terragrunt

README

Terragrunt

Terragrunt is a thin wrapper for Terraform that provides extra tools for working with multiple Terraform modules, remote state, and locking.

Quick start

1. Install Terraform.

2. Install Terragrunt by going to the Releases Page, downloading the binary for your OS, renaming it to terragrunt, and adding it to your PATH.

3. Go into a folder with your Terraform configurations (.tf files) and create a terraform.tfvars file with a terragrunt = { ... } block that contains the configuration for Terragrunt (check out the Use cases section for the types of configuration Terragrunt supports):

   terragrunt = {
     # (put your Terragrunt configuration here)
   }
   

4. Now, instead of running terraform directly, run all the standard Terraform commands using terragrunt:

   terragrunt get
   terragrunt plan
   terragrunt apply
   terragrunt output
   terragrunt destroy
   

   Terragrunt forwards almost all commands, arguments, and options directly to Terraform, using whatever version of Terraform you already have installed. However, based on the settings in your terraform.tfvars file, Terragrunt can configure remote state, locking, extra arguments, and lots more.

Use cases

Terragrunt supports the following use cases:

1. Keep your Terraform code DRY
2. Keep your remote state configuration DRY
3. Keep your CLI flags DRY
4. Work with multiple Terraform modules

Keep your Terraform code DRY

Motivation

Consider the following file structure, which defines three environments (prod, qa, stage) with the same infrastructure in each one (an app, a MySQL database, and a VPC):

└── live
    ├── prod
    │   ├── app
    │   │   └── main.tf
    │   ├── mysql
    │   │   └── main.tf
    │   └── vpc
    │       └── main.tf
    ├── qa
    │   ├── app
    │   │   └── main.tf
    │   ├── mysql
    │   │   └── main.tf
    │   └── vpc
    │       └── main.tf
    └── stage
        ├── app
        │   └── main.tf
        ├── mysql
        │   └── main.tf
        └── vpc
            └── main.tf

The contents of each environment will be more or less identical, except perhaps for a few settings (e.g. the prod environment may run bigger or more servers). As the size of the infrastructure grows, having to maintain all of this
duplicated code between environments becomes more error prone. You can reduce the amount of copy paste using Terraform modules, but even the code to instantiate a module and set up input variables, output variables, providers, and remote state can still create a lot of maintenance overhead.

How can you keep your Terraform code DRY so that you only have to define it once, no matter how many environments you have?

Remote Terraform configurations

Terragrunt has the ability to download remote Terraform configurations. The idea is that you define the Terraform code for your infrastructure just once, in a single repo, called, for example, modules:

└── modules
    ├── app
    │   └── main.tf
    ├── mysql
    │   └── main.tf
    └── vpc
        └── main.tf

This repo contains typical Terraform code, with one difference: anything in your code that should be different between environments should be exposed as an input variable. For example, the app module might expose the following variables:

variable "instance_count" {
  description = "How many servers to run"
}

variable "instance_type" {
  description = "What kind of servers to run (e.g. t2.large)"
}

These variables allow you to run smaller/fewer servers in qa and stage to save money and larger/more servers in prod to ensure availability and scalability.

In a separate repo, called, for example, live, you define the code for all of your environments, which now consists of just one .tfvars file per component (e.g. app/terraform.tfvars, mysql/terraform.tfvars, etc). This gives you the following file layout:

└── live
    ├── prod
    │   ├── app
    │   │   └── terraform.tfvars
    │   ├── mysql
    │   │   └── terraform.tfvars
    │   └── vpc
    │       └── terraform.tfvars
    ├── qa
    │   ├── app
    │   │   └── terraform.tfvars
    │   ├── mysql
    │   │   └── terraform.tfvars
    │   └── vpc
    │       └── terraform.tfvars
    └── stage
        ├── app
        │   └── terraform.tfvars
        ├── mysql
        │   └── terraform.tfvars
        └── vpc
            └── terraform.tfvars

Notice how there are no Terraform configurations (.tf files) in any of the folders. Instead, each .tfvars file specifies a terraform { ... } block that specifies from where to download the Terraform code, as well as the environment-specific values for the input variables in that Terraform code. For example, stage/app/terraform.tfvars may look like this:

terragrunt = {
  terraform {
    source = "git::git@github.com:foo/modules.git//app?ref=v0.0.3"
  }
}

instance_count = 3
instance_type = "t2.micro"

(Note: the double slash (//) is intentional and required. It's part of Terraform's Git syntax for module sources.)

And prod/app/terraform.tfvars may look like this:

terragrunt = {
  terraform {
    source = "git::git@github.com:foo/modules.git//app?ref=v0.0.1"
  }
}

instance_count = 10
instance_type = "m2.large"

Notice how the two terraform.tfvars files set the source URL to the same app module, but at different versions (i.e. stage is testing out a newer version of the module). They also set the parameters for the app module to different values that are appropriate for the environment: smaller/fewer servers in stage to save money, larger/more instances in prod for scalability and high availability.

