What is a Resource? #

# resource — Terraform CREATES and MANAGES this
# Terraform is responsible for its lifecycle: create, update, destroy
resource "aws_vpc" "main" {
  cidr_block = "10.0.0.0/16"

  tags = {
    Name = "main-vpc"
  }
}

# data — Terraform READS this, doesn't create it
# The resource already exists outside Terraform, you just want to reference it
data "aws_ami" "ubuntu" {
  most_recent = true
  owners      = ["099720109477"]  # Canonical

  filter {
    name   = "name"
    values = ["ubuntu/images/hvm-ssd/ubuntu-*-22.04-amd64-server-*"]
  }
}

# Using both together
resource "aws_instance" "web" {
  ami           = data.aws_ami.ubuntu.id  # AMI from the data source
  instance_type = "t3.micro"
  vpc_id        = aws_vpc.main.id         # VPC from the resource
}

# Complete resource block structure with all its parts

resource "<PROVIDER>_<TYPE>" "<NAME>" {
  # ─────────────────────────────────────────
  # ARGUMENTS — configuration specific to this resource
  # ─────────────────────────────────────────
  ami           = "ami-0abcdef1234567890"
  instance_type = "t3.micro"

  # Nested blocks
  tags = {
    Name        = "web-server"
    Environment = var.environment
  }

  # ─────────────────────────────────────────
  # META-ARGUMENTS — apply to all resources
  # ─────────────────────────────────────────

  # Explicit dependency
  depends_on = [aws_iam_role_policy_attachment.node]

  # Create multiple instances
  count = 3
  # or: for_each = var.instance_map

  # Use a non-default provider
  provider = aws.us_east

  # Customize the lifecycle
  lifecycle {
    create_before_destroy = true
    prevent_destroy       = false
    ignore_changes        = [tags["LastModified"]]
  }
}

# View all resources in state
terraform state list

# Output:
# aws_instance.web
# aws_instance.workers[0]
# aws_instance.workers[1]
# aws_instance.workers[2]
# module.vpc.aws_vpc.main
# module.vpc.aws_subnet.public["ap-southeast-1a"]

# View details of one resource in state
terraform state show aws_instance.web

# Output:
# # aws_instance.web:
# resource "aws_instance" "web" {
#     ami                    = "ami-0abcdef1234567890"
#     id                     = "i-0abcdef1234567890"
#     instance_type          = "t3.micro"
#     private_ip             = "10.0.1.42"
#     public_ip              = "54.123.45.67"
#     ... (all attributes, including the ones AWS generated)
# }

# Reference format: <TYPE>.<NAME>.<ATTRIBUTE>

resource "aws_vpc" "main" {
  cidr_block = "10.0.0.0/16"
}

resource "aws_subnet" "public" {
  vpc_id = aws_vpc.main.id  # Reference to the "id" attribute of aws_vpc.main
}

# For resources with count:
resource "aws_subnet" "private" {
  count  = 3
  vpc_id = aws_vpc.main.id
  cidr_block = "10.0.${count.index + 10}.0/24"
}

resource "aws_route_table_association" "private" {
  count          = 3
  subnet_id      = aws_subnet.private[count.index].id  # Access per index
  route_table_id = aws_route_table.private.id
}

# All subnets at once using a splat expression:
output "private_subnet_ids" {
  value = aws_subnet.private[*].id  # ["subnet-001", "subnet-002", "subnet-003"]
}

# RECOMMENDED PATTERNS:

resource "aws_instance" "web" {
  # 1. Required arguments (mandatory) at the top
  ami           = var.ami_id
  instance_type = var.instance_type

  # 2. Important optional arguments
  subnet_id              = aws_subnet.public.id
  vpc_security_group_ids = [aws_security_group.web.id]
  iam_instance_profile   = aws_iam_instance_profile.web.name

  # 3. Nested blocks
  root_block_device {
    volume_size = 20
    volume_type = "gp3"
    encrypted   = true
  }

  # 4. Tags always at the end
  tags = merge(
    var.common_tags,
    {
      Name = "web-server"
      Role = "web"
    }
  )

  # 5. Lifecycle at the very end (if any)
  lifecycle {
    create_before_destroy = true
  }
}

# Format: module.module_name.resource_type.resource_name[index]

# Root level resource
aws_instance.web

# Resource in a module
module.networking.aws_vpc.main

# Resource in a count
aws_instance.web[0]
aws_instance.web[1]

# Resource in a for_each
aws_instance.web["app-server"]
aws_instance.web["db-server"]

# Nested module
module.networking.module.subnets.aws_subnet.private[0]

# Resource reference for dependencies
resource "aws_instance" "app" {
  subnet_id = module.networking.private_subnet_ids[0]
  # Implicit dependency on module.networking
}

# Every resource has metadata you can access

resource "aws_instance" "web" {
  ami           = "ami-12345"
  instance_type = "t3.micro"
  
  tags = {
    Name = "web-server"
  }
}

# Available metadata:
# aws_instance.web.id           → Instance ID
# aws_instance.web.arn          → ARN
# aws_instance.web.private_ip   → Private IP
# aws_instance.web.public_ip    → Public IP (if any)
# aws_instance.web.availability_zone → AZ
# aws_instance.web.instance_state → State (running, stopped)

# View all attributes of a resource
terraform state show aws_instance.web
# Shows ALL attributes stored in state

RESOURCE STATES:

1. pending_create: Will be created on apply
2. creating: Currently being created
3. created: Already created (active)
4. pending_update: Will be updated on apply
5. updating: Currently being updated
6. pending_destroy: Will be deleted on apply
7. destroying: Currently being deleted
8. destroyed: Already deleted

TERRAFORM PLAN ICONS:
+  Create (new resource)
~  Update (changed resource)
-  Destroy (deleted resource)
-/+ Replace (destroy + create)
<= Read (data source)

# View a resource's state
terraform state show aws_instance.web

# List all resources in state
terraform state list

# List with a filter
terraform state list | grep aws_instance