09-03-Terraform-Variables-list

Terraform Variables

Implement variable type as list

In Terraform, you can use complex type constructors like list and map to define variables.

• list (or tuple): a sequence of values, like ["t2.micro", "t2.small"].
• Elements in a list or tuple are identified by consecutive whole numbers, starting with zero. [0], [1]

Let's consider the ec2_instance_type variable as an example and Implement List function

Previous Definition:

variable "ec2_instance_type" {
    description = "EC2 Instance Type"
    type        = string
    default     = "t2.micro"
}

To utilize a list as a complex type constructor for instance_type, you can modify it like this:

list type

variable "ec2_instance_type" {
    description = "EC2 Instance Types"
    type        = list(string)
    default     = ["t2.micro", "t2.small", "t2.large"]
}

Here's what changed:

• Type Declaration (type): It is now specified as list(string), indicating that the variable is expected to be a list of strings.

• Default Value (default): The default value is now specified as ["t2.micro", "t2.small, "t2.large"], indicating that by default, it's a list containing a three string element, which is element[0]="t2.micro", element[1]="t2.small, and element[2]="t2.large".

• In order to use this list type variable in 01_ec2.tf you will have to call it using below syntax

  instance_type = var.ec2_instance_type[0] # for t2.micro
  instance_type = var.ec2_instance_type[1] # for t2.small
  instance_type = var.ec2_instance_type[2] # for t2.large

• Example:

  00_provider.tf

  terraform {
  required_providers {
      aws = {
      source  = "hashicorp/aws"
      version = "~> 5.0"
      }
  }
  }
  
  provider "aws" {
  #region = "us-east-1"
  region = var.aws_region
  
  default_tags {
      tags = {
      Terraform = "yes"
      #Owner = "Venkatesh"
      Owner = var.owner
      }
  }
  }

  01_ec2.tf

  resource "aws_instance" "myec2" {
  # terraform arguments without variables
  # ami = "ami-0df435f331839b2d6"
  # instance_type = "t2.micro"
  # count = 1
  
  # using variables for arguments
  ami           = var.ec2_ami
  instance_type = var.ec2_instance_type[1]
  count         = var.instance_count
  
  tags = {
      Name = "Linux2023"
      env  = var.env
  }
  }

  02_variables.tf

  variable "aws_region" {
  description = "AWS Region In Which Resources will be Created"
  type        = string
  default     = "us-east-1"
  }
  
  variable "owner" {
  description = "Name of the Engineer who is creating Resources"
  type        = string
  default     = "Venkatesh"
  }
  
  variable "ec2_ami" {
  description = "AWS EC2 AMI Amazon Linux 2023"
  type        = string
  default     = "ami-0df435f331839b2d6" # Amazon Linux 2023
  }
  
  variable "ec2_instance_type" {
  description = "EC2 Instance Type"
  type        = list(string)
  default     = ["t2.micro", "t2.small", "t2.large"]
  }
  
  variable "instance_count" {
  description = "Number of EC2"
  type        = number
  default     = 1
  }
  
  variable "env" {
  description = "Environment Type"
  type        = string
  default     = "dev"
  }

• In the above example, We've defined six variables:

  1. aws_region: default AWS Region to be used
  2. owner: Name of the Engineer who is creating Resources
  3. ec2_ami: AWS EC2 AMI
  4. ec2_instance_type: List of AWS EC2 Instance type
  5. instance_count: Number of EC2 to be Created
  6. env : Environment Type (PRE, PRD, DEV, UAT)

• terraform plan Output for var.ec2_instance_type[1] (for t2.small):

  • You can notice the change in instance_type to t2.small when var.ec2_instance_type[1] is used.

    $ terraform plan
    
    Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
    + create
    
    Terraform will perform the following actions:
    
    # aws_instance.myec2[0] will be created
    + resource "aws_instance" "myec2" {
        + ami                                  = "ami-0df435f331839b2d6"
        + arn                                  = (known after apply)
        + associate_public_ip_address          = (known after apply)
        + availability_zone                    = (known after apply)
        + cpu_core_count                       = (known after apply)
        + cpu_threads_per_core                 = (known after apply)
        + disable_api_stop                     = (known after apply)
        + disable_api_termination              = (known after apply)
        + ebs_optimized                        = (known after apply)
        + get_password_data                    = false
        + host_id                              = (known after apply)
        + host_resource_group_arn              = (known after apply)
        + iam_instance_profile                 = (known after apply)
        + id                                   = (known after apply)
        + instance_initiated_shutdown_behavior = (known after apply)
        + instance_lifecycle                   = (known after apply)
        + instance_state                       = (known after apply)
        + instance_type                        = "t2.small"
        + ipv6_address_count                   = (known after apply)
        + ipv6_addresses                       = (known after apply)
        + key_name                             = (known after apply)
        + monitoring                           = (known after apply)
        + outpost_arn                          = (known after apply)
        + password_data                        = (known after apply)
        + placement_group                      = (known after apply)
        + placement_partition_number           = (known after apply)
        + primary_network_interface_id         = (known after apply)
        + private_dns                          = (known after apply)
        + private_ip                           = (known after apply)
        + public_dns                           = (known after apply)
        + public_ip                            = (known after apply)
        + secondary_private_ips                = (known after apply)
        + security_groups                      = (known after apply)
        + source_dest_check                    = true
        + spot_instance_request_id             = (known after apply)
        + subnet_id                            = (known after apply)
        + tags                                 = {
            + "Name" = "Linux2023"
            + "env"  = "dev"
            }
        + tags_all                             = {
            + "Name"      = "Linux2023"
            + "Owner"     = "Venkatesh"
            + "Terraform" = "yes"
            + "env"       = "dev"
            }
        + tenancy                              = (known after apply)
        + user_data                            = (known after apply)
        + user_data_base64                     = (known after apply)
        + user_data_replace_on_change          = false
        + vpc_security_group_ids               = (known after apply)
        }
    
    Plan: 1 to add, 0 to change, 0 to destroy.

• Similarly if you pass var.ec2_instance_type[0] or var.ec2_instance_type[2], the instance_type will be t2.micro and t2.large respectively

References :

Types and Values

Type Constraints list