Outputs for module communication in Terraform - Mini Project: Build & Apply

Outputs for module communication

📖 Scenario: You are building a Terraform project with two modules: network and server. The network module creates a virtual network and outputs its ID. The server module needs to use this network ID to launch a server inside the network.

🎯 Goal: Create Terraform outputs in the network module to share the network ID. Then, in the root module, capture this output and pass it as an input variable to the server module.

📋 What You'll Learn

Create a Terraform output called vnet_id in the network module that outputs the virtual network ID.

In the root module, declare the network module and capture its vnet_id output.

Declare the server module in the root module and pass the captured vnet_id as an input variable called network_id.

In the server module, declare an input variable called network_id.

💡 Why This Matters

🌍 Real World

In real cloud projects, modules often need to share information like network IDs, security group IDs, or subnet IDs. Outputs and inputs help modules communicate cleanly.

💼 Career

Understanding module communication is essential for building scalable and maintainable Terraform infrastructure code, a key skill for cloud engineers and DevOps professionals.

1

Create the network module output

In the network module, create an output called vnet_id that outputs the value of the resource azurerm_virtual_network.main.id.

Terraform

# Your code here

Hint

Use the output block with the name vnet_id and set its value to azurerm_virtual_network.main.id.

2

Declare the network module in the root module

In the root module, declare a module called network with the source path ./modules/network. Capture its output vnet_id by referencing module.network.vnet_id in a locals block as network_id.

Terraform

# Your code here

Hint

Declare the network module with source = "./modules/network". Then create a locals block with network_id = module.network.vnet_id.

3

Declare the server module and pass network_id

In the root module, declare a module called server with the source path ./modules/server. Pass the captured network_id local value as an input variable called network_id to the server module.

Terraform

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

locals {
  network_id = module.network.vnet_id
}

# Your code here

Hint

Declare the server module with source = "./modules/server" and pass network_id = local.network_id.

4

Declare the input variable network_id in the server module

In the server module, declare an input variable called network_id of type string.

Terraform

# Your code here

Hint

Use a variable block with the name network_id and set type = string.

Practice

1. What is the main purpose of output blocks in Terraform modules?

easy

A. To share values from one module to another or to the user

B. To create new resources in the cloud

C. To define variables for user input

D. To delete resources after deployment

Solution

1. Step 1: Understand the role of output blocks

   Output blocks are used to expose values from a module so other modules or users can access them.

2. Step 2: Differentiate outputs from other blocks

   Unlike resource or variable blocks, outputs do not create or accept input but share information.

3. Final Answer:

   To share values from one module to another or to the user -> Option A

4. Quick Check:

   Outputs share values = D [OK]

Hint: Outputs share info between modules or users [OK]

Common Mistakes:

• Confusing outputs with resource creation
• Thinking outputs accept user input
• Assuming outputs delete resources

2. Which of the following is the correct syntax to define an output named instance_ip with value aws_instance.web.private_ip?

easy

A. output instance_ip = aws_instance.web.private_ip

B. output "instance_ip" { value = aws_instance.web.private_ip }

C. output "instance_ip" = aws_instance.web.private_ip

D. output { instance_ip = aws_instance.web.private_ip }

Solution

1. Step 1: Recall Terraform output block syntax

   Outputs use the syntax: output "name" { value = expression }

2. Step 2: Match syntax with options

   output "instance_ip" { value = aws_instance.web.private_ip } matches the correct syntax with quotes and value assignment inside braces.

3. Final Answer:

   output "instance_ip" { value = aws_instance.web.private_ip } -> Option B

4. Quick Check:

   Correct output block syntax = B [OK]

Hint: Output blocks need quotes and value inside braces [OK]

Common Mistakes:

• Missing quotes around output name
• Using equals sign outside braces
• Placing value outside output block

3. Given this module output block:

output "db_endpoint" {
  value = aws_db_instance.main.endpoint
}

And this root module code:

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

output "database_url" {
  value = module.database.db_endpoint
}

What will terraform output database_url show after deployment?

medium

A. The endpoint address of the AWS database instance

B. The string 'module.database.db_endpoint'

C. An error because outputs cannot be nested

D. The IP address of the AWS database instance

Solution

1. Step 1: Understand module output forwarding

   The module outputs 'db_endpoint' which is accessed in root as 'module.database.db_endpoint'.

2. Step 2: Check root output value

   The root output 'database_url' uses the module output value, so it will show the actual endpoint string.

3. Final Answer:

   The endpoint address of the AWS database instance -> Option A

4. Quick Check:

   Module output forwarded = A [OK]

Hint: Root output uses module output value directly [OK]

Common Mistakes:

• Thinking output shows variable name as string
• Assuming outputs cannot be nested
• Confusing endpoint with IP address

4. You wrote this output block inside a module:

output "server_ip" {
  value = aws_instance.app.private_ip
}

But when you run terraform apply, you get an error: Reference to undeclared resource. What is the likely cause?

medium

A. The output block is missing a description

B. Output blocks cannot reference resource attributes

C. Output names cannot contain underscores

D. The resource aws_instance.app is not defined in the module

Solution

1. Step 1: Analyze the error message

   'Reference to undeclared resource' means Terraform cannot find 'aws_instance.app' in the module.

2. Step 2: Check output references

   Outputs must reference existing resources; if resource is missing, error occurs.

3. Final Answer:

   The resource aws_instance.app is not defined in the module -> Option D

4. Quick Check:

   Missing resource causes reference error = A [OK]

Hint: Check resource exists before referencing in output [OK]

Common Mistakes:

• Assuming outputs can't reference resources
• Thinking underscores are invalid in output names
• Believing description is mandatory

5. You have a module that creates multiple AWS instances and outputs a list of their private IPs:

output "instance_ips" {
  value = [for i in aws_instance.app : i.private_ip]
}

How can you use this output in the root module to create a security group rule allowing SSH from all these IPs?

hard

A. Use count = length(module.app.instance_ips) but no for_each

B. Directly assign cidr_blocks = module.app.instance_ips without conversion

C. Use for_each = toset(module.app.instance_ips) in the security group rule resource

D. Outputs cannot be used to create security group rules

Solution

1. Step 1: Understand output is a list of IPs

   The output returns a list of private IP addresses from multiple instances.

2. Step 2: Use for_each with a set for unique IPs

   Security group rules can be created per IP using for_each with toset() to avoid duplicates.

3. Final Answer:

   Use for_each = toset(module.app.instance_ips) in the security group rule resource -> Option C

4. Quick Check:

   Use for_each with toset for multiple IP rules = C [OK]

Hint: Use for_each with toset() for list outputs in resources [OK]

Common Mistakes:

• Assigning list directly to cidr_blocks without for_each
• Using count without mapping IPs
• Thinking outputs can't be used in resource blocks