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Keep your Terraform code DRY

Dry Use cases Backend

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 terragrunt.hcl file per component (e.g. app/terragrunt.hcl, mysql/terragrunt.hcl, etc). This gives you the following file layout:

└── live
    ├── prod
    │   ├── app
    │   │   └── terragrunt.hcl
    │   ├── mysql
    │   │   └── terragrunt.hcl
    │   └── vpc
    │       └── terragrunt.hcl
    ├── qa
    │   ├── app
    │   │   └── terragrunt.hcl
    │   ├── mysql
    │   │   └── terragrunt.hcl
    │   └── vpc
    │       └── terragrunt.hcl
    └── stage
        ├── app
        │   └── terragrunt.hcl
        ├── mysql
        │   └── terragrunt.hcl
        └── vpc
            └── terragrunt.hcl

Notice how there are no Terraform configurations (.tf files) in any of the folders. Instead, each terragrunt.hcl 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/terragrunt.hcl may look like this:

terraform {
  # Deploy version v0.0.3 in stage
  source = "git::git@github.com:foo/modules.git//app?ref=v0.0.3"
}

inputs = {
  instance_count = 3
  instance_type  = "t2.micro"
}

(Note: the double slash (//) in the source parameter is intentional and required. It’s part of Terraform’s Git syntax for module sources. Terraform may display a “Terraform initialized in an empty directory” warning, but you can safely ignore it.)

And prod/app/terragrunt.hcl may look like this:

terraform {
  # Deploy version v0.0.1 in prod
  source = "git::git@github.com:foo/modules.git//app?ref=v0.0.1"
}

inputs = {
  instance_count = 10
  instance_type  = "m2.large"
}

You can now deploy the modules in your live repo. For example, to deploy the app module in stage, you would do the following:

cd live/stage/app
terragrunt apply

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

  1. Download the configurations specified via the source parameter into the --terragrunt-download-dir folder (by default .terragrunt-cache in the working directory, which we recommend adding to .gitignore). This downloading is done by using the same go-getter library Terraform uses, 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.

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

  4. Pass any variables defined in the inputs = { …​ } block as environment variables (prefixed with TF_VAR_ to your Terraform code. Notice how the inputs block in stage/app/terragrunt.hcl deploys fewer and smaller instances than prod.

Check out the terragrunt-infrastructure-modules-example and terragrunt-infrastructure-live-example repos for fully-working sample code that demonstrates this new folder structure.

Achieve DRY Terraform code and immutable infrastructure

With this new approach, copy/paste between environments is minimized. The terragrunt.hcl files contain solely the source URL of the module to deploy and the inputs to set for that module in the current environment. To create a new environment, you copy an old one and update just the environment-specific inputs in the terragrunt.hcl 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/terragrunt.hcl and prod/app/terragrunt.hcl 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 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. Terraform may display a “Terraform initialized in an empty directory” warning, but you can safely ignore it.)

Important gotcha: Terragrunt caching

The first time you set the source parameter to a remote URL, Terragrunt will download the code from that URL into a tmp folder. It will NOT download it again afterwords unless you change that URL. That’s because downloading code—and more importantly, reinitializing remote state, redownloading provider plugins, and redownloading modules—can take a long time. To avoid adding 10-90 seconds of overhead to every Terragrunt command, Terragrunt assumes all remote URLs are immutable, and only downloads them once.

Therefore, when working locally, you should use the --terragrunt-source parameter and point it at a local file path as described in the previous section. Terragrunt will copy the local files every time you run it, which is nearly instantaneous, and doesn’t require reinitializing everything, so you’ll be able to iterate quickly.

If you need to force Terragrunt to redownload something from a remote URL, run Terragrunt with the --terragrunt-source-update flag and it’ll delete the tmp folder, download the files from scratch, and reinitialize everything. This can take a while, so avoid it and use --terragrunt-source when you can!

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 .terragrunt-cache/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 (terragrunt.hcl), 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 utilize the Terragrunt built-in function get_terragrunt_dir() to use a relative file path:

    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_terragrunt_dir() function, you can use relative paths!
        arguments = [
          "-var-file=${get_terragrunt_dir()}/../common.tfvars",
          "-var-file=example.tfvars"
        ]
      }
    }
    

    See the get_terragrunt_dir() documentation for more details.

Using Terragrunt with private Git repos

The easiest way to use Terragrunt with private Git repos is to use SSH authentication. Configure your Git account so you can use it with SSH (see the guide for GitHub here) and use the SSH URL for your repo, prepended with git::ssh://:

terraform {
  source = "git::ssh://git@github.com/foo/modules.git//path/to/module?ref=v0.0.1"
}

Look up the Git repo for your repository to find the proper format.

Note: In automated pipelines, you may need to run the following command for your Git repository prior to calling terragrunt to ensure that the ssh host is registered locally, e.g.:

$ ssh -T -oStrictHostKeyChecking=accept-new git@github.com || true

DRY common Terraform code with Terragrunt generate blocks

Terragrunt has the ability to generate code in to the downloaded remote Terraform modules before calling out to terraform using the generate block. This can be used to inject common terraform configurations into all the modules that you use.

For example, it is common to have custom provider configurations in your code to customize authentication. Consider a setup where you want to always assume a specific role when calling out to the terraform module. However, not all modules expose the right variables for configuring the aws provider so that you can assume the role through Terraform.

In this situation, you can use Terragrunt generate blocks to generate a tf file called provider.tf that includes the provider configuration. Add a root terragrunt.hcl file for each of the environments in the file layout for the live repo:

└── live
    ├── prod
    │   ├── terragrunt.hcl
    │   ├── app
    │   │   └── terragrunt.hcl
    │   ├── mysql
    │   │   └── terragrunt.hcl
    │   └── vpc
    │       └── terragrunt.hcl
    ├── qa
    │   ├── terragrunt.hcl
    │   ├── app
    │   │   └── terragrunt.hcl
    │   ├── mysql
    │   │   └── terragrunt.hcl
    │   └── vpc
    │       └── terragrunt.hcl
    └── stage
        ├── terragrunt.hcl
        ├── app
        │   └── terragrunt.hcl
        ├── mysql
        │   └── terragrunt.hcl
        └── vpc
            └── terragrunt.hcl

Each root terragurnt.hcl file (the one at the environment level, e.g prod/terragrunt.hcl) should define a generate block to generate the AWS provider configuration to assume the role for that environment. For example, if you wanted to assume the role arn:aws:iam::0123456789:role/terragrunt in all the modules for the prod account, you would put the following in prod/terragrunt.hcl:

generate "provider" {
  path = "provider.tf"
  if_exists = "overwrite_terragrunt"
  contents = <<EOF
provider "aws" {
  assume_role {
    role_arn = "arn:aws:iam::0123456789:role/terragrunt"
  }
}
EOF
}

This instructs Terragrunt to create the file provider.tf in the working directory (where Terragrunt calls terraform) before it calls any of the Terraform commands (e.g plan, apply, validate, etc). This allows you to inject this provider configuration in all the modules that includes the root file.

To include this in the child configurations (e.g mysql/terragrunt.hcl), you would update all the child modules to include this configuration using the include block:

include {
  path = find_in_parent_folders()
}

The include block tells Terragrunt to use the exact same Terragrunt configuration from the terragrunt.hcl file specified via the path parameter. It behaves exactly as if you had copy/pasted the Terraform configuration from the included file generate configuration into the child, but this approach is much easier to maintain!