Terraform notes

## I. Important references
https://developer.hashicorp.com/terraform/language/
https://developer.hashicorp.com/terraform/cloud-docs
https://registry.terraform.io/providers/hashicorp/aws/latest/docs

### 1. Some points
Terraform HCL is similar to C syntax in some ways
- Comment: #, //, /* ... */
- Block: <type> <label> { ... }
- As I understand: 
    - Block: <primary/terraform type> <secondary/provider type> <label> { ... }
        resource "terraform_data" "replacement" {
          input = var.revision
        }

### 2. Useful tips

#### a. Remove a resource from your Terraform configuration without destroying the real infrastructure object it manages

removed {
  from = aws_instance.example

  lifecycle {
    destroy = false
  }
}

### 2. Example:

#### Resource block
resource "aws_instance" "web" {
  ami           = "ami-a1b2c3d4"
  instance_type = "t2.micro"
}

#### Accessing Resource Attributes
<RESOURCE TYPE>.<NAME>.<ATTRIBUTE>

### Meta-arguments

#### Lifecycle
https://developer.hashicorp.com/terraform/language/meta-arguments/lifecycle

Within Resource block

#### create_before_destroy (bool)

resource "azurerm_resource_group" "example" {
  # ...

  lifecycle {
    create_before_destroy = true
  }
}

#### prevent_destroy (bool)

#### ignore_changes (list of attribute names)

resource "aws_instance" "example" {
  # ...

  lifecycle {
    ignore_changes = [
      # Ignore changes to tags, e.g. because a management agent
      # updates these based on some ruleset managed elsewhere.
      tags,
    ]
  }
}

#### replace_triggered_by (list of resource or attribute references)

Useful when we need constrains between resources 

resource "aws_appautoscaling_target" "ecs_target" {
  # ...
  lifecycle {
    replace_triggered_by = [
      # Replace `aws_appautoscaling_target` each time this instance of
      # the `aws_ecs_service` is replaced.
      aws_ecs_service.svc.id
    ]
  }
}

#### Custom Condition Checks: precondition & postcondition

resource "aws_instance" "example" {
  instance_type = "t2.micro"
  ami           = "ami-abc123"

  lifecycle {
    # The AMI ID must refer to an AMI that contains an operating system
    # for the `x86_64` architecture.
    precondition {
      condition     = data.aws_ami.example.architecture == "x86_64"
      error_message = "The selected AMI must be for the x86_64 architecture."
    }
  }
}

data "aws_ami" "example" {
  id = var.aws_ami_id

  lifecycle {
    # The AMI ID must refer to an existing AMI that has the tag "nomad-server".
    postcondition {
      condition     = self.tags["Component"] == "nomad-server"
      error_message = "tags[\"Component\"] must be \"nomad-server\"."
    }
  }
}