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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
  • standard
  • standard
  • standard
  • 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                    =
  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("")

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

Step-03: Define Outputs from a EC2 Instance Module

  • 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

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