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Terraform Variables

Custom validation rules for Terraform input variables

  • Custom validation rules are a way to enforce specific conditions or constraints on the values of your resource attributes.

  • These rules allow you to define checks that must be satisfied before Terraform accepts the configuration.

  • Validation Rules: These are conditions that you set for your resource attributes. They can be as straightforward or as complex as your requirements.

  • Custom Validation: Terraform allows you to create your own validation rules using the validation block within a resource definition.

  • Use cases :

    • Ensuring required parameters are set.
    • Checking that values meet specific criteria.
    • Verifying that certain conditions are met before resource creation.

  • Benefits :

    • Adds an extra layer of control over your infrastructure.
    • Helps catch potential misconfigurations early in the deployment process.
    • Provides meaningful error messages for easier troubleshooting.

  • Example:

    00_provider.tf

    terraform {
required_providers {
    aws = {
    source  = "hashicorp/aws"
    version = "~> 5.0"
    }
}
}

provider "aws" {
region = var.aws_region

default_tags {
    tags = {
    Terraform = "yes"
    Owner = var.owner
    }
}
}

    01_ec2.tf

    resource "aws_instance" "myec2" {
ami = var.ec2_ami
instance_type = var.ec2_instance_type

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
validation {
    condition = length(var.ec2_ami) > 4 && substr(var.ec2_ami, 0,4) == "ami-"
    error_message = "The EC2 AMI ID value must be a valid AMI id, starting with \"ami-\"."
}
}

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

variable "env" {
description = "Environment Type"
type        = string
default     = "dev"
}

  • In the above example, variable ec2_ami: AWS EC2 AMI variable we define custom validation rules that signifies the ami id should be id of length > 4 and always start with ami-

To test the custom validation rules lets change the ami id to start differently and see what terraform reports

    variable "ec2_ami" {
    description = "AWS EC2 AMI Amazon Linux 2023"
    type        = string
    #default     = "ami-0df435f331839b2d6" # Amazon Linux 2023
    default     = "xyz-0df435f331839b2d6"
    validation {
        condition = length(var.ec2_ami) > 4 && substr(var.ec2_ami, 0,4) == "ami-"
        error_message = "The EC2 AMI ID value must be a valid AMI id, starting with \"ami-\"."
    }
    }

  • terraform plan Output

    • You can notice the validation rules in effect and re error_message being displayed to make correction
    $ terraform plan

Planning failed. Terraform encountered an error while generating this plan.

╷
│ Error: Invalid value for variable
│
│   on 02_variables.tf line 13:
│   13: variable "ec2_ami" {
│     ├────────────────
│     │ var.ec2_ami is "xyz-0df435f331839b2d6"
│
│ The EC2 AMI ID value must be a valid AMI id, starting with "ami-".
│
│ This was checked by the validation rule at 02_variables.tf:18,3-13.

References :

Terraform Custom Conditions