How to use remote configurations

Once you've set up your live and modules repositories, all you need to do is run terragrunt commands in the live repository. For example, to deploy the app module in qa, you would do the following:

cd live/qa/app
terragrunt apply

When Terragrunt finds the terraform block with a source parameter in live/qa/app/terraform.tfvars file, it will:

1. Download the configurations specified via the source parameter into a temporary folder. This downloading is done by using the terraform init command, so the source parameter supports the exact same syntax as the module source parameter, including local file paths, Git URLs, and Git URLs with ref parameters (useful for checking out a specific tag, commit, or branch of Git repo). Terragrunt will download all the code in the repo (i.e. the part before the double-slash //) so that relative paths work correctly between modules in that repo.

2. Copy all files from the current working directory into the temporary folder. This way, Terraform will automatically read in the variables defined in the terraform.tfvars file.

3. Execute whatever Terraform command you specified in that temporary folder.

DRY Terraform code and immutable infrastructure

With this new approach, copy/paste between environments is minimized. The .tfvars files contain solely the variables that are different between environments. To create a new environment, you copy an old one and update just the environment-specific values in the .tfvars files, which is about as close to the "essential complexity" of the problem as you can get.

Just as importantly, since the Terraform module code is now defined in a single repo, you can version it (e.g., using Git tags and referencing them using the ref parameter in the source URL, as in the stage/app/terraform.tfvars and prod/app/terraform.tfvars examples above), and promote a single, immutable version through each environment (e.g., qa -> stage -> prod). This idea is inspired by Kief Morris' blog post Using Pipelines to Manage Environments with Infrastructure as Code.

Working locally

If you're testing changes to a local copy of the modules repo, you you can use the --terragrunt-source command-line option or the TERRAGRUNT_SOURCE environment variable to override the source parameter. This is useful to point Terragrunt at a local checkout of your code so you can do rapid, iterative, make-a-change-and-rerun development:

cd live/stage/app
terragrunt apply --terragrunt-source ../../../modules//app

(Note: the double slash (//) here too is intentional and required. Terragrunt downloads all the code in the folder before the double-slash into the temporary folder so that relative paths between modules work correctly.)

Important gotcha: working with relative file paths

One of the gotchas with downloading Terraform configurations is that when you run terragrunt apply in folder foo, Terraform will actually execute in some temporary folder such as /tmp/foo. That means you have to be especially careful with relative file paths, as they will be relative to that temporary folder and not the folder where you ran Terragrunt!

In particular:

• Command line: When using file paths on the command line, such as passing an extra -var-file argument, you should use absolute paths:

  # Use absolute file paths on the CLI!
  terragrunt apply -var-file /foo/bar/extra.tfvars
  

• Terragrunt configuration: When using file paths directly in your Terragrunt configuration (terraform.tfvars), such as in an extra_arguments block, you can't use hard-coded absolute file paths, or it won't work on your teammates' computers. Therefore, you should use a relative file path with the get_tfvars_dir() helper:

  terragrunt = {
    terraform {
      source = "git::git@github.com:foo/modules.git//frontend-app?ref=v0.0.3"
  
      extra_arguments "custom_vars" {
        commands = [
          "apply",
          "plan",
          "import",
          "push",
          "refresh"
        ]
  
        # With the get_tfvars_dir helper, you can use relative paths! 
        arguments = [
          "-var-file=${get_tfvars_dir()}/../common.tfvars",
          "-var-file=terraform.tfvars"
        ]
      }    
    }
  }
  

  See the get_tfvars_dir() documentation for more details.

Keep your remote state configuration DRY

Motivation

Terraform supports remote state storage via a variety of backends that you configure as follows:

terraform {
  backend "s3" {
    bucket     = "my-terraform-state"
    key        = "frontend-app/terraform.tfstate"
    region     = "us-east-1"
    encrypt    = true
    lock_table = "my-lock-table"
  }
}

Unfortunately, the backend configuration does not support interpolation. This makes it hard to keep your code DRY if you have multiple Terraform modules. For example, consider the following folder structure, which uses different Terraform modules to deploy a backend app, frontend app, MySQL database, and a VPC:

├── backend-app
│   └── main.tf
├── frontend-app
│   └── main.tf
├── mysql
│   └── main.tf
└── vpc
    └── main.tf

To use remote state with each of these modules, you would have to copy/paste the exact same backend configuration into each of the main.tf files. The only thing that would differ between the configurations would be the key parameter: e.g., the key for mysql/main.tf might be mysql/terraform.tfstate and the key for frontend-app/main.tf might be frontend-app/terraform.tfstate.

