Conditional expressions (ternary) in Terraform - Mini Project: Build & Apply

Using Conditional Expressions (Ternary) in Terraform

📖 Scenario: You are setting up a Terraform configuration to deploy cloud resources. You want to control the size of a virtual machine based on an environment variable. If the environment is 'production', the VM should be large; otherwise, it should be small.

🎯 Goal: Build a Terraform configuration that uses a conditional expression (ternary) to set the VM size based on the environment variable.

📋 What You'll Learn

Create a variable called environment with default value 'development'.

Create a local variable called vm_size that uses a conditional expression to set 'large' if environment is 'production', else 'small'.

Create a resource block for a virtual machine that uses the vm_size local variable.

Ensure the configuration is valid Terraform syntax.

💡 Why This Matters

🌍 Real World

Conditional expressions help automate infrastructure decisions based on environment or other inputs, making deployments flexible and reusable.

💼 Career

Cloud engineers often use Terraform conditionals to manage different deployment environments efficiently.

1

Define the environment variable

Create a Terraform variable called environment with type string and default value "development".

Terraform

# Your code here

Hint

Use the variable block with type and default attributes.

2

Create a local variable vm_size with a conditional expression

Add a locals block and inside it create a local variable called vm_size that uses a conditional expression: if var.environment equals "production", set vm_size to "large", else set it to "small".

Terraform

variable "environment" {
  type    = string
  default = "development"
}

# Your code here

Hint

Use the syntax condition ? true_value : false_value inside the locals block.

3

Create a resource using the vm_size local variable

Add a resource block named aws_instance with resource name example. Set the instance_type attribute to use the local variable local.vm_size. Use ami attribute with a placeholder value "ami-12345678".

Terraform

variable "environment" {
  type    = string
  default = "development"
}

locals {
  vm_size = var.environment == "production" ? "large" : "small"
}

# Your code here

Hint

Use the resource block with the correct resource type and name. Reference the local variable with local.vm_size.

4

Add a provider block for AWS

Add a provider block for aws with region set to "us-east-1".

Terraform

variable "environment" {
  type    = string
  default = "development"
}

locals {
  vm_size = var.environment == "production" ? "large" : "small"
}

resource "aws_instance" "example" {
  ami           = "ami-12345678"
  instance_type = local.vm_size
}

# Your code here

Hint

The provider block tells Terraform which cloud provider to use and the region.

Practice

1. What does this Terraform conditional expression do?
var.is_production ? "prod" : "dev"

easy

A. Returns true or false

B. Always returns "prod"

C. Returns "prod" if var.is_production is true, otherwise "dev"

D. Causes a syntax error

Solution

1. Step 1: Understand the condition

   The expression checks if var.is_production is true or false.

2. Step 2: Evaluate the ternary expression

   If true, it returns "prod"; if false, it returns "dev".

3. Final Answer:

   Returns "prod" if var.is_production is true, otherwise "dev" -> Option C

4. Quick Check:

   Condition ? true_value : false_value = C [OK]

Hint: Remember: condition ? value_if_true : value_if_false [OK]

Common Mistakes:

• Thinking it returns a boolean
• Confusing true and false branches
• Assuming it always returns the first value

2. Which of these is the correct syntax for a Terraform conditional expression?

easy

A. condition : true_val ? false_val

B. condition ? true_val : false_val

C. condition ? (true_val, false_val)

D. if (condition) then true_val else false_val

Solution

1. Step 1: Recall Terraform conditional syntax

   Terraform uses the format: condition ? true_val : false_val.

2. Step 2: Compare options

   Only condition ? true_val : false_val matches this syntax exactly.

3. Final Answer:

   condition ? true_val : false_val -> Option B

4. Quick Check:

   Correct ternary syntax = D [OK]

Hint: Look for question mark and colon in correct order [OK]

Common Mistakes:

• Using if-then-else syntax from other languages
• Swapping question mark and colon
• Using commas inside the expression

3. Given this Terraform code:

variable "env" { default = "test" }
output "message" {
  value = var.env == "prod" ? "Deploy to production" : "Deploy to staging"
}

What will be the output value?

medium

A. Error: unknown variable

B. "Deploy to production"

C. "test"

D. "Deploy to staging"

Solution

1. Step 1: Check variable value

   var.env is set to "test" by default.

2. Step 2: Evaluate condition

   var.env == "prod" is false, so the false branch "Deploy to staging" is chosen.

3. Final Answer:

   "Deploy to staging" -> Option D

4. Quick Check:

   Condition false -> false branch output = A [OK]

Hint: Check variable value before condition evaluation [OK]

Common Mistakes:

• Assuming default is "prod"
• Confusing true and false branches
• Expecting variable value as output

4. Identify the error in this Terraform conditional expression:

output "result" {
  value = var.enabled ? "yes" "no"
}

medium

A. Missing colon ':' between true and false values

B. Variable var.enabled is undefined

C. Quotes around values are incorrect

D. Ternary operator not supported in outputs

Solution

1. Step 1: Review ternary syntax

   The correct syntax requires a colon ':' between true and false values.

2. Step 2: Identify missing colon

   The expression has "? \"yes\" \"no\"" missing the ':' separator.

3. Final Answer:

   Missing colon ':' between true and false values -> Option A

4. Quick Check:

   Syntax error due to missing ':' = A [OK]

Hint: Check for '?' and ':' pair in ternary expressions [OK]

Common Mistakes:

• Omitting the colon ':'
• Misplacing quotes
• Assuming ternary not allowed in outputs

5. You want to assign a resource name based on environment, but if the environment variable is empty or null, use "default" instead. Which expression correctly does this in Terraform?

var.env == "" || var.env == null ? "default" : var.env

hard

A. var.env == "" || var.env == null ? "default" : var.env

B. var.env == "" && var.env == null ? "default" : var.env

C. var.env ? "default" : var.env

D. var.env == null ? var.env : "default"

Solution

1. Step 1: Understand the requirement

   If var.env is empty string or null, use "default"; else use var.env.

2. Step 2: Analyze each option

   var.env == "" || var.env == null ? "default" : var.env checks if var.env is empty or null, then returns "default", else var.env, matching the requirement exactly.

3. Final Answer:

   var.env == "" || var.env == null ? "default" : var.env -> Option A

4. Quick Check:

   Check empty or null -> default else env = B [OK]

Hint: Check both empty string and null explicitly in condition [OK]

Common Mistakes:

• Using && instead of || causing wrong logic
• Assuming var.env alone checks for empty or null
• Swapping true and false branches