Local Values

Locals = ตัวแปรท้องถิ่นใน Terraform ที่ใช้ลด duplication และทำให้ code อ่านง่ายขึ้น — ไม่ใช่ input จากภายนอก

Locals Block

locals {
  name_prefix = "${var.environment}-${var.project}"

  common_tags = {
    Environment = var.environment
    Project     = var.project
    ManagedBy   = "terraform"
  }
}

ใช้ผ่าน local.<NAME>:

resource "aws_s3_bucket" "data" {
  bucket = "${local.name_prefix}-data"
  tags   = local.common_tags
}

resource "aws_s3_bucket" "logs" {
  bucket = "${local.name_prefix}-logs"
  tags   = local.common_tags
}

Locals vs Variables

Aspect	variable	locals
Source	external (CLI, .tfvars, env)	internal (computed)
Type	typed	inferred
Override	ได้	ไม่ได้
เปลี่ยนตอน apply	ผ่าน input	recompute เอง
ใช้ทำอะไร	parameterize	DRY + readable

Mental Model

• variable = "input parameter" ของ module/project
• locals = "private helper" คำนวณค่าใน module

Use Cases

1. Lessen Duplication

❌ ไม่ใช้ locals:

resource "aws_s3_bucket" "data" {
  bucket = "${var.env}-${var.project}-data"
  tags = {
    Environment = var.env
    Project     = var.project
    ManagedBy   = "terraform"
  }
}

resource "aws_s3_bucket" "logs" {
  bucket = "${var.env}-${var.project}-logs"
  tags = {
    Environment = var.env
    Project     = var.project
    ManagedBy   = "terraform"
  }
}

✅ ใช้ locals:

locals {
  name_prefix = "${var.env}-${var.project}"
  common_tags = {
    Environment = var.env
    Project     = var.project
    ManagedBy   = "terraform"
  }
}

resource "aws_s3_bucket" "data" {
  bucket = "${local.name_prefix}-data"
  tags   = local.common_tags
}

resource "aws_s3_bucket" "logs" {
  bucket = "${local.name_prefix}-logs"
  tags   = local.common_tags
}

2. Computed Values

variable "vpc_cidr" {
  type    = string
  default = "10.0.0.0/16"
}

locals {
  # คำนวณ subnet CIDR จาก VPC CIDR
  public_subnets  = [for i in range(2) : cidrsubnet(var.vpc_cidr, 8, i)]
  private_subnets = [for i in range(2) : cidrsubnet(var.vpc_cidr, 8, i + 100)]
}

resource "aws_subnet" "public" {
  count      = length(local.public_subnets)
  vpc_id     = aws_vpc.main.id
  cidr_block = local.public_subnets[count.index]
}

3. Conditional Logic

locals {
  is_prod = var.environment == "prod"

  instance_type = local.is_prod ? "t3.large" : "t3.micro"
  backup_count  = local.is_prod ? 30 : 7
  multi_az      = local.is_prod
}

resource "aws_db_instance" "main" {
  instance_class          = local.instance_type
  backup_retention_period = local.backup_count
  multi_az                = local.multi_az
}

4. Transform Data

variable "users" {
  type = list(object({
    name  = string
    email = string
    admin = bool
  }))
}

locals {
  # Convert list → map
  user_map = { for u in var.users : u.name => u }

  # Filter
  admins = [for u in var.users : u if u.admin]

  # Map (transform)
  admin_emails = [for u in local.admins : u.email]
}

resource "aws_iam_user" "users" {
  for_each = local.user_map
  name     = each.value.name
}

Multiple Locals Blocks

มีหลาย locals block ในไฟล์เดียวก็ได้ — Terraform merge ให้:

locals {
  region = "ap-southeast-1"
}

locals {
  full_name = "${local.region}-app"
}

→ access local.region และ local.full_name

ใช้แบ่งกลุ่ม locals ตาม theme เช่น:

locals {
  # Naming
  name_prefix = "${var.env}-${var.project}"
  bucket_name = "${local.name_prefix}-data"
}

locals {
  # Networking
  vpc_cidr        = "10.0.0.0/16"
  public_subnets  = [for i in range(2) : cidrsubnet(local.vpc_cidr, 8, i)]
}

locals {
  # Tags
  common_tags = {
    Environment = var.env
    ManagedBy   = "terraform"
  }
}

ตัวอย่าง Real-World

locals.tf

locals {
  # Project metadata
  project_name = "my-app"
  environment  = var.environment
  region       = data.aws_region.current.name
  account_id   = data.aws_caller_identity.current.account_id

  # Naming
  name_prefix  = "${local.project_name}-${local.environment}"
  bucket_name  = "${local.name_prefix}-${local.account_id}-${local.region}"

  # Tags (merge default with user tags)
  common_tags = merge(
    {
      Project     = local.project_name
      Environment = local.environment
      ManagedBy   = "terraform"
      Region      = local.region
    },
    var.extra_tags
  )

  # Network (computed from VPC CIDR)
  vpc_cidr        = var.vpc_cidr
  public_subnets  = [for i in range(var.az_count) : cidrsubnet(local.vpc_cidr, 8, i)]
  private_subnets = [for i in range(var.az_count) : cidrsubnet(local.vpc_cidr, 8, i + 100)]

  # Conditional
  is_prod                = local.environment == "prod"
  enable_deletion_protection = local.is_prod
  backup_retention       = local.is_prod ? 30 : 7
}

data "aws_region" "current" {}
data "aws_caller_identity" "current" {}

Anti-Patterns

# ❌ Locals ที่อ้างอิง resource attribute (slow, fragile)
locals {
  vpc_id = aws_vpc.main.id    # อันที่อ้าง .id ตรงๆ ดีกว่า
}

# ❌ Locals ที่คัดลอกค่ามาจาก variable
locals {
  region = var.region    # ใช้ var.region ตรงๆ ดีกว่า
}

# ❌ Locals ใหญ่เกินไป (อ่านยาก)
locals {
  config = {
    # 50+ keys ที่ลึกซ้อน
  }
}
# ดีกว่าแยกเป็น object variable

สรุป

• locals = private helper variable
• ใช้ลด duplication + ทำ computed values + conditional logic
• ไม่รับ input จากภายนอก (ต่างจาก variable)
• มีหลาย locals block ในไฟล์เดียวได้
• หลีกเลี่ยง locals ที่ "ก็คือ variable เหมือนกัน" — ใช้ var.<x> ตรงๆ ดีกว่า

ต่อไป → Section 6: Outputs