A Terraform module is a collection of configuration files that encapsulate resources used together to achieve a specific outcome. Modules promote reusability, organization, and maintainability in infrastructure as code by allowing you to group related resources and manage them as a single unit.

Understanding the Module Block

In Terraform, you define a module block to incorporate the contents of another module into your configuration. This allows you to reuse existing configurations and standardize resource provisioning across your infrastructure.

Syntax of a Module Block

A typical module block has the following structure:

module "<MODULE_NAME>" {
  source  = "<SOURCE>"
  version = "<VERSION>"
  # Additional arguments corresponding to the module's input variables
}



• <MODULE_NAME>: A unique identifier for the module instance within your configuration.
• <SOURCE>: Specifies the location of the module’s source code. This can be a local path, a Git repository, or a Terraform Registry address.
• <VERSION>: Defines the version of the module to use, applicable when sourcing modules from registries.

Example: Using a Module from the Terraform Registry

Suppose you want to deploy an AWS Virtual Private Cloud (VPC) using a community-maintained module from the Terraform Registry. You can define the module block as follows:

module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "3.19.0"

  name = "my-vpc"
  cidr = "10.0.0.0/16"

  azs             = ["us-west-1a", "us-west-1b", "us-west-1c"]
  private_subnets = ["10.0.1.0/24", "10.0.2.0/24", "10.0.3.0/24"]
  public_subnets  = ["10.0.101.0/24", "10.0.102.0/24", "10.0.103.0/24"]

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

In this example:

• source: Specifies the module’s location in the Terraform Registry.
• version: Ensures that Terraform uses version 3.19.0 of the module, maintaining consistency across deployments.
• The subsequent arguments (name, cidr, azs, etc.) correspond to the input variables defined by the module, allowing customization of the VPC’s configuration.

Example: Using a Local Module

You can also create and reference local modules within your project directory. Assume you have a module that sets up an AWS EC2 instance, located in the modules/ec2-instance directory:

module "web_server" {
  source = "./modules/ec2-instance"

  instance_type = "t2.micro"
  ami_id        = "ami-0c55b159cbfafe1f0"
  subnet_id     = "subnet-abc12345"
}



Here:

• source: Points to the relative path of the local module.
• The arguments (instance_type, ami_id, subnet_id) are input variables defined within the ec2-instance module, allowing you to customize the EC2 instance’s properties.

Key Components of a Module

A well-structured Terraform module typically includes the following files:

• main.tf: Contains the primary resource definitions.
• variables.tf: Declares input variables to parameterize the module.
• outputs.tf: Defines output values to expose information about the resources.
• versions.tf: Specifies the required Terraform version and provider constraints.
• README.md: Provides documentation on the module’s purpose and usage.

Best Practices for Using Modules

• Encapsulation: Modules should encapsulate their resources, exposing only necessary inputs and outputs.
• Reusability: Design modules to be reusable across different configurations and environments.
• Versioning: Implement version control for modules to manage changes and ensure stability.
• Documentation: Provide clear documentation within the module directory to explain its purpose and usage.
• Consistency: Use consistent naming conventions and file structures across modules.

By effectively utilizing module blocks, you can create modular, reusable, and maintainable infrastructure configurations, enhancing the scalability and manageability of your Terraform projects.

$ tree minimal-module/
.
├── README.md
├── main.tf
├── variables.tf
├── outputs.tf

resource "azurerm_resource_group" "example" {
  name     = "my-resources66"
  location = "West Europe"
}

module "apache"{
	source              = "./modules/install_apache"
}

module "nginx"{
	source              = "./modules/install_nginx"
	instances = ["${module.web.instance_ids}"]
}

module "network" {
  source              = "Azure/network/azurerm"
  resource_group_name = azurerm_resource_group.example.name
  address_spaces      = ["10.0.0.0/16", "10.2.0.0/16"]
  subnet_prefixes     = ["10.0.1.0/24", "10.0.2.0/24", "10.0.3.0/24"]
  subnet_names        = ["subnet1", "subnet2", "subnet3"]

  subnet_service_endpoints = {
    "subnet1" : ["Microsoft.Sql"], 
    "subnet2" : ["Microsoft.Sql"],
    "subnet3" : ["Microsoft.Sql"]
  }

  tags = {
    environment = "dev"
    costcenter  = "it"
  }

  depends_on = [azurerm_resource_group.example]
}

key parameters available for a Terraform module block

Certainly! Here’s a table summarizing the key parameters available for a Terraform module block:

*Note: Input variables are user-defined parameters that allow customization of the module’s behavior. Each input variable must be defined within the module’s variables.tf file. When calling the module, you provide values for these variables.*

By utilizing these parameters, you can effectively manage and customize the behavior of your Terraform modules, leading to more modular and maintainable infrastructure configurations.

Terraform Module Block Source Arguments Style

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Rajesh Kumar

I’m a DevOps/SRE/DevSecOps/Cloud Expert passionate about sharing knowledge and experiences. I am working at Cotocus. I blog tech insights at DevOps School, travel stories at Holiday Landmark, stock market tips at Stocks Mantra, health and fitness guidance at My Medic Plus, product reviews at I reviewed , and SEO strategies at Wizbrand. 

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