Use registry modules in configuration

In the previous tutorial, you learned when and why to use Terraform modules. In this tutorial, you will use modules from the public Terraform Registry to provision an example environment on AWS by referencing the modules in Terraform configuration. The concepts you use in this tutorial will apply to any modules from any source.

Prerequisites

You can complete this tutorial using the same workflow with either Terraform Community Edition or HCP Terraform. HCP Terraform is a platform that you can use to manage and execute your Terraform projects. It includes features like remote state and execution, structured plan output, workspace resource summaries, and more.

This tutorial assumes that you are familiar with the Terraform workflow. If you are new to Terraform, complete the Get Started tutorials first.

In order to complete this tutorial, you will need the following:

• Terraform v1.1+ installed locally.
• An AWS account with local credentials configured for use with Terraform.

Use the Terraform Registry

Open the Terraform Registry page for the VPC module.

This page displays information about the module and a link to the source repository. The page also has a dropdown interface to select the module version, module usage metrics, and example configuration.

The example configuration sets two arguments: source and version.

1. The source argument is required when you use a Terraform module. In the example configuration, Terraform will search for a module in the Terraform Registry that matches the given string. You could also use a URL or local module. Refer to the Terraform documentation for a full list of possible module sources.

2. The version argument is not required, but we highly recommend you include it when using a Terraform module. For supported sources, this argument specifies the module version Terraform will load. Without the version argument, Terraform will load the latest version of the module. In this tutorial, you will specify an exact version number for the modules you use. Refer to the module documentation for more methods to specify module versions.

Terraform treats other arguments in the module blocks as input variables for the module.

Clone the example configuration

Clone the example repository. The configuration in this repository uses modules to create an example AWS environment using a Virtual Private Cloud (VPC) and two EC2 instances.

$ git clone https://github.com/hashicorp-education/learn-terraform-modules-use

Change to the repository directory.

$ cd learn-terraform-modules-use

Review configuration

Open terraform.tf. This file defines the terraform block, which Terraform uses to configures itself. This block specifies this Terraform configuration must use the aws provider that is within the v4.49.0 minor release. It also requires that you use a Terraform version greater than v1.1.0.

terraform {
  /* Uncomment this block to use HCP Terraform for this tutorial
  cloud {
    organization = "organization-name"
    workspaces {
      name = "learn-terraform-module-use"
    }
  }
  */

  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 4.49.0"
    }
  }
  required_version = ">= 1.1.0"
}

Open main.tf. This file contains the resource configuration.

provider "aws" {
  region = "us-west-2"

  default_tags {
    tags = {
      hashicorp-learn = "module-use"
    }
  }
}

module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "3.18.1"

  name = var.vpc_name
  cidr = var.vpc_cidr

  azs             = var.vpc_azs
  private_subnets = var.vpc_private_subnets
  public_subnets  = var.vpc_public_subnets

  enable_nat_gateway = var.vpc_enable_nat_gateway

  tags = var.vpc_tags
}

module "ec2_instances" {
  source  = "terraform-aws-modules/ec2-instance/aws"
  version = "4.3.0"

  count = 2
  name  = "my-ec2-cluster-${count.index}"

  ami                    = "ami-0c5204531f799e0c6"
  instance_type          = "t2.micro"
  vpc_security_group_ids = [module.vpc.default_security_group_id]
  subnet_id              = module.vpc.public_subnets[0]

  tags = {
    Terraform   = "true"
    Environment = "dev"
  }
}

This configuration includes three blocks:

1. The provider "aws" block configures the AWS provider. Depending on the authentication method you use, you may need to include additional arguments in the provider block.
2. The module "vpc" block configures a Virtual Private Cloud (VPC) module, which provisions networking resources such as a VPC, subnets, and internet and NAT gateways based on the arguments provided.
3. The module "ec2_instances" block defines two EC2 instances provisioned within the VPC created by the module.

Set values for module input variables

Modules can contain both required and optional arguments. You must specify all required arguments to use the module. Most module arguments correspond to the module's input variables. Optional inputs will use the module's default values if not explicitly defined.

On the Terraform Registry page for the AWS VPC module, click on the Inputs tab to find the input arguments that the module supports.

Review each argument defined in the module "vpc" block.

module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "3.18.1"

  name = var.vpc_name
  cidr = var.vpc_cidr

  azs             = var.vpc_azs
  private_subnets = var.vpc_private_subnets
  public_subnets  = var.vpc_public_subnets

  enable_nat_gateway = var.vpc_enable_nat_gateway

  tags = var.vpc_tags
}

Next, review the module "ec2_instances" block.

module "ec2_instances" {
  source  = "terraform-aws-modules/ec2-instance/aws"
  version = "4.3.0"

  count = 2
  name  = "my-ec2-cluster-${count.index}"

  ami                    = "ami-0c5204531f799e0c6"
  instance_type          = "t2.micro"
  vpc_security_group_ids = [module.vpc.default_security_group_id]
  subnet_id              = module.vpc.public_subnets[0]

  tags = {
    Terraform   = "true"
    Environment = "dev"
  }
}

• The count meta-argument defines two EC2 instances. For a full list of module meta-arguments, refer to the module documentation.
• The required vpc_security_group_ids and subnet_id arguments reference resources created by the vpc module. The Terraform Registry module page contains the full list of arguments for the ec2-instance module.