To keep your remote state configuration DRY, you can use Terragrunt. You still have to specify the backend you want to use in each module, but instead of copying and pasting the configuration settings over and over again into each main.tf file, you can leave them blank:

terraform {
  # The configuration for this backend will be filled in by Terragrunt
  backend "s3" {}
}

Filling in remote state settings with Terragrunt

To fill in the settings via Terragrunt, create a terraform.tfvars file in the root folder and in each of the Terraform modules:

├── terraform.tfvars
├── backend-app
│   ├── main.tf
│   └── terraform.tfvars
├── frontend-app
│   ├── main.tf
│   └── terraform.tfvars
├── mysql
│   ├── main.tf
│   └── terraform.tfvars
└── vpc
    ├── main.tf
    └── terraform.tfvars

In your root terraform.tfvars file, you can define your entire remote state configuration just once in a remote_state block, as follows:

terragrunt = {
  remote_state {
    backend = "s3"
    config {
      bucket     = "my-terraform-state"
      key        = "${path_relative_to_include()}/terraform.tfstate"
      region     = "us-east-1"
      encrypt    = true
      lock_table = "my-lock-table"
    }
  }
}

The remote_state block supports all the same backend types as Terraform. The next time you run terragrunt, it will automatically configure all the settings in the remote_state.config block, if they aren't configured already, by calling terraform init.

In each of the child terraform.tfvars files, such as mysql/terraform.tfvars, you can tell Terragrunt to automatically include all the settings from the root terraform.tfvars file as follows:

terragrunt = {
  include {
    path = "${find_in_parent_folders()}"
  }
}

The include block tells Terragrunt to use the exact settings from the terraform.tfvars file specified via the path parameter. It behaves exactly as if you had copy/pasted the contents of the root terraform.tfvars file into mysql/terraform.tfvars, but this approach is much easier to maintain! Note that if you include any other settings in the terragrunt block of a child .tfvars file, it will override the settings in the parent.

The terraform.tfvars files above use two helper functions:

• find_in_parent_folders(): This helper returns the path to the first terraform.tfvars file it finds in the parent folders above the current terraform.tfvars file. In the example above, the call to find_in_parent_folders() in mysql/terraform.tfvars will return ../terraform.tfvars. This way, you don't have to hard code the path parameter in every module.

• path_relative_to_include(): This helper function returns the relative path between the current terraform.tfvars file and the path specified in its include block. We typically use this in a root terraform.tfvars file so that each Terraform child module stores its Terraform state at a different key. For example, the mysql module will have its key parameter resolve to mysql/terraform.tstate and the frontend-app module will have its key parameter resolve to frontend-app/terraform.tfstate.

See the helper functions docs for more info.

Create remote state and locking resources automatically

When you run terragrunt with remote_state configuration, it will automatically create the following resources if they don't already exist:

• S3 bucket: If you are using the S3 backend for remote state storage and the bucket you specify in remote_state.config doesn't already exist, Terragrunt will create it automatically, with versioning enabled.

• DynamoDB table: If you are using the S3 backend for remote state storage and you specify a lock_table (a DynamoDB table used for locking) in remote_state.config, if that table doesn't already exist, Terragrunt will create it automatically, including a primary key called LockID.

Note: If you specify a profile key in remote_state.config, Terragrunt will automatically use this AWS profile when creating the S3 bucket or DynamoDB table.

Keep your CLI flags DRY

Motivation

Sometimes you may need to pass extra CLI arguments every time you run certain terraform commands. For example, you may want to set the lock-timeout setting to 20 minutes for all commands that may modify remote state so that Terraform will keep trying to acquire a lock for up to 20 minutes if someone else already has the lock rather than immediately exiting with an error.

You can configure Terragrunt to pass specific CLI arguments for specific commands using an extra_arguments block in your terraform.tfvars file:

terragrunt = {
  terraform {
    # Force Terraform to keep trying to acquire a lock for up to 20 minutes if someone else already has the lock 
    extra_arguments "retry_lock" {
      commands = [
        "init",
        "apply",
        "refresh",
        "import",
        "plan",
        "taint",
        "untaint"
      ]
      
      arguments = [
        "-lock-timeout=20m"
      ]    
    } 
  }
}

Each extra_arguments block includes an arbitrary name (in the example above, retry_lock), a list of commands to which the extra arguments should be add, a list of arguments or required_var_files or optional_var_files to add. With the configuration above, when you run terragrunt apply, Terragrunt will call Terraform as follows:

> terragrunt apply

terraform apply -lock-timeout=20m

Multiple extra_arguments blocks

You can specify one or more extra_arguments blocks. The arguments in each block will be applied any time you call terragrunt with one of the commands in the commands list. If more than one extra_arguments block matches a command, the arguments will be added in the order of of appearance in the configuration. For example, in addition to lock settings, you may also want to pass custom -var-file arguments to several commands:

terragrunt = {
  terraform {
    # Force Terraform to keep trying to acquire a lock for up to 20 minutes if someone else already has the lock 
    extra_arguments "retry_lock" {
      commands = [
        "init",
        "apply",
        "refresh",
        "import",
        "plan",
        "taint",
        "untaint"
      ]
      
      arguments = [
        "-lock-timeout=20m"
      ]    
    } 
  
    # Pass custom var files to Terraform
    extra_arguments "custom_vars" {
      commands = [
        "apply",
        "plan",
        "import",
        "push",
        "refresh"
      ]
      
      arguments = [
        "-var", "foo=bar",
        "-var", "region=us-west-1"
      ]
    }
  }
}

With the configuration above, when you run terragrunt apply, Terragrunt will call Terraform as follows:

