Understanding terraform init, plan, and apply: Behind the Scenes

Workflow Terraform Commands

Welcome to the advanced Terraform guide! Whether you’re already familiar with the basics or looking to level up, we’re going to explore three essential Terraform commands: terraform init, terraform plan, and terraform apply. We’ll break down what happens behind the scenes, provide real-world examples, and explain the output step by step

Let’s get started! 🔥

1. What Happens When You Run terraform init?

The terraform init command is the first step in any Terraform workflow. It sets up your project and ensures Terraform is ready to execute your configurations.

Behind the Scenes

When you run terraform init, several things happen:

• Provider Plugins: Terraform downloads the necessary provider plugins (like AWS, Azure, Google Cloud) for your project. These plugins allow Terraform to interact with cloud platforms and provision resources.
• Backend Initialization: If you’re using a remote backend (e.g., storing your state files in S3), Terraform configures it during this step.
• Lock File Creation: A .terraform.lock.hcl file is generated, locking the versions of your provider plugins, ensuring consistency across environments.

Example

Let’s say you have the following configuration in your main.tf:

provider "aws" {
  region = "us-east-1"
}

Running terraform init will look something like this:

$ terraform init

Initializing the backend...
Initializing provider plugins...
- Finding latest version of hashicorp/aws...
- Installing hashicorp/aws v4.0.0...
- Installed hashicorp/aws v4.0.0 (signed by HashiCorp)

Terraform has been successfully initialized!

The init command is like preparing your toolbox before you start a project. It makes sure you have everything you need to proceed: the right plugins, correct versions, and any backend configuration.

2. What Happens When You Run terraform plan?

The terraform plan command is where Terraform shows you what changes it would make to your infrastructure before actually making any changes. It compares the current state of your infrastructure (what exists) with the desired state (your configuration files).

Behind the Scenes

Here’s what happens when you run terraform plan:

• State Comparison: Terraform compares the current state of your resources (e.g., EC2 instances, databases) with the desired state defined in your configuration files.
• Execution Plan Creation: It creates a plan that shows what will be added, modified, or destroyed.
• No Changes Yet: This is just a preview. Nothing is modified during terraform plan.

Example

Let’s extend our previous example and say we want to add an EC2 instance:

provider "aws" {
  region = "us-east-1"
}

resource "aws_instance" "my_instance" {
  ami           = "ami-12345678"
  instance_type = "t2.micro"
}

Running terraform plan will produce something like this:

$ terraform plan

Terraform will perform the following actions:

  # aws_instance.my_instance will be created
  + resource "aws_instance" "my_instance" {
      + ami           = "ami-12345678"
      + instance_type = "t2.micro"
    }

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

The plan command is like looking at a blueprint before you start building. It tells you exactly what Terraform will do, allowing you to review changes and avoid surprises.

3. What Happens When You Run terraform apply?

Now comes the exciting part — making changes! The terraform apply command executes the plan and applies the desired state to your infrastructure.

Behind the Scenes

When you run terraform apply, here’s what happens:

• Execution of the Plan: Terraform takes the plan generated by terraform plan and applies the changes to your infrastructure.
• Resource Creation/Modification/Deletion: Depending on the plan, Terraform will add, update, or destroy resources.
• State Update: Once the changes are applied, Terraform updates the terraform.tfstate file, which keeps track of your infrastructure's current state.

Example

Continuing with our EC2 example, here’s what happens when you run terraform apply:

$ terraform apply

Terraform will perform the following actions:

  # aws_instance.my_instance will be created
  + resource "aws_instance" "my_instance" {
      + ami           = "ami-12345678"
      + instance_type = "t2.micro"
    }

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

Do you want to perform these actions? (yes/no)

If you type yes, Terraform will create the EC2 instance:

aws_instance.my_instance: Creating...
aws_instance.my_instance: Creation complete after 1m12s [id=i-0abcdef1234567890]

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

The apply command is like starting the actual construction based on your blueprint. It makes the changes for real and updates your infrastructure to match your configuration.

To recap, here’s what each command does:

1. terraform init

Setup: Initializes your project by setting up the working directory.

Download Plugins: Installs the necessary provider plugins (e.g., AWS, Azure).

Initialize Backends: Configures any remote state storage (e.g., S3).

Lock Files: Creates a .terraform.lock.hcl file to lock provider versions for consistency.

2. terraform plan

State Refresh: Updates the current state by pulling the latest data from your cloud provider.

Compare States: Compares the current infrastructure with your desired configuration.

Execution Plan: Shows what will be added, changed, or destroyed.

Dry Run: No actual changes are made — this is just a preview.

3. terraform apply

State Refresh (Optional): Optionally refreshes the state before making changes.

Execute the Plan: Applies the planned changes to the infrastructure.

Create/Modify/Destroy: Updates resources according to the plan.

Update State: Saves the new state to the terraform.tfstate file for future use.

In short: init prepares your environment, plan shows what will change, and apply makes those changes!

Photo by Azwedo L.LC on Unsplash

And that’s it — happy Terraforming! 😄