Why Terragrunt for DRY configurations in Terraform? - Purpose & Use Cases

The Big Idea

What if you could fix one file and update all your cloud environments instantly?

The Scenario

Imagine you manage many cloud environments, each needing similar setup but with slight differences. You copy-paste configuration files for each environment and tweak them manually.

The Problem

This manual copying is slow and risky. One small mistake in one file can cause failures. Updating common settings means editing many files, increasing errors and wasted time.

The Solution

Terragrunt lets you write shared configuration once and reuse it everywhere. It keeps your setup clean, consistent, and easy to update, saving time and avoiding mistakes.

Before vs After

✗ Before

resource "aws_s3_bucket" "bucket" {
  bucket = "my-app-dev"
  acl    = "private"
}

# Repeat for prod, staging with slight changes

✓ After

include {
  path = find_in_parent_folders()
}

inputs = {
  bucket_name = "my-app-${local.env}"
}

What It Enables

With Terragrunt, you can manage many environments confidently, making updates once and applying everywhere automatically.

Real Life Example

A company managing dev, staging, and production clouds uses Terragrunt to keep all environments aligned and quickly roll out changes without errors.

Key Takeaways

Manual copying of configs is slow and error-prone.

Terragrunt centralizes shared settings for easy reuse.

It boosts reliability and speeds up cloud management.

Practice

1. What is the main purpose of using Terragrunt with Terraform?

easy

A. To reuse Terraform configurations and avoid repeating code

B. To replace Terraform with a new tool

C. To write Terraform code in a different programming language

D. To deploy applications without infrastructure

Solution

1. Step 1: Understand Terragrunt's role

   Terragrunt is designed to help reuse and share Terraform code, making it easier to manage infrastructure without repeating code.

2. Step 2: Compare options

   Options B, C, and D describe incorrect uses or misunderstandings of Terragrunt's purpose.

3. Final Answer:

   To reuse Terraform configurations and avoid repeating code -> Option A

4. Quick Check:

   Terragrunt = DRY Terraform code reuse [OK]

Hint: Terragrunt helps avoid repeating Terraform code [OK]

Common Mistakes:

• Thinking Terragrunt replaces Terraform
• Believing Terragrunt changes Terraform language
• Confusing Terragrunt with application deployment tools

2. Which of the following is the correct way to include a Terraform module using Terragrunt?

easy

A. module "app" { source = "./app" }

B. include { path = find_in_parent_folders() }

C. terraform { backend = "s3" }

D. resource "aws_instance" "web" {}

Solution

1. Step 1: Identify Terragrunt syntax for including configs

   Terragrunt uses the include block with path = find_in_parent_folders() to reuse parent configs.

2. Step 2: Differentiate from Terraform syntax

   Options A, C, and D are Terraform syntax: A is a module block, C is backend config, D is a resource block, not Terragrunt includes.

3. Final Answer:

   include { path = find_in_parent_folders() } -> Option B

4. Quick Check:

   Terragrunt include uses find_in_parent_folders() [OK]

Hint: Terragrunt includes parent config with include + find_in_parent_folders() [OK]

Common Mistakes:

• Using Terraform module syntax instead of Terragrunt include
• Confusing backend config with include
• Writing resource blocks inside Terragrunt files

3. Given this Terragrunt configuration snippet:

include {
  path = find_in_parent_folders()
}

inputs = {
  region = "us-east-1"
  env    = "prod"
}

What will happen when you run terragrunt apply in this folder?

medium

A. Terragrunt will apply Terraform code without any variables

B. Terragrunt will fail because inputs cannot be used with include

C. Terragrunt will ignore the inputs block and only use parent config

D. Terragrunt will apply Terraform code using the parent config and inputs region=us-east-1, env=prod

Solution

1. Step 1: Understand include and inputs usage

   The include block imports parent config. The inputs block adds or overrides variables for this folder.

2. Step 2: Predict Terragrunt behavior on apply

   Terragrunt merges parent config with local inputs, so region and env variables are set as given.

3. Final Answer:

   Terragrunt will apply Terraform code using the parent config and inputs region=us-east-1, env=prod -> Option D

4. Quick Check:

   Include + inputs = merged config applied [OK]

Hint: Inputs override or add variables when using include [OK]

Common Mistakes:

• Thinking inputs are ignored with include
• Assuming inputs cause errors
• Believing variables are not passed to Terraform

4. You wrote this Terragrunt config:

include {
  path = find_in_parent_folders()
}

inputs = {
  region = "us-west-2"
  env = "dev"
}

terraform {
  source = "../modules/app"
}

When running terragrunt apply, you get an error: "Error: Unsupported block type". What is the likely cause?

medium

A. The terraform block cannot be nested inside the inputs block

B. The inputs block must come before include

C. The source path is incorrect

D. The region variable is invalid

Solution

1. Step 1: Check Terragrunt config block usage

   Terragrunt requires the terraform { source = ... } block at the root level. It cannot be nested inside other blocks like inputs.

2. Step 2: Identify error cause

   The error "Unsupported block type" usually means the block is misplaced or invalid in Terragrunt config.

3. Final Answer:

   The terraform block cannot be nested inside the inputs block -> Option A

4. Quick Check:

   Misplaced terraform block causes error [OK]

Hint: Terraform block must be correctly placed in Terragrunt config [OK]

Common Mistakes:

• Placing terraform block inside inputs or include
• Wrong order of blocks causing syntax errors
• Incorrect source path causing unrelated errors

5. You manage multiple environments (dev, staging, prod) with Terragrunt. You want to avoid repeating the backend configuration for each environment. Which approach best follows DRY principles?

hard

A. Define backend only in Terraform modules, not in Terragrunt

B. Copy the backend block into each environment's Terragrunt config

C. Create a root Terragrunt config with backend settings and use include in each environment folder

D. Use different backend types for each environment

Solution

1. Step 1: Understand DRY with Terragrunt

   Terragrunt allows sharing common config via a root config and include blocks in child folders.

2. Step 2: Evaluate options for backend reuse

   Create a root Terragrunt config with backend settings and use include in each environment folder uses root config for backend, avoiding repetition. Copy the backend block into each environment's Terragrunt config repeats code, violating DRY. Define backend only in Terraform modules, not in Terragrunt is incorrect because backend is configured in Terragrunt for remote state. Use different backend types for each environment adds complexity without reuse.

3. Final Answer:

   Create a root Terragrunt config with backend settings and use include in each environment folder -> Option C

4. Quick Check:

   Root config + include = DRY backend config [OK]

Hint: Put shared backend in root config, include it in environments [OK]

Common Mistakes:

• Copying backend config to each environment
• Configuring backend only in Terraform modules
• Using different backends unnecessarily