> terragrunt apply

terraform apply -lock-timeout=20m -var foo=bar -var region=us-west-1

Required and optional var-files

One common usage of extra_arguments is to include tfvars files. instead of using arguments, it is simpler to use either required_var_files or optional_var_files. Both options require only to provide the list of file to include. The only difference is that required_var_files will add the extra argument -var-file=<your file> for each file specified and if they don't exist, terraform will complain. Using optional_var_files instead, terragrunt will only add the -var-file=<your file> for existing files. This allows many conditional configurations based on environment variables as you can see in the following example:

/my/tf
├── terraform.tfvars
├── prod.tfvars
├── us-west-2.tfvars
├── backend-app
│   ├── main.tf
│   ├── dev.tfvars
│   └── terraform.tfvars
├── frontend-app
│   ├── main.tf
│   ├── us-east-1.tfvars
│   └── terraform.tfvars

terragrunt = {
  terraform {
    extra_arguments "conditional_vars" {
      commands = [
        "apply",
        "plan",
        "import",
        "push",
        "refresh"
      ]

      required_var_files = [
        "${get_parent_tfvars_dir()}/terraform.tfvars"
      ]

      optional_var_files = [
        "${get_parent_tfvars_dir()}/${get_env("TF_VAR_env", "dev")}.tfvars",
        "${get_parent_tfvars_dir()}/${get_env("TF_VAR_region", "us-east-1")}.tfvars"
        "${get_tfvars_dir()}/${get_env("TF_VAR_env", "dev")}.tfvars",
        "${get_tfvars_dir()}/${get_env("TF_VAR_region", "us-east-1")}.tfvars"
      ]
    }
  }

See the get_tfvars_dir() and get_parent_tfvars_dir() documentation for more details.

Note that terragrunt cannot interpolate terraform variables (${var.xxx}) in the terragrunt configuration, your variables have to be defined through TF_VAR_xxx environment variable to be referred by terragrunt.

With the configuration above, when you run terragrunt apply-all, Terragrunt will call Terraform as follows:

> terragrunt apply-all
[backend-app]  terraform apply -var-file=/my/tf/terraform.tfvars -var-file=/my/tf/backend-app/dev.tfvars
[frontend-app] terraform apply -var-file=/my/tf/terraform.tfvars -var-file=/my/tf/frontend-app/us-east-1.tfvars

> TF_VAR_env=prod terragrunt apply-all
[backend-app]  terraform apply -var-file=/my/tf/terraform.tfvars -var-file=/my/tf/prod.tfvars
[frontend-app] terraform apply -var-file=/my/tf/terraform.tfvars -var-file=/my/tf/prod.tfvars -var-file=/my/tf/frontend-app/us-east-1.tfvars

> TF_VAR_env=prod TF_VAR_region=us-west-2 terragrunt apply-all
[backend-app]  terraform apply -var-file=/my/tf/terraform.tfvars -var-file=/my/tf/prod.tfvars -var-file=/my/tf/us-west-2.tfvars
[frontend-app] terraform apply -var-file=/my/tf/terraform.tfvars -var-file=/my/tf/prod.tfvars -var-file=/my/tf/us-west-2.tfvars

Handling whitespace

The list of arguments cannot include whitespaces, so if you need to pass command line arguments that include spaces (e.g. -var bucket=example.bucket.name), then each of the arguments will need to be a separate item in the arguments list:

terragrunt = {
  terraform {
    extra_arguments "bucket" {
      arguments = [
        "-var", "bucket=example.bucket.name",
      ]
      commands = [
        "apply",
        "plan",
        "import",
        "push",
        "refresh"
      ]
    }
  }
}

With the configuration above, when you run terragrunt apply, Terragrunt will call Terraform as follows:

> terragrunt apply

terraform apply -var bucket=example.bucket.name

Work with multiple Terraform modules

Motivation

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

root
├── backend-app
│   └── main.tf
├── frontend-app
│   └── main.tf
├── mysql
│   └── main.tf
├── redis
│   └── main.tf
└── vpc
    └── main.tf

There is one module to deploy a frontend-app, another to deploy a backend-app, another for the MySQL database, and so on. To deploy such an environment, you'd have to manually run terraform apply in each of the subfolder, wait for it to complete, and then run terraform apply in the next subfolder. How do you avoid this tedious and time-consuming process?

The apply-all, destroy-all, and output-all commands

To be able to deploy multiple Terraform modules in a single command, add a terraform.tfvars file to each module:

root
├── backend-app
│   ├── main.tf
│   └── terraform.tfvars
├── frontend-app
│   ├── main.tf
│   └── terraform.tfvars
├── mysql
│   ├── main.tf
│   └── terraform.tfvars
├── redis
│   ├── main.tf
│   └── terraform.tfvars
└── vpc
    ├── main.tf
    └── terraform.tfvars

Inside each terraform.tfvars file, add a terragrunt = { ... } block to identify this as a module managed by Terragrunt (the block can be empty or include any of the configs described in this documentation):

terragrunt = {
  # Put your Terragrunt configuration here
}

Now you can go into the root folder and deploy all the modules within it by using the apply-all command:

cd root
terragrunt apply-all

When you run this command, Terragrunt will recursively look through all the subfolders of the current working directory, find all terraform.tfvars files with a terragrunt = { ... } block, and run terragrunt apply in each one concurrently.

