Variable validation blocks in Terraform - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of a variable validation block in Terraform?

A variable validation block checks if the input value for a variable meets certain rules before applying the configuration. It helps catch errors early by enforcing constraints.

beginner

How do you define a validation block inside a Terraform variable?

Inside the variable block, you add a validation block with a condition expression and an error message. For example:

variable "age" {
  type = number
  validation {
    condition     = age >= 18
    error_message = "Age must be 18 or older."
  }
}

beginner

What happens if a variable value does not meet the validation condition?

Terraform stops the plan or apply process and shows the error message defined in the validation block. This prevents invalid configurations from being deployed.

intermediate

Can validation blocks use complex expressions?

Yes, validation conditions can use any valid Terraform expression, including functions, comparisons, and logical operators to create complex rules.

intermediate

Why is using variable validation blocks considered a best practice?

They improve reliability by catching invalid inputs early, reduce debugging time, and make configurations easier to understand and maintain.

What keyword starts a validation block inside a Terraform variable?

A validation

B check

C assert

D verify

The correct keyword to define a validation block inside a variable is 'validation'.

What must a validation block always include?

A description and default

B type and default

C condition and error_message

D condition and default

A validation block requires a 'condition' expression and an 'error_message' to show if the condition fails.

If a variable fails validation, what does Terraform do?

A Ignores the error and continues

B Stops and shows the error message

C Uses the default value automatically

D Logs a warning but applies anyway

Terraform stops the process and shows the error message to prevent invalid inputs.

Can you use functions inside a validation condition?

A No, only simple comparisons are allowed

B Only string functions are allowed

C Only arithmetic functions are allowed

D Yes, any valid Terraform expression is allowed

Validation conditions can use any valid Terraform expression, including functions.

Which of these is a good reason to use variable validation blocks?

A To catch invalid inputs early

B To speed up Terraform apply

C To automatically fix errors

D To reduce variable declarations

Validation blocks help catch invalid inputs early, improving reliability.

Explain how to create a variable validation block in Terraform and what it does.

Think about how you check if a value is allowed before using it.

Why should you use variable validation blocks in your Terraform configurations?

Consider how catching mistakes early helps in real life.

Practice

1. What is the main purpose of a validation block inside a Terraform variable?

easy

A. To check if the input value meets specific rules before applying the configuration

B. To assign a default value to the variable

C. To declare the variable type

D. To output the variable value after deployment

Solution

1. Step 1: Understand variable validation purpose

   The validation block is used to enforce rules on input values to prevent invalid configurations.

2. Step 2: Differentiate from other variable features

   Default values assign fallback values, type declares data type, and output shows results, but validation specifically checks input correctness.

3. Final Answer:

   To check if the input value meets specific rules before applying the configuration -> Option A

4. Quick Check:

   Validation block purpose = input checking [OK]

Hint: Validation blocks check inputs before use [OK]

Common Mistakes:

• Confusing validation with default value assignment
• Thinking validation outputs variable values
• Mixing validation with type declaration

2. Which of the following is the correct syntax to add a validation block inside a Terraform variable?

easy

A. variable "example" { validate { condition = length(var.example) > 0 error = "Must not be empty" } }

B. variable "example" { validation { condition = length(var.example) > 0 error_message = "Must not be empty" } }

C. variable "example" { validation { check = length(var.example) > 0 message = "Must not be empty" } }

D. variable "example" { validation { condition = length(example) > 0 error_message = "Must not be empty" } }

Solution

1. Step 1: Identify correct block and attribute names

   The correct block is validation with attributes condition and error_message.

2. Step 2: Check variable references and syntax

   Inside the condition, use var.example to refer to the variable value. variable "example" { validation { condition = length(var.example) > 0 error_message = "Must not be empty" } } matches this exactly.

