What is Local Value? | DevOps Dictionary

What Is a Terraform Local Value?

A local value is scratch paper. You write a calculation once — like "${var.project}-${var.environment}" — give it a name (name_prefix), and reference that name everywhere you need the result instead of repeating the calculation.

Locals are the DRY (Don't Repeat Yourself) principle for Terraform expressions. If you find yourself writing the same expression in three resource blocks, that expression belongs in a local.

At Swiggy, the infrastructure team uses a single common_tags local that applies consistently to every resource. When the cost accounting team needs a new tag, one line changes in the locals block — not sixty lines across sixty resources.

◈ DIAGRAM

+------------------------------------------+
| Without locals — expression repeated:    |
| bucket = "swiggy-${var.env}-data"        |  <- in S3 resource
| name   = "swiggy-${var.env}-app"         |  <- in EC2 resource
| id     = "swiggy-${var.env}-rds"         |  <- in RDS resource
| 3 places to update when naming changes   |
+------------------------------------------+
                    |
                    v
+------------------------------------------+
| With locals — computed once:             |
| locals {                                 |
|   prefix = "swiggy-${var.environment}"   |
| }                                        |
| bucket = "${local.prefix}-data"          |
| name   = "${local.prefix}-app"           |
| id     = "${local.prefix}-rds"           |
| 1 place to update                        |
+------------------------------------------+

Locals Block Syntax

NSIS

# locals.tf — or add locals block anywhere in .tf files
# All locals in one block are evaluated together
 
locals {
  # ── String construction ────────────────────────────────────────────────
  name_prefix = "${var.project}-${var.environment}"   # swiggy-payments-prod
 
  # ── Tagging ───────────────────────────────────────────────────────────
  # Define once, use on every resource
  common_tags = {
    Project     = var.project
    Environment = var.environment
    ManagedBy   = "terraform"
    Repository  = "github.com/swiggy/infrastructure"
    CostCenter  = "platform-${var.environment}"
  }
 
  # ── Conditional logic ──────────────────────────────────────────────────
  is_production   = var.environment == "prod"
  instance_type   = local.is_production ? "t3.large" : "t3.micro"
  backup_retention = local.is_production ? 30 : 3
  multi_az        = local.is_production
 
  # ── Map lookups ────────────────────────────────────────────────────────
  # Look up instance type for the current environment
  ec2_size = lookup(var.instance_types_map, var.environment, "t3.micro")
 
  # ── Computed names ──────────────────────────────────────────────────────
  # Locals CAN reference other locals in the same block
  s3_bucket_name = "${local.name_prefix}-data-${data.aws_caller_identity.current.account_id}"
  rds_id         = "${local.name_prefix}-postgres"
  sg_name        = "${local.name_prefix}-app-sg"
  log_group      = "/aws/${data.aws_region.current.name}/${local.name_prefix}/app"
 
  # ── List manipulation ──────────────────────────────────────────────────
  az_count = length(data.aws_availability_zones.available.names)
  azs      = slice(data.aws_availability_zones.available.names, 0, 2)
 
  # Flatten list of lists into one list
  all_private_cidrs = flatten([var.private_subnet_cidrs, var.db_subnet_cidrs])
 
  # ── Map operations ──────────────────────────────────────────────────────
  # Merge common tags with resource-specific tags
  rds_tags = merge(local.common_tags, {
    Name      = local.rds_id
    Component = "database"
    Engine    = "postgres"
  })
}

Using Locals in Resources

APPLESCRIPT

# main.tf — locals keep resource blocks clean and DRY
 
resource "aws_s3_bucket" "data" {
  bucket = local.s3_bucket_name   # computed once in locals
  tags   = local.common_tags      # map from locals — not repeated here
}
 
resource "aws_instance" "app" {
  ami           = data.aws_ami.amazon_linux.id
  instance_type = local.instance_type   # conditional — large in prod, micro in dev
 
  tags = merge(local.common_tags, {
    Name = "${local.name_prefix}-app"
  })
}
 
resource "aws_db_instance" "main" {
  identifier      = local.rds_id
  multi_az        = local.multi_az         # true in prod, false in dev
  backup_retention_period = local.backup_retention
  tags            = local.rds_tags
}

Locals vs Variables — When to Use Which

◈ DIAGRAM

+------------------------------------------+
| Use VARIABLES when:                      |
| - The caller needs to set the value      |
| - The value differs per environment      |
| - Example: instance_type, environment,   |
|   db_password, project_name              |
+------------------------------------------+
 
+------------------------------------------+
| Use LOCALS when:                         |
| - Value is computed from other values    |
| - Not settable by the caller             |
| - Used in more than one place            |
| - Example: name_prefix, common_tags,     |
|   is_production, s3_bucket_name          |
+------------------------------------------+

Advanced Locals Patterns

GAMS

locals {
  # For expressions inside locals
  upper_envs = [for env in ["dev", "staging", "prod"] : upper(env)]
 
  # Convert list to set for for_each
  environment_set = toset(["dev", "staging", "prod"])
 
  # Complex conditional
  log_level = (
    var.environment == "prod" ? "warn" :
    var.environment == "staging" ? "info" :
    "debug"   # dev
  )
 
  # Build IAM policy ARNs dynamically
  ssm_policy_arn = "arn:aws:iam::aws:policy/AmazonSSMManagedInstanceCore"
 
  # CIDR math
  public_subnets = [
    for i in range(2) : cidrsubnet(var.vpc_cidr, 8, i + 1)
  ]
  # Result: ["10.0.1.0/24", "10.0.2.0/24"] when vpc_cidr = "10.0.0.0/16"
}

Troubleshooting Locals

Error	Root Cause	Fix
`Error: Cycle: local.x -> local.y -> local.x`	Two locals referencing each other in a loop	Restructure — one must not depend on the other
`Error: local.x is not defined`	Typo in local name or missing declaration	Check spelling in locals block
`local.x` returns wrong type	Expression result type mismatch	Use `tostring()`, `tolist()`, or `toset()` for type coercion
Large locals block is hard to read	All locals in one block	Split into multiple locals blocks — they can appear anywhere in .tf files

PLACEMENT PRO TIP
**Tip:** Add a comment above each local explaining why it exists — not what it computes. `# common tags applied to every resource` is more useful than `# tags map` as a comment on the `common_tags` local.

REMEMBER THIS
**Remember:** Locals cannot be set by callers. If a value needs to be configurable by the person running Terraform (or a CI/CD pipeline), it must be a variable. If it is computed internally from other values, it is a local.