Similarly, to undeploy all the Terraform modules, you can use the destroy-all command:

cd root
terragrunt destroy-all

Finally, to see the currently applied outputs of all of the subfolders, you can use the output-all command:

cd root
terragrunt output-all

If your modules have dependencies between them—for example, you can't deploy the backend-app until MySQL and redis are deployed—you'll need to express those dependencines in your Terragrunt configuation as explained in the next section.

Dependencies between modules

Consider the following file structure:

root
├── backend-app
│   ├── main.tf
│   └── terraform.tfvars
├── frontend-app
│   ├── main.tf
│   └── terraform.tfvars
├── mysql
│   ├── main.tf
│   └── terraform.tfvars
├── redis
│   ├── main.tf
│   └── terraform.tfvars
└── vpc
    ├── main.tf
    └── terraform.tfvars

Let's assume you have the following dependencies between Terraform modules:

• backend-app depends on mysql, redis, and vpc
• frontend-app depends on backend-app and vpc
• mysql depends on vpc
• redis depends on vpc
• vpc has no dependencies

You can express these dependencies in your terraform.tfvars config files using a dependencies block. For example, in backend-app/terraform.tfvars you would specify:

terragrunt = {
  dependencies {
    paths = ["../vpc", "../mysql", "../redis"]
  }
}

Similarly, in frontend-app/terraform.tfvars, you would specify:

terragrunt = {
  dependencies {
    paths = ["../vpc", "../backend-app"]
  }
}

Once you've specified the dependencies in each terraform.tfvars file, when you run the terragrunt apply-all or terragrunt destroy-all, Terragrunt will ensure that the dependencies are applied or destroyed, respectively, in the correct order. For the example at the start of this section, the order for the apply-all command would be:

1. Deploy the VPC
2. Deploy MySQL and Redis in parallel
3. Deploy the backend-app
4. Deploy the frontend-app

If any of the modules fail to deploy, then Terragrunt will not attempt to deploy the modules that depend on them. Once you've fixed the error, it's usually safe to re-run the apply-all or destroy-all command again, since it'll be a no-op for the modules that already deployed successfully, and should only affect the ones that had an error the last time around.

Terragrunt details

This section contains detailed documentation for the following aspects of Terragrunt:

1. Helper functions
2. CLI options
3. Migrating from Terragrunt v0.11.x and Terraform 0.8.x and older
4. Terragrunt config files
5. Developing Terragrunt
6. License

Helper functions

Terragrunt allows you to use the same inteprolation syntax as Terraform (${...}) to call helper functions. Note that helper functions only work within a terragrunt = { ... } block. Terraform does NOT process interpolations in .tfvars files.

Here are the supported helper functions:

• find_in_parent_folders
• path_relative_to_include
• path_relative_from_include
• get_env
• get_tfvars_dir
• get_parent_tfvars_dir
• get_aws_account_id

find_in_parent_folders

find_in_parent_folders() searches up the directory tree from the current .tfvars file and returns the relative path to to the first terraform.tfvars in a parent folder or exit with an error if no such file is found. This is primarily useful in an include block to automatically find the path to a parent .tfvars file:

terragrunt = {
  include {
    path = "${find_in_parent_folders()}"
  }
}

path_relative_to_include

path_relative_to_include() returns the relative path between the current .tfvars file and the path specified in its include block. For example, consider the following folder structure:

├── terraform.tfvars
└── prod
    └── mysql
        └── terraform.tfvars
└── stage
    └── mysql
        └── terraform.tfvars

Imagine prod/mysql/terraform.tfvars and stage/mysql/terraform.tfvars include all settings from the root terraform.tfvars file:

terragrunt = {
  include {
    path = "${find_in_parent_folders()}"
  }
}

The root terraform.tfvars can use the path_relative_to_include() in its remote_state configuration to ensure each child stores its remote state at a different key:

terragrunt = {
  remote_state {
    backend = "s3" 
    config {
      bucket = "my-terraform-bucket"
      region = "us-east-1"
      key    = "${path_relative_to_include()}/terraform.tfstate"
    }
  }
}

The resulting key will be prod/mysql/terraform.tfstate for the prod mysql module and stage/mysql/terraform.tfstate for the stage mysql module.

path_relative_from_include

path_relative_from_include() returns the relative path between the path specified in its include block and the current .tfvars file (it is the counterpart of path_relative_to_include()). For example, consider the following folder structure:

├── sources
|  ├── mysql
|  |  └── *.tf
|  └── secrets
|     └── mysql
|         └── *.tf
└── terragrunt
  └── common.tfvars
  ├── mysql
  |  └── terraform.tfvars
  ├── secrets
  |  └── mysql
  |     └── terraform.tfvars
  └── terraform.tfvars

Imagine terragrunt/mysql/terraform.tfvars and terragrunt/secrets/mysql/terraform.tfvars include all settings from the root terraform.tfvars file:

terragrunt = {
  include {
    path = "${find_in_parent_folders()}"
  }
}

The root terraform.tfvars can use the path_relative_from_include() in combination with path_relative_to_include() in its source configuration to retrieve the relative terraform source code from the terragrunt configuration file:

terragrunt = {
  terraform {
    source = "${path_relative_from_include()}/../sources//${path_relative_to_include()}"
  }
  ...
}

The resulting source will be ../../sources//mysql for mysql module and ../../../sources//secrets/mysql for secrets/mysql module.

