---
title: Explicit Stacks
description: Define stacks using terragrunt.stack.hcl files for reusable patterns.
slug: features/stacks/explicit
sidebar:
  order: 3
---

import FileTree from '@components/vendored/starlight/FileTree.astro';
import { Aside } from '@astrojs/starlight/components';
import Since from '@components/Since.astro';
import Before from '@components/Before.astro';

For an alternate approach (that is more flexible, but not necessarily always the better solution), you can define explicit stacks using `terragrunt.stack.hcl` files. These are **blueprints** that programmatically generate units at runtime.

### What is a terragrunt.stack.hcl file?

A `terragrunt.stack.hcl` file is a blueprint that defines how to generate Terragrunt configurations programmatically. It tells Terragrunt:

- What units to create.
- Where to get their configurations from.
- Where to place them in the directory structure.
- What values to pass to each unit.

<Aside type="caution">
It is invalid for a unit to contain a `terragrunt.stack.hcl` file and for a stack to contain a `terragrunt.hcl` file. If either is true, Terragrunt will throw an error.

This is to prevent ambiguity in deciding whether a component is a unit or a stack.
</Aside>

### Supported Configuration Blocks

#### `unit` blocks - Define Individual Infrastructure Components

- **Purpose**: Define a single, deployable piece of infrastructure.
- **Use case**: When you want to create a single piece of isolated infrastructure (e.g. a specific VPC, database, or application).
- **Result**: Generates a directory with a single `terragrunt.hcl` file in the specified `path` from the specified `source`.

#### `stack` blocks - Define Reusable Infrastructure Patterns

- **Purpose**: Define a stack of units to be deployed together.
- **Use case**: When you have a common, multi-unit pattern (like "dev environment" or "three-tier web application") that you want to deploy multiple times.
- **Result**: Generates a directory with another `terragrunt.stack.hcl` file in the specified `path` from the specified `source`.

### Example: Simple Stack with Units

```hcl
# terragrunt.stack.hcl

unit "vpc" {
  source = "git::git@github.com:acme/infrastructure-catalog.git//units/vpc?ref=v0.0.1"
  path   = "vpc"
  values = {
    vpc_name = "main"
    cidr     = "10.0.0.0/16"
  }
}

unit "database" {
  source = "git::git@github.com:acme/infrastructure-catalog.git//units/database?ref=v0.0.1"
  path   = "database"
  values = {
    engine   = "postgres"
    version  = "13"
    vpc_path = "../vpc"
  }
}
```

Running `terragrunt stack generate` in the directory containing that `terragrunt.stack.hcl` file generates:

<FileTree>

- .terragrunt-stack
  - vpc
    - terragrunt.hcl
    - terragrunt.values.hcl
  - database
    - terragrunt.hcl
    - terragrunt.values.hcl

</FileTree>

<Aside type="note">
Note that the contents are generated into a `.terragrunt-stack` directory. This is to make it convenient to add `.terragrunt-stack` to your `.gitignore` file, and always generate the stack on demand instead of checking it in.

Also note the `terragrunt.values.hcl` files generated next to the `terragrunt.hcl` files of the units. These files contain the values specified in the `values` block for the unit.
</Aside>

### Example: Nested Stack with Reusable Patterns

```hcl
# terragrunt.stack.hcl

stack "dev" {
  source = "git::git@github.com:acme/infrastructure-catalog.git//stacks/environment?ref=v0.0.1"
  path   = "dev"
  values = {
    environment = "development"
    cidr        = "10.0.0.0/16"
  }
}

stack "prod" {
  source = "git::git@github.com:acme/infrastructure-catalog.git//stacks/environment?ref=v0.0.1"
  path   = "prod"
  values = {
    environment = "production"
    cidr        = "10.1.0.0/16"
  }
}
```

The referenced stack might contain:

```hcl
# stacks/environment/terragrunt.stack.hcl

unit "vpc" {
  source = "git::git@github.com:acme/infrastructure-catalog.git//units/vpc?ref=v0.0.1"
  path   = "vpc"
  values = {
    vpc_name = values.environment
    cidr     = values.cidr
  }
}

unit "database" {
  source = "git::git@github.com:acme/infrastructure-catalog.git//units/database?ref=v0.0.1"
  path   = "database"
  values = {
    environment = values.environment
    vpc_path    = "../vpc"
  }
}
```

