In this article, we are going to create an AWS EC2 Linux instance and security group, examine a state file, and replace a resource using CLI to show how you manipulate the resources.

In the next article - How to manage and manipulate resources in Terraform state file - 2, we will manipulate resources using these commands - move, remove and refresh to observe how important state is to your Terraform operations.

Terraform state file - terraform.tfstate

Terraform stores information about your infrastructure in a state file. This state file keeps track of resources created by your configuration and maps them to real-world resources, and keep track of metadata.

This state is stored by default in a local file named "terraform.tfstate", but it can also be stored remotely.

Terraform compares your configuration with the state file and your existing infrastructure to create plans and make changes to your infrastructure. When you run terraform apply or terraform destroy against your initialized configuration, Terraform writes metadata about your configuration to the state file and updates your infrastructure resources accordingly. Prior to any operation, Terraform does a refresh to update the state with the real infrastructure.

Please visit my GitHub Repository for Terraform articles on various topics being updated on constant basis.

Let’s get started!

Objectives:

1. Create infrastructure and a state file

2. Examine the state file with CLI

3. Replace a resource with CLI

Pre-requisites:

• AWS user account with admin access, not a root account.

• Cloud9 IDE with AWS CLI, and Terraform installed.

Resources Used:

For building this EC2 module, I have used a data source for pulling in an AMI ID instead of a hard-coded value. I have spent so much time to make this work, and after many, many attempts, I succeeded! I have also used Terraform documentation for this purpose.

Terraform documentation for AMI.

data source for pulling in an AMI ID.

Steps for implementation to this project:

1. Create infrastructure and a state file

• Let’s create the following organizational structure as shown below.

• Create a main.tf file. This will deploy a Linux EC2 instance "ec2_orig", with the security group*"ec2_sg"*.

# PROVIDER BLOCK

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 4.23"
    }
  }
  required_version = ">= 1.2.0"
}

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


# EC2 BLOCK

data "aws_ami" "linux" {
   most_recent = true
   owners      = ["amazon"]

  filter {
    name   = "name"
    values = ["amzn2-ami-hvm-*-x86_64-gp2"]
  }

  filter {
    name   = "virtualization-type"
    values = ["hvm"]
  }
}



resource "aws_instance" "ec2_orig" {
  ami                = data.aws_ami.linux.id
  instance_type      = "t2.micro"
  vpc_security_group_ids = [aws_security_group.ec2_sg.id]

  tags = {
    Name = "ec2_orig"
  }
}



# security group 
resource "aws_security_group" "ec2_sg" {
   name        = "ec2_sg"
   description = "allow inbound HTTP traffic"

   # HTTP from vpc
   ingress {
      from_port   = 80
      to_port     = 80
      protocol    = "tcp"
      cidr_blocks = ["0.0.0.0/0"]     
   }


  # outbound rules
  # internet access to anywhere
  egress {
     from_port   = 0
     to_port     = 0
     protocol    = "-1"
     cidr_blocks = ["0.0.0.0/0"]
  }

  tags = {
     name = "ec2_sg"
  }
}



# OUTPUTS BLOCK

output "instance_id" {
  value = aws_instance.ec2_orig.id
}

output "public_ip" {
  value       = aws_instance.ec2_orig.public_ip
  description = "The public IP of the ec2 server"
}

output "security_group" {
  value = aws_security_group.ec2_sg.id
}

• Run terraform init to initialize Terraform.

• Run terraform apply to apply the configuration and type yes when prompted.

• Wait for 4-5 minutes for the EC2 instance to be created.

2. Examine the state file with CLI

• Review the resources in the state file with the Terraform CLI without interacting with the .tfstate file. This is how you should interact with your state.

• Run terraform show to get a human-friendly output of the resources contained in your state.

instance_id, public_ip

security_group

data "aws_ami" "linux"

outputs

• Run terraform state list to get the list of resource names and local identifiers in your state file.

3. Replace a resource with CLI

• Terraform updates your infrastructure if it does not match your configuration.

• Hence, you can use the -replace command for terraform plan and terraform apply operations to safely recreate resources in your environment even if you have not edited the configuration.

• Replacing a resource is also useful in cases where a user manually changes a setting on a resource or when you need to update a script.

• This allows you to rebuild specific resources and avoid a full terraform destroy operation on your configuration.

• Run terraform plan -replace="aws_instance.ec2_orig" to see the resources that Terraform would create..

• Run terraform apply -replace="aws_instance.ec2_orig" to force Terraform to destroy and recreate the resource and type yes when prompted.

• Wait 4-5 minutes for the new EC2 instance to be re-created.

• Run terraform state list to get the list of new resource names and local identifiers in your state file.

What we have done so far

you created an EC2 Linux instance and corresponding security group. Then, you examined your local state file and replaced a resource using CLI to show how you manipulated the resources.