Another use case would be to add extra argument to include the common.tfvars file for all subdirectories:

terragrunt = {
  terraform = {
    ...

    extra_arguments "common_var" {
      commands = [
        "apply",
        "plan",
        "import",
        "push",
        "refresh"
      ]

      arguments = [
        "-var-file=${get_tfvars_dir()}/${path_relative_from_include()}/common.tfvars",
      ]
    }
  }
}

This allows proper retrieval of the common.tfvars from whatever the level of subdirectories we have.

get_env

get_env(NAME, DEFAULT) returns the value of the environment variable named NAME or DEFAULT if that environment variable is not set. Example:

terragrunt = {
  remote_state {
    backend = "s3"
    config {
      bucket = "${get_env("BUCKET", "my-terraform-bucket")}"
    }
  }  
}

Note that Terraform will read environment variables that start with the prefix TF_VAR_, so one way to share the a variable named foo between Terraform and Terragrunt is to set its value as the environment variable TF_VAR_foo and to read that value in using this get_env helper function.

get_tfvars_dir

get_tfvars_dir() returns the directory where the Terragrunt configuration file (by default, terraform.tfvars) lives. This is useful when you need to use relative paths with remote Terraform configurations and you want those paths relative to your Terragrunt configuration file and not relative to the temporary directory where Terragrunt downloads the code.

For example, imagine you have the following file structure:

/terraform-code
├── common.tfvars
├── frontend-app
│   └── terraform.tfvars

Inside of /terraform-code/frontend-app/terraform.tfvars you might try to write code that looks like this:

terragrunt = {
  terraform {
    source = "git::git@github.com:foo/modules.git//frontend-app?ref=v0.0.3"
    
    extra_arguments "custom_vars" {
      commands = [
        "apply",
        "plan",
        "import",
        "push",
        "refresh"
      ]
      
      arguments = [
        "-var-file=../common.tfvars", # Note: This relative path will NOT work correctly!
        "-var-file=terraform.tfvars"
      ]
    }    
  }
}

Note how the source parameter is set, so Terragrunt will download the frontend-app code from the modules repo into a temporary folder and run terraform in that temporary folder. Note also that there is an extra_arguments block that is trying to allow the frontend-app to read some shared variables from a common.tfvars file. Unfortunately, the relative path (../common.tfvars) won't work, as it will be relative to the temporary folder! Moreover, you can't use an absolute path, or the code won't work on any of your teammates' computers.

To make the relative path work, you need to use the get_tfvars_dir() helper to combine the path with the folder where the .tfvars file lives:

terragrunt = {
  terraform {
    source = "git::git@github.com:foo/modules.git//frontend-app?ref=v0.0.3"
    
    extra_arguments "custom_vars" {
      commands = [
        "apply",
        "plan",
        "import",
        "push",
        "refresh"
      ]
      
      # With the get_tfvars_dir helper, you can use relative paths! 
      arguments = [
        "-var-file=${get_tfvars_dir()}/../common.tfvars",
        "-var-file=terraform.tfvars"
      ]
    }    
  }
}

For the example above, this path will resolve to /terraform-code/frontend-app/../common.tfvars, which is exactly what you want.

get_parent_tfvars_dir

get_parent_tfvars_dir() returns the absolute directory where the Terragrunt parent configuration file (by default, terraform.tfvars) lives. This is useful when you need to use relative paths with remote Terraform configurations and you want those paths relative to your parent Terragrunt configuration file and not relative to the temporary directory where Terragrunt downloads the code.

This function is very similar to get_tfvars_dir() except it returns the root instead of the leaf of your terragrunt configuration folder.

/terraform-code
├── terraform.tfvars
├── common.tfvars
├── app1
│   └── terraform.tfvars
├── tests
│   ├── app2
│   |   └── terraform.tfvars
│   └── app3
│       └── terraform.tfvars

terragrunt = {
  terraform {
    extra_arguments "common_vars" {
      commands = [
        "apply",
        "plan",
        "import",
        "push",
        "refresh"
      ]

      arguments = [
        "-var-file=${get_parent_tfvars_dir()}/common.tfvars"
      ]
    }
  }
}

The common.tfvars located in the terraform root folder will be included by all applications, whatever their relative location to the root.

get_aws_account_id

get_aws_account_id() returns the AWS account id associated with the current set of credentials. Example:

terragrunt = {
  remote_state {
    backend = "s3"
    config {
      bucket = "mycompany-${get_aws_account_id())"
    }
  }
}

This allows uniqueness of the storage bucket per AWS account (since bucket name must be globally unique).

