Local Values

Hands-on: Try the Simplify Terraform Configuration with Locals tutorial.

A local value assigns a name to an expression, so you can use the name multiple times within a module instead of repeating the expression.

If you're familiar with traditional programming languages, it can be useful to compare Terraform modules to function definitions:

• Input variables are like function arguments.
• Output values are like function return values.
• Local values are like a function's temporary local variables.

Declaring a Local Value

A set of related local values can be declared together in a single locals block:

locals {
  service_name = "forum"
  owner        = "Community Team"
}

The expressions in local values are not limited to literal constants; they can also reference other values in the module in order to transform or combine them, including variables, resource attributes, or other local values:

locals {
  # Ids for multiple sets of EC2 instances, merged together
  instance_ids = concat(aws_instance.blue.*.id, aws_instance.green.*.id)
}

locals {
  # Common tags to be assigned to all resources
  common_tags = {
    Service = local.service_name
    Owner   = local.owner
  }
}

Ephemeral values

Local values implicitly become ephemeral if you reference an ephemeral value when you assign that local a value. For example, you can create a local that references an ephemeral service_token.

variable "service_name" {
  type    = string
  default = "forum"
}

variable "environment" {
  type    = string
  default = "dev"
}

variable "service_token" {
  type      = string
  ephemeral = true
}

locals {
  service_tag   = "${var.service_name}-${var.environment}"
  session_token = "${var.service_name}:${var.service_token}"
}

The local.session_token value is implicitly ephemeral because it relies on an ephemeral variable.

Using Local Values

Once a local value is declared, you can reference it in expressions as local.<NAME>.

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

  tags = local.common_tags
}

A local value can only be accessed in expressions within the module where it was declared.

When To Use Local Values

Local values can be helpful to avoid repeating the same values or expressions multiple times in a configuration, but if overused they can also make a configuration hard to read by future maintainers by hiding the actual values used.

Use local values only in moderation, in situations where a single value or result is used in many places and that value is likely to be changed in future. The ability to easily change the value in a central place is the key advantage of local values.