Running `terragrunt stack generate` in the directory containing that `terragrunt.stack.hcl` file generates:

<FileTree>

- .terragrunt-stack
  - dev
    - terragrunt.stack.hcl
    - .terragrunt-stack
      - vpc
        - terragrunt.hcl
        - terragrunt.values.hcl
      - database
        - terragrunt.hcl
        - terragrunt.values.hcl
  - prod
    - terragrunt.stack.hcl
    - .terragrunt-stack
      - vpc
        - terragrunt.hcl
        - terragrunt.values.hcl
      - database
        - terragrunt.hcl
        - terragrunt.values.hcl

</FileTree>

### Working with Explicit Stacks

```bash
# Generate units from the `terragrunt.stack.hcl` file in the current
# working directory (and all stacks in child directories).
terragrunt stack generate

# Deploy all generated units defined using the `terragrunt.stack.hcl` file
# in the current working directory (and any units generated by stacks in this file).
#
# Note that this will also automatically generate the stack if it is not already generated.
terragrunt stack run apply
```

### Advantages of Explicit Stacks

- **Reusability**: Define patterns once, reuse them across environments.
- **Consistency**: Ensure all environments follow the same structure.
- **Version Control**: Version collections of infrastructure patterns alongside the units of infrastructure that make them up.
- **Automation**: Generate complex infrastructure from simple blueprints.
- **Flexibility**: Easy to create variations with different values.

### Limitations of Explicit Stacks

- **Complexity**: Requires understanding another configuration file.
- **Generation Overhead**: Units must be generated before use.
- **Debugging**: Generated files can be harder to debug if you accidentally generate files that are not what you intended.

## Stack Outputs

When defining a stack using a `terragrunt.stack.hcl` file, you also have the ability to interact with the aggregated outputs of all the units in the stack from the CLI.

To do this, use the [`stack output`](/reference/cli/commands/stack/output) command (not the [`stack run output`](/reference/cli/commands/stack/run) command).

```bash
$ terragrunt stack output
backend_app = {
  domain = "backend-app.example.com"
}
frontend_app = {
  domain = "frontend-app.example.com"
}
mysql = {
  endpoint = "terraform-20250504140737772400000001.abcdefghijkl.us-east-1.rds.amazonaws.com"
}
valkey = {
  endpoint = "serverless-valkey-01.amazonaws.com"
}
vpc = {
  vpc_id = "vpc-1234567890"
}
```

This returns a single aggregated HCL object aggregating all the outputs for all the units within the stack.

<Aside type="tip">
You can use the `--format json` flag to get the output in JSON format, which can be useful for accessing values programmatically.
</Aside>

## Using Local State with Stacks

When using Explicit Stacks, you might want to use local state files instead of remote state for development, testing, or specific use cases. However, this presents a challenge because the generated `.terragrunt-stack` directory can be safely deleted and regenerated using `terragrunt stack clean && terragrunt stack generate`, which would normally cause local state files to be lost.

To solve this problem, you can configure your stack to store state files outside of the `.terragrunt-stack` directory, in a persistent location that survives stack regeneration.

### Configuration

Here's how to configure local state that persists across stack regeneration:

**1. Create a `root.hcl` file with local backend configuration:**

```hcl
# root.hcl
remote_state {
  backend = "local"

  generate = {
    path      = "backend.tf"
    if_exists = "overwrite_terragrunt"
  }

  config = {
    path = "${get_parent_terragrunt_dir()}/.terragrunt-local-state/${path_relative_to_include()}/tofu.tfstate"
  }
}
```

**2. Create your stack definition:**

```hcl
# live/terragrunt.stack.hcl
unit "vpc" {
  source = "${find_in_parent_folders("units/vpc")}"
  path   = "vpc"
}

unit "database" {
  source = "${find_in_parent_folders("units/database")}"
  path   = "database"
}

unit "app" {
  source = "${find_in_parent_folders("units/app")}"
  path   = "app"
}
```

**3. Configure your units to include the root configuration:**

```hcl
# units/vpc/terragrunt.hcl
include "root" {
  path = find_in_parent_folders("root.hcl")
}

terraform {
  source = "."
}
```

**4. Add a `.gitignore` file to exclude state files from version control:**

```text
# .gitignore
.terragrunt-local-state
```

**Important:** Local state files should never be committed to version control as they may contain sensitive information and can cause conflicts when multiple developers work on the same infrastructure.

