terraform.tfvars vs variables.tf: What is the Difference?

TLDR

The variables.tf file defines input variables and their types, while the terraform.tfvars file provides values for those variables. Use variables.tf for declarations and terraform.tfvars for environment-specific values.

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Terraform uses variables to make configurations dynamic and reusable. Two common files for managing variables are variables.tf and terraform.tfvars. This guide will explain the difference between these files and how to use them effectively.

What is variables.tf?

The variables.tf file is used to declare input variables and their types. It defines the structure and constraints for variables.

Example: variables.tf

Generally, variables.tf contains variable declarations with descriptions and types. Here's a simple example:

variable "region" {
  description = "The AWS region to deploy resources in."
  type        = string
}

variable "instance_type" {
  description = "The type of EC2 instance."
  type        = string
  default     = "t2.micro"
}

What is terraform.tfvars?

The terraform.tfvars file is used to provide values for the variables declared in variables.tf. It is typically used for environment-specific configurations.

Example: terraform.tfvars

A terraform.tfvars file contains key-value pairs that correspond to the variables declared in variables.tf. Here's an example:

region         = "us-east-1"
instance_type  = "t3.medium"

Key Differences

Feature	variables.tf	terraform.tfvars
Purpose	Declares variables and their types	Provides values for variables
Required in Project	Yes	No
Environment-Specific	No	Yes
Default Values	Can define defaults	Overrides defaults

How Terraform Uses These Files

1. Declaration: Terraform reads variables.tf to understand the variables and their constraints.
2. Value Assignment: Terraform reads terraform.tfvars to assign values to the variables.
3. Override: Command-line flags or environment variables can override values in terraform.tfvars.

Best Practices

• Use variables.tf for Declarations: Keep all variable declarations in variables.tf for consistency.
• Use terraform.tfvars for Environment-Specific Values: Store values that change between environments in terraform.tfvars.
• Avoid Hardcoding: Do not hardcode values in variables.tf. Use terraform.tfvars or other methods to provide values.
• Document Variables: Add descriptions to all variables in variables.tf for better maintainability.

By understanding the difference between variables.tf and terraform.tfvars, you can manage variables in Terraform more effectively, making your configurations dynamic and reusable.