terraform/website/docs/configuration/locals.html.md

layout	page_title	sidebar_current	description
docs	Configuring Local Values	docs-config-locals	Local values assign a name to an expression that can then be used multiple times within a module.

Local Values

A local value assigns a name to an expression, allowing it to be used multiple times within a module without repeating it.

Comparing modules to functions in a traditional programming language: if input variables are analogous to function arguments and outputs values are analogous to function return values, then local values are comparable to a function's local temporary symbols.

-> Note: For brevity, local values are often referred to as just "locals" when the meaning is clear from context.

Declaring a Local Value

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

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

The expressions assigned to local value names can either be simple constants like the above, allowing these values to be defined only once but used many times, or they can be more complex expressions that transform or combine values from elsewhere in the module:

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
  }
}

As shown above, local values can be referenced from elsewhere in the module with an expression like local.common_tags, and locals can reference each other in order to build more complex values from simpler ones.

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

  tags = local.common_tags
}

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.