### How It Works

The key insight is using `path_relative_to_include()` in the state path configuration. This function returns the relative path from each unit to the `root.hcl` file, creating unique state file paths like:

```text
.terragrunt-local-state/live/.terragrunt-stack/vpc/tofu.tfstate
.terragrunt-local-state/live/.terragrunt-stack/database/tofu.tfstate
.terragrunt-local-state/live/.terragrunt-stack/app/tofu.tfstate
```

Since these state files are stored in `.terragrunt-local-state/` (outside of `.terragrunt-stack/`), they persist when you run:

```bash
terragrunt stack clean && terragrunt stack generate
```

### Directory Structure

After running the stack, your directory structure will look like this:

<FileTree>

- .
  - root.hcl
  - .gitignore (Excludes .terragrunt-local-state)
  - .terragrunt-local-state/ (Persistent state files - ignored by git)
    - live/
      - .terragrunt-stack/
        - vpc/
          - tofu.tfstate
        - database/
          - tofu.tfstate
        - app/
          - tofu.tfstate
  - live/
    - terragrunt.stack.hcl
    - .terragrunt-stack/ (Generated stack - can be deleted)
      - vpc/
        - terragrunt.hcl
        - main.tf
      - database/
        - terragrunt.hcl
        - main.tf
      - app/
        - terragrunt.hcl
        - main.tf
  - units/ (Reusable unit definitions)
    - vpc/
    - database/
    - app/

</FileTree>

## CAS Integration

<Before version="1.0.3">

<Aside type="tip">
Experimental integration between CAS and Stacks is coming soon. Follow the progress in [#5558](https://github.com/gruntwork-io/terragrunt/issues/5558).
</Aside>

</Before>

<Since version="1.0.3">

<Aside type="tip">
This functionality is part of the [`cas` experiment](/reference/experiments/active#cas). Enable it with `--experiment cas`. See the [CAS feature documentation](/features/caching/cas) for more details.
</Aside>

When authoring stacks in a catalog, you can use the `update_source_with_cas` attribute on nested blocks to replace remote Git URLs with relative paths.

Without CAS, catalog authors need to plumb remote Git URLs through `values` expressions so that generated files reference the correct remote sources:

```hcl
# stacks/service/terragrunt.stack.hcl
unit "service" {
  source = "git::git@github.com:acme/catalog.git//units/service?ref=${values.version}"
  path   = "service"
  values = {
    version = values.version
  }
}
```

This is error-prone and creates unnecessary diffs when the version changes. It also requires a network fetch for every nested source during generation.

With `update_source_with_cas`, you can use relative paths instead:

```hcl
# stacks/service/terragrunt.stack.hcl
unit "service" {
  source = "../..//units/service"

  update_source_with_cas = true

  path = "service"
}
```

The `//` separator works the same as with any Terragrunt source. The part before `//` is the base path, and the part after `//` is the subdirectory.

The attribute can also be used in `terragrunt.hcl` files within the `terraform` block:

```hcl
# units/service/terragrunt.hcl
terraform {
  source = "../..//modules/service"

  update_source_with_cas = true
}
```

When `update_source_with_cas = true` is set:

1. Terragrunt clones the catalog repository once via CAS and stores the content.
2. It follows relative source paths within the cloned repository.
3. At each level, it rewrites the `source` attribute to a `cas::` reference (e.g., `cas::sha1:abc123...`) that points to stored CAS content.
4. Generated `.terragrunt-stack` files contain deterministic CAS references instead of remote URLs.
5. Subsequent generation waves resolve `cas::` references directly from the CAS store, with no network access required.

Consumers do not set `update_source_with_cas` themselves. When the `cas` experiment is enabled and the source is remote, Terragrunt uses the CAS path automatically. The attribute only has effect inside catalog files, where it flags nested `source` attributes for rewriting.

<Before version="1.0.5">

<Aside type="tip">
A `mutable` attribute that opts a `unit` or `stack` block out of CAS hardlinking is coming in v1.0.5.
</Aside>

</Before>

<Since version="1.0.5">

`unit` and `stack` blocks also accept a `mutable` attribute. When `mutable = true`, the unit's or stack's content under `.terragrunt-stack` is copied from the CAS store and the working tree is editable. The default is `false`: files are materialized read-only so an accidental edit cannot reach back into the shared CAS store.

