
        # (28 unchanged attributes hidden)# (6 unchanged blocks hidden)
    }Plan: 0 to add, 1 to change, 0 to destroy.

As we can see terraform has detected the change.

Remember, terraform tries to ensure that the deployed infra is based on the desired state. If there is a difference between the two, terraform plan will show the necessary changes required to achieve the desired state.

Let’s do the terraform apply

This will update the instance type to t2.micro

Hope this clarifies what the desired state and current state is and how the terraform handles the difference between them.

In the subsequent, article we will explore more of terraform.

Thanks for reading. If you have some feedback, please provide your response or reach out to me on Twitter or Github.

Happy Coding!!!