Variable validation rules in Terraform - Step-by-Step Execution

Process Flow - Variable validation rules

Define variable with validation

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Input value provided

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Check validation rule

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Accept value

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Use variable

Terraform checks the variable's input against validation rules. If valid, it proceeds; if invalid, it stops with an error.

Execution Sample

Terraform

variable "env" {
  type = string
  validation {
    condition     = contains(["dev", "prod"], var.env)
    error_message = "env must be 'dev' or 'prod'"
  }
}

Defines a variable 'env' that only accepts 'dev' or 'prod' values.

Process Table

Step	Input Value	Validation Condition	Result	Action
1	"dev"	contains(["dev", "prod"], "dev")	True	Accept value, continue deployment
2	"test"	contains(["dev", "prod"], "test")	False	Show error: env must be 'dev' or 'prod', stop deployment

💡 Execution stops if validation condition is False, preventing invalid variable usage.

Status Tracker

Variable	Start	After Step 1	After Step 2
var.env	undefined	"dev"	Error - invalid value, deployment halted

Key Moments - 2 Insights

Why does Terraform stop deployment when the variable value is invalid?

Terraform enforces validation rules strictly. As shown in execution_table step 2, when the condition is false, it shows the error message and stops to prevent misconfiguration.

Can the validation condition use complex expressions?

Yes, the condition can use any expression that returns true or false. It runs at plan/apply time to check the input value, as seen in the condition field in execution_table.

Visual Quiz - 3 Questions

Look at the execution table, what happens when the input value is "dev"?

A Terraform stops deployment with an error

B Terraform ignores the validation

C Terraform accepts the value and continues

D Terraform changes the value automatically

💡 Hint

Refer to execution_table row 1 where the condition is true and action is to accept value.

At which step does the validation condition become false?

A Step 2

B Step 1

C Never

D Both steps

💡 Hint

Check execution_table row 2 where input "test" fails the condition.

If we add "stage" to the allowed list in the condition, how would step 2 change?

A Step 2 would pass validation if input is "stage"

B Step 2 would still fail validation

C Step 2 would cause a syntax error

D Step 2 would be skipped

💡 Hint

Step 2 input is "test". Adding "stage" to ["dev", "prod"] results in ["dev", "prod", "stage"], which still does not contain "test", so it fails.

Concept Snapshot

Terraform variable validation rules:
- Define validation block inside variable
- Use condition expression returning true/false
- Provide error_message for invalid input
- Deployment stops if validation fails
- Helps catch config errors early

Full Transcript

Terraform variable validation rules let you check if a variable's input value meets certain conditions before deployment proceeds. You define a validation block inside the variable with a condition expression that returns true or false. If the input value passes the condition, Terraform accepts it and continues deployment. If it fails, Terraform shows the error message and stops deployment to prevent misconfiguration. This ensures only valid values are used, catching errors early. The validation condition can be any expression, such as checking if the value is in a list of allowed strings.