Review root input variables

Using input variables with modules is similar to using variables in any Terraform configuration. A common pattern is to identify which module arguments you may want to change in the future, and create matching variables in your configuration's variables.tf file with sensible default values. You can pass the variables to the module block as arguments.

You do not need to set all module input variables with variables. For example, if your organization requires NAT gateway enabled for all VPCs, you should not use a variable to set the enable_nat_gateway argument.

Open variables.tf to review the input variable declarations and definitions.

variable "vpc_name" {
  description = "Name of VPC"
  type        = string
  default     = "example-vpc"
}

variable "vpc_cidr" {
  description = "CIDR block for VPC"
  type        = string
  default     = "10.0.0.0/16"
}

variable "vpc_azs" {
  description = "Availability zones for VPC"
  type        = list(string)
  default     = ["us-west-2a", "us-west-2b", "us-west-2c"]
}

variable "vpc_private_subnets" {
  description = "Private subnets for VPC"
  type        = list(string)
  default     = ["10.0.1.0/24", "10.0.2.0/24"]
}

variable "vpc_public_subnets" {
  description = "Public subnets for VPC"
  type        = list(string)
  default     = ["10.0.101.0/24", "10.0.102.0/24"]
}

variable "vpc_enable_nat_gateway" {
  description = "Enable NAT gateway for VPC"
  type        = bool
  default     = true
}

variable "vpc_tags" {
  description = "Tags to apply to resources created by VPC module"
  type        = map(string)
  default = {
    Terraform   = "true"
    Environment = "dev"
  }
}

Review root output values

Modules also have output values. You can reference them with the module.MODULE_NAME.OUTPUT_NAME naming convention. In the Terraform Registry for the module, click on the Outputs tab to find all outputs for the module.

You can reference module outputs in other parts of your configuration. Terraform will not display module outputs by default. You must create a corresponding output in your root module and set it to the module's output. This tutorial shows both cases.

Open outputs.tf to find the module outputs.

output "vpc_public_subnets" {
  description = "IDs of the VPC's public subnets"
  value       = module.vpc.public_subnets
}

output "ec2_instance_public_ips" {
  description = "Public IP addresses of EC2 instances"
  value       = module.ec2_instances[*].public_ip
}

In this example, the vpc_public_subnets output references the vpc module's public_subnets output, and ec2_instance_public_ips references the public IP addresses for both EC2 instances created by the module.

Provision infrastructure

Initialize this configuration.

$ terraform init
Initializing the backend...
##...
Terraform has been successfully initialized!
You may now begin working with Terraform. Try running "terraform plan" to see
any changes that are required for your infrastructure. All Terraform commands
should now work.
If you ever set or change modules or backend configuration for Terraform,
rerun this command to reinitialize your working directory. If you forget, other
commands will detect it and remind you to do so if necessary.

Now, apply your configuration to create your VPC and EC2 instances. Respond to the prompt with yes to apply the changes. The vpc and ec2 modules define more resources than just the VPC and EC2 instances.

$ terraform apply

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  + create

  ## ...

Plan: 22 to add, 0 to change, 0 to destroy.

## ...

Do you want to perform these actions?
  Terraform will perform the actions described above.
  Only 'yes' will be accepted to approve.

  Enter a value: yes

## ...

Apply complete! Resources: 22 added, 0 changed, 0 destroyed.

Outputs:

ec2_instance_public_ips = [
  "54.245.140.252",
  "34.219.48.47",
]
vpc_public_subnets = [
  "subnet-0cb9ff659ba66a7dd",
  "subnet-0c2788b6ffb0611c0",
]

Once Terraform completes, it will display the configuration outputs.

If you use HCP Terraform to provision your resources, your workspace now displays the list of all of the resources it manages.

Understand how modules work

When using a new module for the first time, you must run either terraform init or terraform get to install the module. When you run these commands, Terraform will install any new modules in the .terraform/modules directory within your configuration's working directory. For local modules, Terraform will create a symlink to the module's directory. Because of this, any changes to local modules will be effective immediately, without having to reinitialize or re-run terraform get.

After following this tutorial, your .terraform/modules directory will look like the following.

.terraform/modules/
├── ec2_instances
├── modules.json
└── vpc

Clean up your infrastructure

Before moving on to the next tutorial, destroy the infrastructure you created. Respond to the confirmation prompt with a yes.

$ terraform destroy
## ...

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  - destroy

  ## ...

Plan: 0 to add, 0 to change, 22 to destroy.

Changes to Outputs:
  - ec2_instance_public_ips = [
      - "54.245.140.252",
      - "34.219.48.47",
    ] -> null
  - vpc_public_subnets      = [
      - "subnet-0cb9ff659ba66a7dd",
      - "subnet-0c2788b6ffb0611c0",
    ] -> null

Do you really want to destroy all resources?
  Terraform will destroy all your managed infrastructure, as shown above.
  There is no undo. Only 'yes' will be accepted to confirm.

  Enter a value: yes

## ...

Destroy complete! Resources: 22 destroyed.

If you used HCP Terraform for this tutorial, after destroying your resources, delete the learn-terraform-module-use workspace from your HCP Terraform organization.

Next steps

In this tutorial, you learned how to use modules in your Terraform configuration, manage module versions, configure module input variables, and use module output values.

HCP Terraform Plus Edition lets you add modules to the private registry that users can deploy without writing configuration. Follow the Create and Use No-Code Modules tutorial to create a no-code ready module.

In the next tutorial, you will create a module for configuration that hosts a website in an S3 bucket.