Local Values

In this lesson, you’ll discover how to use local values (locals) in OpenTofu to eliminate duplication, improve readability, and keep your configuration DRY (Don’t Repeat Yourself).

The Problem: Repeated Tag Definitions

Imagine you have two AWS EC2 instances—web and db—that share identical tags. Without locals, your HCL might look like this:

resource "aws_instance" "web" {
  ami           = "ami-06178cf087598769c"
  instance_type = "t2.medium"

  tags = {
    Department = "finance"
    Project    = "cerberus"
  }
}

resource "aws_instance" "db" {
  ami           = "ami-0567cf08759818b"
  instance_type = "m5.large"

  tags = {
    Department = "finance"
    Project    = "cerberus"
  }
}

Step 1: Define a locals Block

Create a locals block to hold your common tag set:

locals {
  common_tags = {
    Department = "finance"
    Project    = "cerberus"
  }
}

Step 2: Reference the Local Value

Use local.common_tags within each resource:

resource "aws_instance" "web" {
  ami           = "ami-06178cf087598769c"
  instance_type = "t2.medium"

  tags = local.common_tags
}

resource "aws_instance" "db" {
  ami           = "ami-0567cf08759818b"
  instance_type = "m5.large"

  tags = local.common_tags
}

Step 3: Preview and Apply

Running tofu plan or tofu apply now shows both instances using the shared tags:

$ tofu apply
An execution plan has been generated and is shown below.
Resource actions are indicated with the following symbols:
  + create

OpenTofu will perform the following actions:

# aws_instance.db will be created
+ resource "aws_instance" "db" {
    + tags = {
        + "Department" = "finance"
        + "Project"    = "cerberus"
    }
}

# aws_instance.web will be created
+ resource "aws_instance" "web" {
    + tags = {
        + "Department" = "finance"
        + "Project"    = "cerberus"
    }
}

Plan: 2 to add, 0 to change, 0 to destroy.

Advanced Example: Combining Variables and Resource Attributes

Locals aren’t limited to static maps—they can also concatenate variables, resource attributes, and more. For instance, to generate a globally unique S3 bucket name:

variable "project" {
  description = "Name of the project"
  type        = string
  default     = "cerberus"
}

resource "random_string" "random_suffix" {
  length  = 6
  special = false
  upper   = false
}

locals {
  bucket_prefix = "${var.project}-${random_string.random_suffix.id}-bucket"
}

resource "aws_s3_bucket" "finance_bucket" {
  acl    = "private"
  bucket = local.bucket_prefix
}

When you run tofu apply, you’ll see:

$ tofu apply --auto-approve
random_string.random_suffix: Creating...
random_string.random_suffix: Creation complete after 0s [id=dhiabk]
aws_s3_bucket.finance_bucket: Creating...
aws_s3_bucket.finance_bucket: Creation complete after 0s [id=cerberus-dhiabk-bucket]
Apply complete! Resources: 2 added, 0 changed, 0 destroyed.

When to Use Local Values

Further Reading and References

• OpenTofu Locals Documentation
• Terraform Best Practices
• Random Provider on Terraform Registry
• AWS Provider on Terraform Registry

That’s it for this lesson on local values! In the next lesson, we’ll explore modules and how they can further modularize your infrastructure code.