Dependency inversion with modules in Terraform - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is dependency inversion in Terraform modules?

Dependency inversion means designing modules so that higher-level modules do not depend on lower-level modules directly. Instead, they depend on abstractions or inputs, allowing flexibility and easier changes.

beginner

Why use dependency inversion with Terraform modules?

It helps keep modules reusable and independent. Changes in one module won't break others because dependencies are managed through inputs and outputs, not hard-coded references.

intermediate

How do Terraform modules communicate dependencies without tight coupling?

Modules communicate using input variables and output values. One module outputs data, and another module receives it as input, avoiding direct internal references.

intermediate

Example: How to invert dependency between a network module and an instance module?

The network module outputs subnet IDs. The instance module takes subnet IDs as input variables. The root module connects them by passing outputs to inputs, not by the instance module calling the network module directly.

advanced

What is a benefit of dependency inversion when scaling Terraform projects?

It allows teams to work on modules independently, reduces errors from tight coupling, and makes it easier to replace or upgrade modules without affecting the whole infrastructure.

What does dependency inversion in Terraform modules primarily promote?

A Loose coupling between modules

B Direct module calls inside other modules

C Hard-coded resource IDs

D Ignoring module outputs

Dependency inversion promotes loose coupling by using inputs and outputs instead of direct references.

How do Terraform modules share data without tight dependency?

A Using output values and input variables

B By importing each other's code

C By hardcoding resource names

D By running scripts inside modules

Modules share data through outputs and inputs, which keeps them independent.

Which Terraform component connects module outputs to inputs?

A Backend

B Provider

C Resource block

D Root module

The root module orchestrates passing outputs from one module as inputs to another.

What is a risk of not using dependency inversion in Terraform modules?

A Modules automatically update

B Modules run faster

C Modules become tightly coupled and hard to maintain

D Modules use fewer variables

Without dependency inversion, modules depend directly on each other, making maintenance difficult.

Which practice aligns with dependency inversion in Terraform?

A Hardcoding subnet IDs inside instance module

B Passing subnet IDs as input variables to instance module

C Calling network module inside instance module

D Ignoring module outputs

Passing subnet IDs as inputs keeps modules independent and flexible.

Explain how dependency inversion improves Terraform module design.

Think about how modules avoid direct references and rely on data passed in.

Describe a simple example of dependency inversion between two Terraform modules.

Consider network and instance modules sharing subnet IDs.

Practice

1. What does dependency inversion mean in Terraform modules?

easy

A. Modules cannot accept variables

B. Modules depend on inputs instead of creating resources themselves

C. Modules always create all resources internally

D. Modules must be written in the root configuration

Solution

1. Step 1: Understand module dependency principle

   Dependency inversion means modules should not create resources directly but rely on inputs.

2. Step 2: Identify correct description

   Modules depend on inputs instead of creating resources themselves correctly states modules depend on inputs, making them flexible and reusable.

3. Final Answer:

   Modules depend on inputs instead of creating resources themselves -> Option B

4. Quick Check:

   Dependency inversion = Modules use inputs [OK]

Hint: Modules get resource info via inputs, not by creating resources [OK]

Common Mistakes:

• Thinking modules must create all resources internally
• Assuming modules cannot accept variables
• Believing modules must be in root config

2. Which of the following is the correct way to pass a resource ID to a module in Terraform?

easy

A. module "example" { resource_id = aws_instance.example.id }

B. module "example" { input_id = aws_instance.example.id }

C. module "example" { instance_id = var.instance_id }

D. module "example" { instance_id = aws_instance.example.id }

Solution

1. Step 1: Identify correct variable passing syntax

   Modules accept variables by name; the value can be a resource attribute like aws_instance.example.id.

2. Step 2: Check option correctness

   module "example" { instance_id = aws_instance.example.id } correctly passes instance_id with the resource ID aws_instance.example.id.

3. Final Answer:

   module "example" { instance_id = aws_instance.example.id } -> Option D

4. Quick Check:

   Pass resource ID as variable = module "example" { instance_id = aws_instance.example.id } [OK]

Hint: Use variable name = resource.attribute to pass IDs [OK]

Common Mistakes:

• Using undefined variable names like resource_id or input_id
• Passing resource IDs without variable names
• Confusing variable and resource references

3. Given this Terraform root module snippet:

resource "aws_vpc" "main" {
  cidr_block = "10.0.0.0/16"
}

module "network" {
  source = "./modules/network"
  vpc_id = aws_vpc.main.id
}

What is the expected behavior of the network module?

medium

A. It ignores the VPC ID and creates a subnet only

B. It creates a new VPC inside the module

C. It uses the existing VPC ID passed as input

D. It fails because VPC ID cannot be passed as input

Solution

1. Step 1: Analyze root module resource and module call

   The root module creates an aws_vpc resource and passes its ID to the network module as vpc_id.

2. Step 2: Understand module behavior with input

   The network module uses the passed vpc_id to configure resources inside that VPC, not create a new one.

3. Final Answer:

   It uses the existing VPC ID passed as input -> Option C

4. Quick Check:

   Module uses input vpc_id = It uses the existing VPC ID passed as input [OK]

Hint: Modules use passed IDs to link resources, not recreate them [OK]

Common Mistakes:

• Assuming module creates a new VPC ignoring input
• Thinking passing resource IDs is invalid
• Believing module fails without explicit VPC creation

4. You have this module call:

module "db" {
  source = "./modules/db"
  subnet_id = aws_subnet.app.id
}

Inside the module, the variable is declared as variable "subnet" { type = string }. What error will occur?

medium

A. Error: Unknown variable 'subnet_id' in module

B. Error: Variable 'subnet' not provided

C. No error, variable names can differ

D. Error: aws_subnet.app.id is invalid

Solution

1. Step 1: Compare variable name and input argument

   The module expects a variable named 'subnet' but the input is 'subnet_id'.

2. Step 2: Understand Terraform variable matching

   Terraform matches input arguments to variable names exactly. 'subnet_id' does not match any variable, causing an unsupported argument error.

3. Final Answer:

   Error: Unknown variable 'subnet_id' in module -> Option A

4. Quick Check:

   Variable name mismatch causes unknown variable error [OK]

Hint: Variable names must match exactly between module and call [OK]

Common Mistakes:

• Assuming variable names can differ
• Confusing variable name with resource attribute name
• Ignoring error messages about missing variables

5. You want to create a reusable module for an AWS security group that attaches to any VPC. Which approach follows dependency inversion best?

hard

A. Module accepts a VPC ID as input and creates security group in that VPC

B. Module hardcodes a VPC ID inside the module code

C. Module requires the user to create security group outside and passes its ID

D. Module creates its own VPC and security group inside

Solution

1. Step 1: Understand dependency inversion for modules

   Modules should not create dependent resources like VPCs but accept them as inputs.

2. Step 2: Evaluate options for best practice

   Module accepts a VPC ID as input and creates security group in that VPC accepts VPC ID as input and creates the security group inside that VPC, following dependency inversion.

3. Final Answer:

   Module accepts a VPC ID as input and creates security group in that VPC -> Option A

4. Quick Check:

   Pass dependencies as inputs for flexibility [OK]

Hint: Pass VPC ID as input; module creates resources inside it [OK]

Common Mistakes:

• Hardcoding resource IDs inside modules
• Modules creating dependent resources themselves
• Requiring users to create resources outside without module help