Terraform Module Basics

Step-01: Introduction

1. Introduction - Module Basics
2. Root Module
3. Child Module

4. Published Modules (Terraform Registry)

5. Module Basics

6. Defining a Child Module
   • Source (Mandatory)
   • Version
   • Meta-arguments (count, for_each, providers, depends_on, )
   • Accessing Module Output Values
   • Tainting resources within a module

Step-02: Defining a Child Module

• We need to understand about the following
• Module Source (Mandatory): To start with we will use Terraform Registry
• Module Version (Optional): Recommended to use module version
• Create a simple EC2 Instance module
• c1-versions.tf: standard
• c2-variables.tf: standard
• c3-ami-datasource.tf: standard
• c4-ec2instance-module.tf: We will focus on building this template

# AWS EC2 Instance Module

module "ec2_cluster" {
  source                 = "terraform-aws-modules/ec2-instance/aws"
  version                = "~> 2.0"

  name                   = "my-modules-demo"
  instance_count         = 2

  ami                    = data.aws_ami.amzlinux.id
  instance_type          = "t2.micro"
  key_name               = "terraform-key"
  monitoring             = true
  vpc_security_group_ids = ["sg-08b25c5a5bf489ffa"]  # Get Default VPC Security Group ID and replace
  subnet_id              = "subnet-4ee95470" # Get one public subnet id from default vpc and replace
  user_data               = file("apache-install.sh")

  tags = {
    Name        = "Modules-Demo"
    Terraform   = "true"
    Environment = "dev"
  }
}

Step-03: Define Outputs from a EC2 Instance Module

• c5-outputs.tf: We will output the EC2 Instance Module attributes (Public DNS and Public IP)

# Output variable definitions

output "ec2_instance_public_ip" {
  description = "Public IP addresses of EC2 instances"
  value       = module.ec2_cluster.*.public_ip
}

output "ec2_instance_public_dns" {
  description = "Public IP addresses of EC2 instances"
  value       = module.ec2_cluster.*.public_dns
}

Step-04: Execute Terraform Commands

# Terraform Init
terraform init

# Terraform Validate
terraform validate

# Terraform Format
terraform fmt

# Terraform Plan
terraform plan

# Terraform Apply
terraform apply -auto-apporve

# Verify 
1) Verify in AWS management console , if EC2 Instances got created.
2) Update default security group of a VPC to allow port 80 from internet
3) Access Apache Webserver
http://<Public-IP-VM1>
http://<Public-IP-VM2>

Step-05: Tainting Resources in a Module

• The taint command can be used to taint specific resources within a module
• Very Very Important Note: It is not possible to taint an entire module. Instead, each resource within the module must be tainted separately.

# List Resources from State
terraform state list

# Taint a Resource
terraform taint <RESOURCE-NAME>
terraform taint module.ec2_cluster.aws_instance.this[0]

# Terraform Plan
terraform plan
Observation: First VM will be destroyed and re-created

# Terraform Apply
terraform apply -auto-approve

Step-06: Clean-Up Resources & local working directory

# Terraform Destroy
terraform destroy -auto-approve

# Delete Terraform files 
rm -rf .terraform*
rm -rf terraform.tfstate*

Step-07: Meta-Arguments for Modules

• Meta-Argument concepts are going to be same as how we learned during Resources section.
• count
• for_each
• providers
• depends_on
• Meta-Arguments for Modules

References

• Terraform EC2 Instance Module