3. Final Answer:

   variable "example" { validation { condition = length(var.example) > 0 error_message = "Must not be empty" } } -> Option B

4. Quick Check:

   Validation syntax = correct block and attributes [OK]

Hint: Use 'validation' block with 'condition' and 'error_message' [OK]

Common Mistakes:

• Using 'validate' instead of 'validation'
• Using wrong attribute names like 'check' or 'error'
• Referencing variable without 'var.' prefix

3. Given this variable declaration:

variable "port" {
  type = number
  validation {
    condition     = var.port >= 1024 && var.port <= 65535
    error_message = "Port must be between 1024 and 65535"
  }
}

What happens if you set port = 80 when applying Terraform?

medium

A. Terraform will apply successfully with port 80

B. Terraform will prompt to enter a valid port

C. Terraform will ignore the validation and use default port

D. Terraform will fail with error: Port must be between 1024 and 65535

Solution

1. Step 1: Analyze the validation condition

   The condition requires the port to be between 1024 and 65535 inclusive.

2. Step 2: Check the input value against the condition

   Port 80 is less than 1024, so the condition fails.

3. Step 3: Understand Terraform behavior on validation failure

   Terraform stops and shows the error message from error_message.

4. Final Answer:

   Terraform will fail with error: Port must be between 1024 and 65535 -> Option D

5. Quick Check:

   Validation fails = error message shown [OK]

Hint: Validation blocks stop apply if condition is false [OK]

Common Mistakes:

• Assuming Terraform applies anyway
• Thinking default values are used automatically
• Expecting interactive prompts for invalid input

4. Identify the error in this variable validation block:

variable "env" {
  type = string
  validation {
    condition     = var.env == "dev" || "prod"
    error_message = "env must be 'dev' or 'prod'"
  }
}

medium

A. Validation blocks cannot use logical OR operators

B. The error_message attribute is misspelled

C. The condition syntax is incorrect; it should compare both values explicitly

D. The variable type should be list, not string

Solution

1. Step 1: Review the condition expression

   The condition var.env == "dev" || "prod" is invalid because "prod" alone is always true.

2. Step 2: Correct the condition syntax

   It should be var.env == "dev" || var.env == "prod" to compare both values explicitly.

3. Final Answer:

   The condition syntax is incorrect; it should compare both values explicitly -> Option C

4. Quick Check:

   Logical OR needs full comparisons [OK]

Hint: Use full comparisons on both sides of OR [OK]

Common Mistakes:

• Writing incomplete logical expressions
• Assuming string alone works as condition
• Confusing error_message spelling

5. You want to validate a list variable users so it must have at least 2 unique names and none can be empty strings. Which validation block correctly enforces this?

hard

A. validation { condition = length(var.users) >= 2 && length(distinct(var.users)) == length(var.users) && alltrue([for u in var.users : u != ""]) error_message = "Users must have 2+ unique non-empty names" }

B. validation { condition = length(var.users) > 2 && distinct(var.users) != [] error_message = "Users must have 2+ unique names" }

C. validation { condition = length(var.users) >= 2 && var.users != [""] error_message = "Users must not be empty" }

D. validation { condition = length(var.users) >= 2 && length(var.users) == length(distinct(var.users)) error_message = "Users must have unique names" }

Solution

1. Step 1: Check list length and uniqueness

   Condition requires at least 2 items and all must be unique, so length(var.users) >= 2 and length(distinct(var.users)) == length(var.users) ensure this.

2. Step 2: Ensure no empty strings

   The alltrue([for u in var.users : u != ""]) checks every user is not empty.

3. Step 3: Compare options

   validation { condition = length(var.users) >= 2 && length(distinct(var.users)) == length(var.users) && alltrue([for u in var.users : u != ""]) error_message = "Users must have 2+ unique non-empty names" } includes all these checks correctly; others miss empty string check or uniqueness properly.

4. Final Answer:

   validation { condition = length(var.users) >= 2 && length(distinct(var.users)) == length(var.users) && alltrue([for u in var.users : u != ""]) error_message = "Users must have 2+ unique non-empty names" } -> Option A

5. Quick Check:

   All conditions combined = validation { condition = length(var.users) >= 2 && length(distinct(var.users)) == length(var.users) && alltrue([for u in var.users : u != ""]) error_message = "Users must have 2+ unique non-empty names" } [OK]

Hint: Combine length, distinct, and alltrue for list validation [OK]

Common Mistakes:

• Missing empty string check
• Using > 2 instead of >= 2 for minimum count
• Not verifying uniqueness correctly