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

Process Flow - Variable validation blocks

Define variable with validation block

↓

Input value provided

↓

Check validation condition

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Accept value

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Stop deployment

Terraform checks the variable's input against the validation condition. If it passes, deployment continues; if not, 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'"
  }
}

This variable 'env' only accepts 'dev' or 'prod'. If another value is given, Terraform shows an error and stops.

Process Table

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

💡 Deployment stops if validation condition is false to prevent invalid inputs.

Status Tracker

Variable	Start	After Step 1	After Step 2
var.env	undefined	"dev" (valid)	"test" (invalid, error)

Key Moments - 2 Insights

Why does Terraform stop deployment when the validation fails?

Terraform stops because the validation block's condition returned false (see execution_table step 2), which means the input does not meet the required rules.

Can the validation block change the variable value?

No, the validation block only checks the value. It does not modify it. If invalid, it stops deployment instead (see execution_table).

Visual Quiz - 3 Questions

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

A Deployment stops with an error

B Value is changed to "prod" automatically

C Value is accepted and deployment continues

D Terraform ignores the validation block

💡 Hint

Check execution_table row 1 under 'Action' column

At which step does the validation condition become false?

A Step 1

B Step 2

C Validation never fails

D Before step 1

💡 Hint

Look at execution_table 'Condition Result' column

If the validation condition was changed to allow "test", how would the execution table change?

A Step 2 would accept "test" and continue deployment

B Step 1 would fail instead

C Both steps would stop deployment

D No change in execution table

💡 Hint

Think about how condition result affects action in execution_table

Concept Snapshot

Terraform variable validation blocks check input values.
Syntax: validation { condition = <bool>; error_message = "msg" }
If condition is false, deployment stops with error.
Validation does not change values, only verifies them.
Use to enforce allowed inputs and prevent mistakes.

Full Transcript

Terraform variable validation blocks let you set rules for input values. When you run Terraform, it checks if the input meets the condition. If yes, deployment continues. If no, Terraform stops and shows the error message. This helps avoid mistakes by catching wrong inputs early. The validation block only checks values; it does not change them. For example, if you want a variable to be only 'dev' or 'prod', you write a condition to check that. If someone enters 'test', Terraform will stop and show an error. This process ensures your infrastructure is configured correctly before applying changes.