Module composition patterns in Terraform - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Architecture

intermediate

Understanding Module Composition with Terraform

You have a Terraform root module that calls two child modules: network and compute. The compute module requires the VPC ID output from the network module. Which is the correct way to pass the VPC ID from network to compute?

Terraform

module "network" {
  source = "./modules/network"
}

module "compute" {
  source = "./modules/compute"
  vpc_id = ???
}

A vpc_id = module.network.vpc_id

B vpc_id = output.network.vpc_id

C vpc_id = var.network.vpc_id

D vpc_id = local.network.vpc_id

💡 Hint

Think about how Terraform modules expose outputs and how other modules can access them.

In Terraform, outputs from one module can be accessed using module... Here, module.network.vpc_id correctly references the output from the network module.

❓ Configuration

intermediate

Correct Module Source Usage

You want to reuse a Terraform module stored in a GitHub repository. Which source value correctly references the module?

Terraform

module "storage" {
  source = ???
  version = "1.0.0"
}

A "git::https://github.com/terraform-modules/storage.git"

B "https://github.com/terraform-modules/storage"

C "github.com/terraform-modules/storage"

D "terraform-registry/storage"

💡 Hint

Terraform requires a specific syntax to fetch modules from Git repositories.

To use a module from a Git repository, the source must start with git:: followed by the repository URL. Option A is the correct syntax.

❓ security

advanced

Secure Module Composition with Sensitive Variables

You have a root module that calls a child module which requires a sensitive API key. How should you pass this sensitive variable to avoid exposing it in Terraform state or logs?

Terraform

module "service" {
  source = "./modules/service"
  api_key = ???
}

A api_key = "hardcoded-secret"

B api_key = local.api_key

C api_key = var.api_key

D api_key = output.api_key

💡 Hint

Consider how Terraform handles sensitive variables and best practices for secrets.

Passing sensitive data as input variables marked sensitive (like var.api_key) is best practice. Hardcoding secrets or using outputs can expose them. Locals do not protect sensitivity.

❓ service_behavior

advanced

Effect of Module Dependency on Resource Creation Order

Given two modules, db and app, where app depends on the database endpoint output from db, what is the effect on resource creation order when you run terraform apply?

Terraform

module "db" {
  source = "./modules/db"
}

module "app" {
  source = "./modules/app"
  db_endpoint = module.db.endpoint
}

A Terraform creates <code>app</code> and <code>db</code> resources in parallel without order.

B Terraform creates <code>db</code> resources first, then <code>app</code> resources after <code>db</code> outputs are available.

C Terraform creates <code>app</code> resources first, then <code>db</code> resources.

D Terraform fails because modules cannot depend on each other.

💡 Hint

Think about how Terraform manages dependencies between modules using outputs and inputs.

Terraform automatically creates resources in dependency order. Since app uses an output from db, db resources are created first.

✅ Best Practice

expert

Optimizing Module Composition for Reusability and Maintainability

You are designing a Terraform infrastructure with multiple environments (dev, staging, prod). Which module composition pattern best supports reusability and environment-specific customization?

A Create separate child modules for each environment with no shared code.

B Create one root module per environment, each with duplicated child modules customized per environment.

C Hardcode environment values inside child modules and call them without variables.

D Use a single root module with environment-specific variables passed to shared child modules.

💡 Hint

Consider how to avoid code duplication while allowing environment differences.

Using a single root module with environment variables passed to shared child modules promotes code reuse and easier maintenance. Other options cause duplication or inflexibility.

Practice

1. What is the main purpose of using module composition patterns in Terraform?

easy

A. To combine multiple modules by passing outputs as inputs for better organization

B. To write all resources in a single large file

C. To avoid using variables in Terraform configurations

D. To deploy resources without any dependencies

Solution

1. Step 1: Understand module composition

   Module composition means connecting modules by using outputs from one as inputs to another.

2. Step 2: Recognize benefits

   This approach keeps infrastructure code organized, reusable, and easier to manage.

3. Final Answer:

   To combine multiple modules by passing outputs as inputs for better organization -> Option A

4. Quick Check:

   Module composition = combining modules with outputs as inputs [OK]

Hint: Think of modules as building blocks connected by outputs [OK]

Common Mistakes:

• Thinking modules should be combined by copying code
• Believing variables are avoided in module composition
• Assuming modules work independently without connections

2. Which of the following is the correct syntax to pass an output from one module as an input to another in Terraform?

easy

A. output.moduleA.output_name

B. module.moduleA.output_name

C. var.moduleA.output_name

D. resource.moduleA.output_name

Solution

1. Step 1: Recall Terraform module output reference syntax

   Outputs from a module are accessed using module.<module_name>.<output_name>.

2. Step 2: Compare options

   Only module.moduleA.output_name matches the correct syntax for accessing module outputs.

