Variable validation rules in Terraform - Mini Project: Build & Apply

Terraform Variable Validation Rules

📖 Scenario: You are setting up a Terraform configuration to deploy cloud resources. To avoid mistakes, you want to add rules that check if the input variables meet certain conditions before applying the configuration.

🎯 Goal: Create a Terraform variable with validation rules that ensure the input is a string with a length between 3 and 10 characters.

📋 What You'll Learn

Define a variable named environment of type string

Add a validation block that checks the length of environment is at least 3 characters

Add a validation block that checks the length of environment is at most 10 characters

Provide a custom error message if the validation fails

💡 Why This Matters

🌍 Real World

Terraform variable validation helps prevent mistakes by checking user inputs before deploying cloud infrastructure, saving time and avoiding errors.

💼 Career

Cloud engineers and DevOps professionals use variable validation to create reliable and user-friendly Terraform modules and configurations.

1

Create the environment variable

Create a Terraform variable named environment of type string.

Terraform

# Your code here

Hint

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

2

Add minimum length validation

Add a validation block inside the environment variable that checks if the length of var.environment is at least 3 characters using length(var.environment) >= 3.

Terraform

variable "environment" {
  type = string
  # Your code here
}

Hint

Use a validation block with condition and error_message inside the variable.

3

Add maximum length validation

Add another validation block inside the environment variable that checks if the length of var.environment is at most 10 characters using length(var.environment) <= 10. Keep the previous validation block as well.

Terraform

variable "environment" {
  type = string
  validation {
    condition     = length(var.environment) >= 3
    error_message = "The environment name must be at least 3 characters long."
  }
  # Your code here
}

Hint

Add a second validation block with the maximum length condition and error message.

4

Complete the variable with both validations

Ensure the environment variable block includes both validation blocks for minimum and maximum length with their respective error messages.

Terraform

variable "environment" {
  type = string
  validation {
    condition     = length(var.environment) >= 3
    error_message = "The environment name must be at least 3 characters long."
  }
  validation {
    condition     = length(var.environment) <= 10
    error_message = "The environment name must be at most 10 characters long."
  }
  # Your code here
}

Hint

Review the entire variable block to confirm both validations are present.

Practice

1. What is the main purpose of variable validation rules in Terraform?

easy

A. To speed up the Terraform apply process

B. To automatically fix errors in the Terraform code

C. To create new variables dynamically during runtime

D. To check if input values meet specific conditions before applying configuration

Solution

1. Step 1: Understand variable validation role

   Variable validation rules ensure inputs are correct before Terraform uses them.

2. Step 2: Identify the purpose

   They check conditions and show errors if inputs are invalid, preventing mistakes.

3. Final Answer:

   To check if input values meet specific conditions before applying configuration -> Option D

4. Quick Check:

   Validation checks inputs = C [OK]

Hint: Validation rules check inputs before use to avoid errors [OK]

Common Mistakes:

• Thinking validation fixes errors automatically
• Confusing validation with variable creation
• Assuming validation speeds up apply

2. Which of the following is the correct syntax to add a validation rule for a variable in Terraform?

easy

A. variable "name" { validation { condition = length(var.name) > 3 error_message = "Name too short" } }

B. variable "name" { validate { condition = length(var.name) > 3 message = "Name too short" } }

C. variable "name" { validation_rule { check = length(var.name) > 3 error = "Name too short" } }

D. variable "name" { validate_rule { condition = length(var.name) > 3 error_message = "Name too short" } }

Solution

1. Step 1: Recall Terraform variable validation syntax

   Terraform uses validation block inside variable with condition and error_message.

2. Step 2: Match syntax with options

   variable "name" { validation { condition = length(var.name) > 3 error_message = "Name too short" } } matches correct keywords and structure exactly.

3. Final Answer:

   variable "name" { validation { condition = length(var.name) > 3 error_message = "Name too short" } } -> Option A

4. Quick Check:

   Correct keywords: validation, condition, error_message = A [OK]

Hint: Look for 'validation' block with 'condition' and 'error_message' keys [OK]

Common Mistakes:

• Using 'validate' instead of 'validation'
• Using wrong keys like 'message' or 'error'
• Incorrect block names like 'validation_rule'

3. Given this variable definition, what happens if the input is "ab"?

variable "username" {
  type = string
  validation {
    condition = length(var.username) >= 3
    error_message = "Username must be at least 3 characters"
  }
}

medium

A. Terraform apply ignores the validation and warns only

B. Terraform apply succeeds and uses "ab" as username

C. Terraform apply fails with error: Username must be at least 3 characters

D. Terraform apply replaces "ab" with default username

Solution

1. Step 1: Check validation condition

   The condition requires username length >= 3.

2. Step 2: Evaluate input "ab" length

   "ab" length is 2, which is less than 3, so condition fails.

3. Final Answer:

   Terraform apply fails with error: Username must be at least 3 characters -> Option C

4. Quick Check:

   Input length < 3 triggers error = A [OK]

Hint: Check if input meets condition; if not, apply fails with error [OK]

Common Mistakes:

• Assuming apply succeeds despite validation failure
• Thinking validation only warns, not errors
• Believing default values replace invalid input

4. Identify the error in this variable validation block:

variable "port" {
  type = number
  validation {
    condition = var.port > 0 && var.port < 65536
    error_message = "Port must be between 1 and 65535"
  }
}

medium

A. Using 'number' type instead of 'number' is invalid

B. Validation condition uses 'var.port' instead of 'self'

C. Error message is missing a period at the end

D. Validation block must be outside the variable block

Solution

1. Step 1: Understand validation condition context

   Inside validation, use self to refer to the variable value, not var.port.

2. Step 2: Identify incorrect usage

   The condition incorrectly uses var.port, which is not best practice.

3. Final Answer:

   Validation condition uses 'var.port' instead of 'self' -> Option B

4. Quick Check:

   Use 'self' in validation condition = B [OK]

Hint: Use 'self' to refer to variable value inside validation [OK]

Common Mistakes:

• Using 'var.variable_name' inside validation condition
• Placing validation block outside variable block
• Ignoring syntax errors in condition

5. You want to validate a list variable so it only accepts lists with exactly 3 strings, each at least 2 characters long. Which validation condition is correct?

hard

A. condition = length(self) == 3 && all([for s in self : length(s) >= 2])

B. condition = length(self) == 3 && all([for s in self : s > 2])

C. condition = length(self) == 3 && alltrue([for s in self : s >= 2])

D. condition = length(self) == 3 && alltrue([for s in self : length(s) >= 2])

Solution

1. Step 1: Check list length condition

   We want exactly 3 items, so length(self) == 3 is correct.

2. Step 2: Validate each string length

   Use all() to ensure all elements satisfy length(s) >= 2.

3. Step 3: Identify correct function and condition

   all() is the modern function; alltrue() is legacy.

4. Final Answer:

   condition = length(self) == 3 && all([for s in self : length(s) >= 2]) -> Option A

5. Quick Check:

   Use 'all' with length checks and list length = D [OK]

Hint: Use 'all' function and 'self' for list validation [OK]

Common Mistakes:

• Using 'alltrue' instead of 'all'
• Comparing strings directly to numbers
• Using 's > 2' instead of 'length(s) >= 2'