It is also possible to configure variables specifically based on the account used:

terragrunt = {
  terraform = {
    ...

    extra_arguments "common_var" {
      commands = [
        "apply",
        "plan",
        "import",
        "push",
        "refresh"
      ]

      arguments = [
        "-var-file=${get_aws_account_id()}.tfvars",
      ]
    }
  }
}

CLI Options

Terragrunt forwards all arguments and options to Terraform. The only exceptions are --version and arguments that start with the prefix --terragrunt-. The currently available options are:

• --terragrunt-config: A custom path to the terraform.tfvars file. May also be specified via the TERRAGRUNT_CONFIG environment variable. The default path is terraform.tfvars in the current directory (see Terragrunt config files for a slightly more nuanced explanation). This argument is not used with the apply-all, destroy-all, and output-all commands.

• --terragrunt-tfpath: A custom path to the Terraform binary. May also be specified via the TERRAGRUNT_TFPATH environment variable. The default is terraform in a directory on your PATH.

• --terragrunt-non-interactive: Don't show interactive user prompts. This will default the answer for all prompts to 'yes'. Useful if you need to run Terragrunt in an automated setting (e.g. from a script).

• --terragrunt-working-dir: Set the directory where Terragrunt should execute the terraform command. Default is the current working directory. Note that for the apply-all and destroy-all directories, this parameter has a different meaning: Terragrunt will apply or destroy all the Terraform modules in the subfolders of the terragrunt-working-dir, running terraform in the root of each module it finds.

• --terragrunt-source: Download Terraform configurations from the specified source into a temporary folder, and run Terraform in that temporary folder. May also be specified via the TERRAGRUNT_SOURCE environment variable. The source should use the same syntax as the Terraform module source parameter. This argument is not used with the apply-all, destroy-all, and output-all commands.

• --terragrunt-source-update: Delete the contents of the temporary folder before downloading Terraform source code into it.

Terragrunt config files

The current version of Terragrunt expects configuration to be defined in a terraform.tfvars file. Previous versions defined the config in a .terragrunt file. The .terragrunt format is now deprecated!

For backwards compatibility, Terragrunt will continue to support the .terragrunt file format for a short period of time. Check out the next section for how this works. Note that you will get a warning in your logs every time you run Terragrunt with a .terragrunt file, and we will eventually stop supporting this older format, so we recommend migrating to the terraform.tfvars format ASAP!

Config file search paths

Terragrunt figures out the path to its config file according to the following rules:

1. The value of the --terragrunt-config command-line option, if specified.
2. The value of the TERRAGRUNT_CONFIG environment variable, if defined.
3. A .terragrunt file in the current working directory, if it exists.
4. A terraform.tfvars file in the current working directory, if it exists.
5. If none of these are found, exit with an error.

The --terragrunt-config parameter is only used by Terragrunt and has no effect on which variable files are loaded by Terraform. Terraform will automatically read variables from a file named terraform.tfvars, but if you want it to read variables from some other .tfvars file, you must pass it in using the --var-file argument:

terragrunt plan --terragrunt-config example.tfvars --var-file example.tfvars

Migrating from .terragrunt to terraform.tfvars

The configuration in a .terragrunt file is identical to that of the terraform.tfvars file, except the terraform.tfvars file requires you to wrap that configuration in a terragrunt = { ... } block.

For example, if this is your .terragrunt file:

include {
  path = "${find_in_parent_folders()}"
}

dependencies {
  paths = ["../vpc", "../mysql", "../redis"]
}

The equivalent terraform.tfvars file is:

terragrunt = {
  include {
    path = "${find_in_parent_folders()}"
  }
  
  dependencies {
    paths = ["../vpc", "../mysql", "../redis"]
  }
}

To migrate, all you need to do is:

1. Copy all the contents of the .terragrunt file.
2. Paste those contents into a terragrunt = { ... } block in a terraform.tfvars file.
3. Delete the .terragrunt file.

Migrating from Terragrunt v0.11.x and Terraform 0.8.x and older

Background

Terragrunt was originally created to support two features that were not available in Terraform: defining remote state configuration in a file (rather than via CLI commands) and locking. As of version 0.9.0, Terraform now supports both of these features natively, so we have had to make changes to Terragrunt:

1. Terragrunt still supports remote state configuration so you can take advantage of Terragrunt's interpolation functions.
2. Terragrunt no longer supports locking.

Migration instructions

If you were using Terragrunt <= v0.11.x and Terraform <= 0.8.x, here is how to migrate:

1. In your Terraform code (the .tf files), you must now define a backend. For example, to use S3 as a remote state backend, you will need to add the following to your Terraform code:

   # main.tf
   terraform {
     # The configuration for this backend will be filled in by Terragrunt
     backend "s3" {}
   }
   

   Note that you can leave the configuration of the backend empty and allow Terragrunt to provide that configuration
   instead. This allows you to keep your remote state configuration more DRY by taking advantage of Terragrunt's interpolation functions:

   # terraform.tfvars
   terragrunt = {
     remote_state {
       backend = "s3"
       config {
         bucket  = "my-terraform-state"
         key     = "${path_relative_to_include()}/terraform.tfstate"
         region  = "us-east-1"
         encrypt = true
       }
     }
   }
   

