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Terraform Input Variables with Validation Rules

Step-01: Introduction

  • Learn some Terraform Functions
  • length()
  • substr()
  • contains()
  • lower()
  • regex()
  • can()
  • Implement Custom Validation Rules in Variables

Step-02: Learn Terraform Length Function

# Go to Terraform Console
terraform console

# Test length function
Template: length()

# String
length("hi")
length("hello")

# List
length(["a", "b", "c"]) 

# Map
length({"key" = "value"}) 
length({"key1" = "value1", "key2" = "value2" })

Step-03: Learn Terraform SubString Function

# Go to Terraform Console
terraform console

# Test substr function
Template: substr(string, offset, length)
substr("stack simplify", 1, 4)
substr("stack simplify", 0, 6)
substr("stack simplify", 0, 1)
substr("stack simplify", 0, 0)
substr("stack simplify", 0, 10)

Step-04: Learn Terraform contains() Function

# Go to Terraform Console
terraform console

# Test contains() function
Template: contains(list, value)
contains(["a", "b", "c"], "a")
contains(["a", "b", "c"], "d")
contains(["eastus", "eastus2"], "westus2")

Step-05: Learn Terraform lower() and upper() Function

# Go to Terraform Console
terraform console

# Test lower() function
Template: lower("STRING")
lower("KALYAN REDDY")
lower("STACKSIMPLIFY")

# Test upper() function
Template: lower("string")
upper("kalyan reddy")
upper("stacksimplify")

Step-06: Create Resource Group Variable with Validation Rules

  • Understand and implement custom validation rules in variables
  • condition: Defines the expression used to evaluate the Input Variable value. Must return either true for valid, or false for invalid value.
  • error_message: Defines the error message displayed by Terraform when the condition expression returns false for an invalid value. Must be ended with period or question mark
  • c2-variables.tf
# 4. Resource Group Location
variable "resoure_group_location" {
  description = "Resource Group Location"
  type = string
  default = "eastus"
  validation {
    condition  = var.resoure_group_location == "eastus" || var.resoure_group_location == "eastus2"
    #condition = contains(["eastus", "eastus2"], lower(var.resoure_group_location))
    error_message = "We only allow Resources to be created in eastus or eastus2 Locations."
  }  
}

Step-07: Run Terraform commands

# Initialize Terraform
terraform init

# Validate Terraform configuration files
terraform validate

# Format Terraform configuration files
terraform fmt

# Review the terraform plan
terraform plan

# Observation
1. When `resoure_group_location = "eastus"`, terraform plan should pass
2. When `resoure_group_location = "eastus2"`, terraform plan should pass
3. When `resoure_group_location = "westus"`, terraform plan should fail with error message as validation rule failed. 

# Uncomment validation rule with contains() function and comment previous one
condition = contains(["eastus", "eastus2"], lower(var.resoure_group_location))

# Review the terraform plan
terraform plan

# Observation
1. When `resoure_group_location = "eastus"`, terraform plan should pass
2. When `resoure_group_location = "eastus2"`, terraform plan should pass
3. When `resoure_group_location = "westus"`, terraform plan should fail with error message as validation rule failed.

Step-08: Learn Terraform regex() and can() Function

# Go to Terraform Console
terraform console

# Test regex() function
Template: regex(pattern, string)
### TRUE CASES
regex("india$", "westindia")
regex("india$", "southindia")
can(regex("india$", "westindia"))
can(regex("india$", "southindia"))

### FAILURE CASES
regex("india$", "eastus")
can(regex("india$", "eastus"))
  • Update Resource Group Location Variable with can() and regex() function related Validation Rule
# 4. Resource Group Location
variable "resoure_group_location" {
  description = "Resource Group Location"
  type = string
  default = "eastus"
  validation {
    #condition  = var.resoure_group_location == "eastus" || var.resoure_group_location == "eastus2"
    #condition = contains(["eastus", "eastus2"], lower(var.resoure_group_location))
    #error_message = "We only allow Resources to be created in eastus or eastus2 Locations."
    condition = can(regex("india$", var.resoure_group_location))
    error_message = "We only allow Resources to be created in westindia and southindia locations."
  }  
}

Step-10: Run Terraform commands

# Validate Terraform configuration files
terraform validate

# Review the terraform plan
terraform plan

# Observation
1. When `resoure_group_location = "westinida"`, terraform plan should pass
2. When `resoure_group_location = "southindia"`, terraform plan should pass
3. When `resoure_group_location = "eastus2"`, terraform plan should fail with error message as validation rule failed.

Step-11: Clean-Up

# Delete Files
rm -rf .terraform*

# Roll back to state as below for Students seamless demo before git check-in
# Change-1: c1-variables.tf
# 4. Resource Group Location
variable "resoure_group_location" {
  description = "Resource Group Location"
  type = string
  default = "eastus"
  validation {
    condition  = var.resoure_group_location == "eastus" || var.resoure_group_location == "eastus2"
    #condition = contains(["eastus", "eastus2"], lower(var.resoure_group_location))
    error_message = "We only allow Resources to be created in eastus or eastus2 Locations."
    #condition = can(regex("india$", var.resoure_group_location))
    #error_message = "We only allow Resources to be created in westindia and southindia locations."
  }  
}

# Change-2: terraform.tfvars
resoure_group_location = "eastus"
#resoure_group_location = "westus2"
#resoure_group_location = "westindia"
#resoure_group_location = "eastus2"

References