</Since>

Given the catalog files above, and an ordinary consumer stack like:

```hcl
# live/terragrunt.stack.hcl
stack "service" {
  source = "git::git@github.com:acme/catalog.git//stacks/service?ref=v1.0.0"

  path = "service"
}
```

Running `terragrunt --experiment cas stack generate` produces:

<FileTree>

- live/
  - terragrunt.stack.hcl
  - .terragrunt-stack/
    - service/
      - terragrunt.stack.hcl (sources rewritten to cas:: references)
      - .terragrunt-stack/
        - service/
          - terragrunt.hcl (terraform source rewritten to cas:: reference)

</FileTree>

The generated stack file at `.terragrunt-stack/service/terragrunt.stack.hcl` contains the rewritten source:

```hcl
# .terragrunt-stack/service/terragrunt.stack.hcl
unit "service" {
  source = "cas::sha1:a1b2c3d4..."

  update_source_with_cas = true

  path = "service"
}
```

And the generated unit file at `.terragrunt-stack/service/.terragrunt-stack/service/terragrunt.hcl` contains:

```hcl
# .terragrunt-stack/service/.terragrunt-stack/service/terragrunt.hcl
terraform {
  source = "cas::sha1:e5f6a7b8...//modules/service"

  update_source_with_cas = true
}
```

The first `cas::` reference points to a synthetic tree in the CAS that contains the exact files needed for the unit. The second points to the full repository tree, with `//modules/service` telling Terragrunt which subdirectory to use as the OpenTofu/Terraform module source.

<Aside type="tip">
The `cas::` reference format includes a hash algorithm prefix (`sha1:` or `sha256:`) to support both SHA-1 and SHA-256 Git repositories.
</Aside>

Requirements:

- The `cas` experiment must be enabled (`--experiment cas`).
- The `--no-cas` flag must not be set.
- Sources using `update_source_with_cas` must be relative paths within the same repository.

### Local catalog sources

The consumer stack's `source` can be a local filesystem path in addition to a remote Git URL. Both go through the same CAS-backed rewrite pipeline:

```hcl
# live/terragrunt.stack.hcl
stack "service" {
  source = "../catalog//stacks/service"

  path = "service"
}
```

Terragrunt copies the referenced directory into a temporary directory, computes a content-addressed root hash over `(relative path, mode, file-content hash)` triples using SHA-256, and rewrites nested `source` attributes against that copy. The original directory is never modified, so the catalog on disk stays clean even though the rewritten `source` values need somewhere to live.

This is useful when iterating on a catalog before tagging a release: the same `update_source_with_cas = true` attributes in the catalog's `terragrunt.stack.hcl` and `terragrunt.hcl` files apply unchanged, whether consumers pull the catalog over Git or point at a local checkout.

</Since>

## Known Limitations of Explicit Stacks

There are currently some known limitations with explicit stacks that you should be aware of as you start to adopt them.

### Dependencies cannot be set on stacks

The `dependency` block cannot set the value of the `config_path` attribute to that of a stack. This is functionality that is planned for the future, but is not currently supported.

As such, if you currently have multiple stacks that need to depend on each other, or on units within each other's stacks, you will need to either use implicit stacks, or work around this limitation by setting the `config_path` attribute to the path of the unit within the stack, and carefully ensuring that all stacks are generated before any units are run.

### Deeply nested stack generation can be slow

Every generation of a stack from a `terragrunt.stack.hcl` file can potentially result in network traffic to fetch the source for the stack and filesystem traffic to copy the generated units to the `.terragrunt-stack` directory. This can result in slow stack generation if you have very deeply nested stacks.

<Before version="1.0.3">
The planned solution for this in the future is to allow for some deduplication in stack generation, but this is not currently implemented.
</Before>

<Since version="1.0.3">
To mitigate this, enable the [`cas` experiment](/reference/experiments/active#cas) and use `update_source_with_cas = true` in your catalog's stack and unit files. This deduplicates repository clones and resolves content from the local CAS store instead of fetching from the network on every generation. See [CAS Integration](#cas-integration) for details.
</Since>

### Includes are not supported in `terragrunt.stack.hcl` files

The `include` block is not supported in `terragrunt.stack.hcl` files. This isn't functionality that is planned for future implementation, but may change based on community feedback, and proven use-cases.

The current design of explicit stacks is that, when necessary, stacks can be nested into other stacks making them better organized and reusable without relying on includes to share configuration between stacks.
