The locals block — named expressions for reuse

Crisp answer: Locals are named expressions computed within a module. They avoid repetition and make complex expressions readable by giving them a name.

Declaring locals:

locals {
  # Simple value
  region = "eu-west-2"

  # Computed from a variable
  name_prefix = "${var.project}-${var.environment}"

  # Conditional
  is_production = var.environment == "prod"

  # Reusable tags applied to all resources
  common_tags = {
    Project     = var.project
    Environment = var.environment
    ManagedBy   = "terraform"
    Team        = "platform"
  }

  # Complex expression you don't want repeated
  private_subnet_cidrs = [
    for i, az in var.availability_zones :
    cidrsubnet(var.vpc_cidr, 8, i)
  ]
}

Using locals:

resource "aws_vpc" "main" {
  cidr_block = var.vpc_cidr
  tags       = merge(local.common_tags, { Name = "${local.name_prefix}-vpc" })
}

resource "aws_subnet" "private" {
  count             = length(var.availability_zones)
  vpc_id            = aws_vpc.main.id
  cidr_block        = local.private_subnet_cidrs[count.index]
  availability_zone = var.availability_zones[count.index]
  tags              = merge(local.common_tags, { Name = "${local.name_prefix}-private-${count.index}" })
}

resource "aws_rds_cluster" "main" {
  cluster_identifier  = "${local.name_prefix}-db"
  deletion_protection = local.is_production
  tags                = local.common_tags
}

locals vs variables:

What to say in the interview:

"Locals are named expressions local to the module. I use them for two things: a common_tags map that gets merged into every resource so all infrastructure has consistent tags, and for complex expressions I would otherwise repeat across multiple resource blocks. The key distinction from variables: variables are inputs set by the caller, locals are computed internally. You cannot set a local from the CLI or a tfvars file."