#90DaysOfDevOps Challenge - Day 69 - Meta-Arguments in Terraform

Welcome back to Day 69 of the #90DaysOfDevOps challenge. In this article, we'll dive into the world of meta-arguments in Terraform, specifically focusing on count and for_each. These powerful meta-arguments allow us to manage multiple instances of resources and modules in an efficient and scalable manner.

Meta-Arguments in Terraform

Meta-arguments in Terraform are special configuration options that provide dynamic and flexible behaviour when defining resources or modules. They enable us to create, modify, and manage multiple instances of resources based on various conditions or inputs.

The count Meta-Argument

The count meta-argument allows us to create multiple instances of a resource based on a specified whole number. Each instance gets its own distinct infrastructure object, enabling individual management of each resource.

Let's see an example of how we can use the count meta-argument:

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 4.16"
    }
  }
  required_version = ">= 1.2.0"
}

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

resource "aws_instance" "server" {
  count = 4

  ami           = "ami-08c40ec9ead489470"
  instance_type = "t2.micro"

  tags = {
    Name = "Server ${count.index + 1}"
  }
}

In this example, we are creating four instances of an AWS EC2 instance, and each instance is tagged with a unique name using the count.index variable.

The for_each Meta-Argument

The for_each meta-argument is similar to the count argument, but instead of using a whole number, it accepts a map or set of strings. This is useful when we need to create multiple resources with different values, such as different AMI IDs or different tags for each resource.

Let's explore the for_each meta-argument with an example:

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 4.16"
    }
  }
  required_version = ">= 1.2.0"
}

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

locals {
  ami_ids = toset([
    "ami-0b0dcb5067f052a63",
    "ami-08c40ec9ead489470",
  ])
}

resource "aws_instance" "server" {
  for_each = local.ami_ids

  ami           = each.key
  instance_type = "t2.micro"

  tags = {
    Name = "Server ${each.key}"
  }
}

In this example, we are creating two instances of an AWS EC2 instance, each with a different AMI ID. The local.ami_ids map defines the AMI IDs and the each.key represents the key (AMI ID) of each iteration.

The depends_on Meta-Argument

The depends_on meta-argument allows us to define explicit dependencies between resources. This ensures that a resource is created or modified only after the specified resource(s) have been successfully created.

resource "aws_s3_bucket" "logs" {
  bucket = "my-logs-bucket"
}

resource "aws_cloudfront_distribution" "cdn" {
  # Configuration for CloudFront distribution

  depends_on = [aws_s3_bucket.logs]
}

The provider Meta-Argument

The provider meta-argument allows us to specify a specific provider configuration for a resource or module. This is useful when working with multiple providers, such as different cloud providers, and we want to use a particular provider for a specific resource or module.

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

provider "google" {
  credentials = file("path/to/google_credentials.json")
}

resource "aws_instance" "server" {
  # Configuration for AWS instance
}

resource "google_compute_instance" "gcp_server" {
  # Configuration for Google Cloud instance

  provider = google
}

Task 1: Demonstrating the Use of count and for_each

To demonstrate the use of count and for_each, follow these steps:

Create a new Terraform configuration file and paste the respective code blocks provided in the examples above.

count

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 4.16"
    }
  }
  required_version = ">= 1.2.0"
}

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

resource "aws_instance" "server" {
  count = 4

  ami           = "ami-08c40ec9ead489470"
  instance_type = "t2.micro"

  tags = {
    Name = "Server ${count.index + 1}"
  }
}

1. Run terraform init to initialize the working directory.

2. Run terraform apply to create the infrastructure as specified in the Terraform configuration.

   

3. Observe how multiple instances of AWS EC2 instances are created based on the value of count.

   

for_each

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 4.16"
    }
  }
  required_version = ">= 1.2.0"
}

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

locals {
  ami_ids = toset([
    "ami-0b0dcb5067f052a63",
    "ami-08c40ec9ead489470",
  ])
}

resource "aws_instance" "server" {
  for_each = local.ami_ids

  ami           = each.key
  instance_type = "t2.micro"

  tags = {
    Name = "Server ${each.key}"
  }
}

1. Run terraform init to initialize the working directory.

2. Run terraform apply to create the infrastructure as specified in the Terraform configuration.

   

3. Observe how multiple instances of AWS EC2 instances are created based on the value of for_each.

   

Meta-arguments like count and for_each are essential tools in Terraform for managing multiple instances of resources and modules. They allow us to dynamically create and configure infrastructure based on our needs, making our infrastructure code more flexible and scalable.

Stay tuned for Day 70 of the #90DaysOfDevOps challenge, where we'll explore Terraform Modules!