3. Final Answer:

   module.moduleA.output_name -> Option B

4. Quick Check:

   Module output reference = module.module_name.output_name [OK]

Hint: Outputs are accessed via module.<name>.<output> [OK]

Common Mistakes:

• Using var or resource instead of module to access outputs
• Confusing output blocks with module references
• Using incorrect keywords like output.moduleA

3. Given the following Terraform code snippet, what will be the value of moduleB's input instance_id?

module "moduleA" {
  source = "./moduleA"
}

module "moduleB" {
  source      = "./moduleB"
  instance_id = module.moduleA.instance_id
}

medium

A. The value of the output named instance_id from moduleA

B. A syntax error because instance_id is not defined

C. The string literal 'module.moduleA.instance_id'

D. Null because moduleB does not have instance_id output

Solution

1. Step 1: Identify module output usage

   The input instance_id for moduleB is assigned from module.moduleA.instance_id, which means it uses the output named instance_id from moduleA.

2. Step 2: Understand value passing

   This passes the actual value of moduleA's output to moduleB's input, not a string or error.

3. Final Answer:

   The value of the output named instance_id from moduleA -> Option A

4. Quick Check:

   moduleB.instance_id = moduleA.instance_id output value [OK]

Hint: module.<name>.<output> passes actual output value, not string [OK]

Common Mistakes:

• Thinking it passes a string literal instead of value
• Assuming syntax error due to missing variable declaration
• Confusing inputs and outputs between modules

4. You have two modules: moduleA outputs vpc_id, and moduleB requires vpc_id as input. The following code causes an error:

module "moduleA" {
  source = "./moduleA"
}

module "moduleB" {
  source = "./moduleB"
  vpc_id = var.vpc_id
}

What is the most likely cause of the error?

medium

A. moduleB source path is incorrect

B. moduleA does not have an output named vpc_id

C. moduleB is using var.vpc_id instead of module.moduleA.vpc_id as input

D. Terraform does not support passing outputs between modules

Solution

1. Step 1: Check input assignment in moduleB

   moduleB expects vpc_id input, but it is assigned from var.vpc_id, which is a variable, not the output from moduleA.

2. Step 2: Identify correct way to pass output

   The correct way is to assign vpc_id = module.moduleA.vpc_id to pass the output from moduleA.

3. Final Answer:

   moduleB is using var.vpc_id instead of module.moduleA.vpc_id as input -> Option C

4. Quick Check:

   Pass outputs via module.moduleA.output, not var [OK]

Hint: Use module.<name>.<output> to pass outputs, not var.<name> [OK]

Common Mistakes:

• Assuming var.vpc_id automatically links to moduleA output
• Ignoring the need to explicitly connect modules
• Thinking Terraform cannot pass outputs between modules

5. You want to create a composed Terraform setup where moduleC depends on outputs from both moduleA and moduleB. Which pattern correctly composes these modules to pass outputs as inputs to moduleC?

// Option A
module "moduleC" {
  source = "./moduleC"
  inputA = module.moduleA.outputA
  inputB = module.moduleB.outputB
}

// Option B
module "moduleC" {
  source = "./moduleC"
  inputA = var.outputA
  inputB = var.outputB
}

// Option C
module "moduleC" {
  source = "./moduleC"
  inputA = module.moduleB.outputB
  inputB = module.moduleA.outputA
}

// Option D
module "moduleC" {
  source = "./moduleC"
  inputA = module.moduleA.outputB
  inputB = module.moduleB.outputA
}

hard

A. Mix outputs incorrectly between modules

B. Use variables instead of module outputs for inputs

C. Swap inputs but still use correct module outputs

D. Pass outputs correctly matching input names from moduleA and moduleB

Solution

1. Step 1: Match inputs to correct module outputs

   Pass outputs correctly matching input names from moduleA and moduleB passes inputA from moduleA.outputA and inputB from moduleB.outputB, matching names correctly.

2. Step 2: Evaluate other options

   Use variables instead of module outputs for inputs uses variables instead of outputs, which breaks composition. Swap inputs but still use correct module outputs swaps the assignments, mismatching inputA (expects moduleA.outputA) with moduleB.outputB and inputB with moduleA.outputA. Mix outputs incorrectly between modules mixes outputs incorrectly, likely causing errors.

3. Final Answer:

   Pass outputs correctly matching input names from moduleA and moduleB -> Option D

4. Quick Check:

   Correct output-to-input mapping = Pass outputs correctly matching input names from moduleA and moduleB [OK]

Hint: Match input names to correct module outputs carefully [OK]

Common Mistakes:

• Swapping outputs between modules without checking input expectations
• Using variables instead of module outputs
• Mixing output names incorrectly causing runtime errors