はじめに

resource "aws_instance" "example" {
  ami           = "ami-020283e959651b381"
  instance_type = "t2.micro"
  subnet_id     = "subnet-xxxxxxx"

  ebs_block_device {
    device_name = "/dev/sdh"
    encrypted   = var.ebs_encryption
  }
}

variable "ebs_encryption" {
  type = bool
  validation {
    condition     = var.ebs_encryption == true
    error_message = "EBS Should Be Encrypted"
  }
  description = "A Boolean of EBS Encryption in the EC2 Instances"
}

╷
│ Error: Invalid value for variable
│
│   on variables.tf line 1:
│    1: variable "ebs_encryption" {
│     ├────────────────
│     │ var.ebs_encryption is false
│
│ EBS Should Be Encrypted
│
│ This was checked by the validation rule at variables.tf:3,3-13.

data "aws_ec2_instance_type" "example" {
  instance_type = "c5.xlarge"
}

resource "aws_instance" "example" {
  ami           = "ami-0f7b55661ecbbe44c"
  instance_type = data.aws_ec2_instance_type.example.instance_type
  subnet_id     = "subnet-xxxxxxxxxx"
  lifecycle {
    precondition {
      condition     = data.aws_ec2_instance_type.example.free_tier_eligible
      error_message = "This instance type is not free in AWS"
    }
  }
}

╷
│ Error: Resource precondition failed
│
│   on main.tf line 18, in resource "aws_instance" "example":
│   18:       condition     = data.aws_ec2_instance_type.example.free_tier_eligible
│     ├────────────────
│     │ data.aws_ec2_instance_type.example.free_tier_eligible is false
│
│ This instance type is not free in AWS

resource "aws_launch_configuration" "example" {
  image_id      = "ami-020283e959651b381"
  instance_type = "t2.micro"
}

resource "aws_autoscaling_group" "example" {
  name                 = "ASG"
  launch_configuration = aws_launch_configuration.example.name
  vpc_zone_identifier  = ["subnet-xxxxxxx"]
  min_size             = 1
  max_size             = 1

  lifecycle {
    postcondition {
      condition     = length(self.availability_zones) > 1
      error_message = "You need to choose more than 1 AZ to ensure high availability"
    }
  }
}

Terraform will perform the following actions:

  # aws_autoscaling_group.example will be created
  + resource "aws_autoscaling_group" "example" {
      + arn                              = (known after apply)
      + availability_zones               = (known after apply)
      + default_cooldown                 = (known after apply)
      + desired_capacity                 = (known after apply)
      + force_delete                     = false
      + force_delete_warm_pool           = false
      + health_check_grace_period        = 300
      + health_check_type                = (known after apply)
      + id                               = (known after apply)
      + ignore_failed_scaling_activities = false
      + launch_configuration             = (known after apply)
      + load_balancers                   = (known after apply)
      + max_size                         = 1
      + metrics_granularity              = "1Minute"
      + min_size                         = 1
      + name                             = "ASG"
      + name_prefix                      = (known after apply)
      + predicted_capacity               = (known after apply)
      + protect_from_scale_in            = false
      + service_linked_role_arn          = (known after apply)
      + target_group_arns                = (known after apply)
      + vpc_zone_identifier              = [
          + "subnet-xxxxxxx",
        ]
      + wait_for_capacity_timeout        = "10m"
      + warm_pool_size                   = (known after apply)
    }

  # aws_launch_configuration.example will be created
  + resource "aws_launch_configuration" "example" {
      + arn                         = (known after apply)
      + associate_public_ip_address = (known after apply)
      + ebs_optimized               = (known after apply)
      + enable_monitoring           = true
      + id                          = (known after apply)
      + image_id                    = "ami-020283e959651b381"
      + instance_type               = "t2.micro"
      + key_name                    = (known after apply)
      + name                        = (known after apply)
      + name_prefix                 = (known after apply)
    }

Plan: 2 to add, 0 to change, 0 to destroy.

╷
│ Error: Resource postcondition failed
│
│   on main.tf line 38, in resource "aws_autoscaling_group" "example":
│   38:       condition     = length(self.availability_zones) > 1
│     ├────────────────
│     │ self.availability_zones is set of string with 1 element
│
│ You need to choose more than 1 AZ to ensure high availability

data "aws_ec2_instance_type" "example" {
  instance_type = "c5.xlarge"
}

resource "aws_instance" "example" {
  ami           = "ami-0f7b55661ecbbe44c"
  instance_type = data.aws_ec2_instance_type.example.instance_type
  subnet_id     = "subnet-xxxxxxx"
  lifecycle {
    postcondition {
      condition     = data.aws_ec2_instance_type.example.free_tier_eligible
      error_message = "This instance type is not free in AWS"
    }
  }
}

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

Terraform planned the following actions, but then encountered a problem:

  # aws_instance.example will be created
  + resource "aws_instance" "example" {
      + ami                                  = "ami-0f7b55661ecbbe44c"
      + 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                        = "c5.xlarge"
      + 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                            = "subnet-xxxxxxx"
      + tags_all                             = (known after apply)
      + 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.
╷
│ Error: Resource postcondition failed
│
│   on main.tf line 11, in resource "aws_instance" "example":
│   11:       condition     = data.aws_ec2_instance_type.example.free_tier_eligible
│     ├────────────────
│     │ data.aws_ec2_instance_type.example.free_tier_eligible is false
│
│ This instance type is not free in AWS