2. Remove any lock { ... } blocks from your Terragrunt configurations, as these are no longer supported.

   If you were storing remote state in S3 and relying on DynamoDB as a locking mechanism, Terraform now supports that natively. To enable it, simply add the lock_table parameter to your S3 backend configuration. If you configure your S3 backend using Terragrunt, then Terragrunt will automatically create the lock_table for you if that table doesn't already exist:

   # terraform.tfvars
   terragrunt = {
     remote_state {
       backend = "s3" 
       config {
         bucket  = "my-terraform-state"
         key     = "${path_relative_to_include()}/terraform.tfstate"
         region  = "us-east-1"
         encrypt = true
   
         # Tell Terraform to do locking using DynamoDB. Terragrunt will automatically create this table for you if
         # it doesn't already exist. 
         lock_table = "my-lock-table"
       }
     }
   }
   

   NOTE: We recommend using a completely new lock table name and NOT reusing the lock table from older versions of Terragrunt, as that older table had a different structure than what Terraform expects, and Terragrunt will not automatically recreate it.

   If you would like Terraform to automatically retry locks like Terragrunt did (this is particularly useful when running Terraform as part of an automated script, such as a CI build), you use an extra_arguments block:

   # terraform.tfvars
   terragrunt = {
     remote_state {
       backend = "s3" 
       config {
         bucket  = "my-terraform-state"
         key     = "${path_relative_to_include()}/terraform.tfstate"
         region  = "us-east-1"
         encrypt = true
   
         # Tell Terraform to do locking using DynamoDB. Terragrunt will automatically create this table for you if
         # it doesn't already exist. 
         lock_table = "my-lock-table"
       }
     }
   
     terraform { 
       # Force Terraform to keep trying to acquire a lock for up to 20 minutes if someone else already has the lock 
       extra_arguments "retry_lock" {
         commands = [
           "init",
           "apply",
           "refresh",
           "import",
           "plan",
           "taint",
           "untaint"
         ]
   
         arguments = [
           "-lock-timeout=20m"
         ]    
       }  
     }
   }     
   

Developing terragrunt

Running locally

To run Terragrunt locally, use the go run command:

go run main.go plan

Running tests

Note: The tests in the dynamodb folder for Terragrunt run against a real AWS account and will add and remove real data from DynamoDB. DO NOT hit CTRL+C while the tests are running, as this will prevent them from cleaning up temporary tables and data in DynamoDB. We are not responsible for any charges you may incur.

Before running the tests, you must configure your AWS credentials as explained in the DynamoDB locking prerequisites section.

To run all the tests:

go test -v -parallel 128 $(glide novendor)

To run only the tests in a specific package, such as the package remote:

cd remote
go test -v -parallel 128

And to run a specific test, such as TestToTerraformRemoteConfigArgsNoBackendConfigs in package remote:

cd remote
go test -v -parallel 128 -run TestToTerraformRemoteConfigArgsNoBackendConfigs

Debug logging

If you set the TERRAGRUNT_DEBUG environment variable to "true", the stack trace for any error will be printed to stdout when you run the app.

Error handling

In this project, we try to ensure that:

1. Every error has a stacktrace. This makes debugging easier.
2. Every error generated by our own code (as opposed to errors from Go built-in functions or errors from 3rd party libraries) has a custom type. This makes error handling more precise, as we can decide to handle different types of errors differently.

To accomplish these two goals, we have created an errors package that has several helper methods, such as errors.WithStackTrace(err error), which wraps the given error in an Error object that contains a stacktrace. Under the hood, the errors package is using the go-errors library, but this may change in the future, so the rest of the code should not depend on go-errors directly.

Here is how the errors package should be used:

1. Any time you want to create your own error, create a custom type for it, and when instantiating that type, wrap it with a call to errors.WithStackTrace. That way, any time you call a method defined in the Terragrunt code, you know the error it returns already has a stacktrace and you don't have to wrap it yourself.
2. Any time you get back an error object from a function built into Go or a 3rd party library, immediately wrap it with errors.WithStackTrace. This gives us a stacktrace as close to the source as possible.
3. If you need to get back the underlying error, you can use the errors.IsError and errors.Unwrap functions.

Formatting

Every source file in this project should be formatted with go fmt. There are few helper scripts and targets in the Makefile that can help with this (mostly taken from the terraform repo):

1. make fmtcheck

   Checks to see if all source files are formatted. Exits 1 if there are unformatted files.

2. make fmt

   Formats all source files with gofmt.

3. make install-pre-commit-hook

   Installs a git pre-commit hook that will run all of the source files through gofmt.

To ensure that your changes get properly formatted, please install the git pre-commit hook with make install-pre-commit-hook.

Releasing new versions

To release a new version, just go to the Releases Page and create a new release. The CircleCI job for this repo has been configured to:

1. Automatically detect new tags.
2. Build binaries for every OS using that tag as a version number.
3. Upload the binaries to the release in GitHub.

See circle.yml and _ci/build-and-push-release-asset.sh for details.

License

This code is released under the MIT License. See LICENSE.txt.