Implicit Stacks

import FileTree from '@components/vendored/starlight/FileTree.astro';

The simplest way to create a stack is to organize your units in a directory structure in your repository. When you have multiple units in a directory, Terragrunt automatically treats that directory as a stack for the purposes of commands like terragrunt run --all apply.

Converting Terraform Modules to Units

Let's say your infrastructure is defined across multiple OpenTofu/Terraform root modules:

• root
  • backend-app
    • main.tf
  • frontend-app
    • main.tf
  • mysql
    • main.tf
  • valkey
    • main.tf
  • vpc
    • main.tf

To convert these to Terragrunt units, simply add a terragrunt.hcl file to each directory:

• root
  • backend-app
    • main.tf
    • terragrunt.hcl
  • frontend-app
    • main.tf
    • terragrunt.hcl
  • mysql
    • main.tf
    • terragrunt.hcl
  • valkey
    • main.tf
    • terragrunt.hcl
  • vpc
    • main.tf
    • terragrunt.hcl

Now you have an implicit stack! The root directory contains all your units and can be managed as a single stack.

Working with Implicit Stacks

Use the --all flag to run an OpenTofu/Terraform command on all units in the implicit stack in the current working directory:

# Deploy all units discovered in the current working directory
terragrunt run --all apply

# Plan changes across all units discovered in the current working directory
terragrunt run --all plan

# Destroy all units discovered in the current working directory
terragrunt run --all destroy

# View outputs from all units discovered in the current working directory
terragrunt run --all output

You can also use the --graph flag to run an OpenTofu/Terraform command on all units in the DAG of the unit in the current working directory.

# Run an OpenTofu/Terraform command on all units in the DAG of the unit in the current working directory
terragrunt run --graph apply

Advantages of Implicit Stacks

• Simple: Just organize units in directory trees.
• Familiar: Organized following best practices for OpenTofu/Terraform repository structures.
• Flexible: Easy to add/remove units by creating/deleting directories.
• Version Control Friendly: Each unit is a separate directory with its own history.
• Backwards Compatible: This has been the default way to work with Terragrunt for over eight years, and the majority of existing Terragrunt configurations use this approach.

Limitations of Implicit Stacks

• Manual Management: Each unit must be manually created and configured.
• No Reusability: Patterns can't be easily shared across environments.
• Repetitive: Similar configurations must be duplicated or referenced from includes.