Variable Keys in Terraform Maps

TLDR

Terraform maps allow you to define key-value pairs for dynamic configurations. You can use variable keys to make your maps more flexible and reusable.

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Maps in Terraform are a powerful way to manage key-value pairs for dynamic configurations. This guide explains how to use variable keys in Terraform maps to create flexible and reusable infrastructure code.

Step 1: Define a Map Variable

Start by defining a map variable in your Terraform configuration.

Example

variable "instance_types" {
  description = "Map of instance types for different environments."
  type        = map(string)
  default = {
    dev  = "t2.micro"
    prod = "t2.large"
  }
}

Explanation

• variable "instance_types": Declares a map variable.
• type = map(string): Specifies that the map contains string values.
• default: Provides default key-value pairs for the map.

Step 2: Access Map Values

Use the map variable to access values dynamically based on keys.

Example

resource "aws_instance" "example" {
  ami           = "ami-12345678"
  instance_type = var.instance_types["dev"]
}

Explanation

• var.instance_types["dev"]: Accesses the value for the dev key in the map.
• instance_type: Sets the instance type dynamically based on the map value.

Step 3: Use Dynamic Keys

Pass dynamic keys to access map values based on other variables.

Example

variable "environment" {
  description = "Deployment environment."
  type        = string
  default     = "dev"
}

resource "aws_instance" "example" {
  ami           = "ami-12345678"
  instance_type = var.instance_types[var.environment]
}

Explanation

• var.environment: Specifies the key dynamically based on the environment variable.
• var.instance_types[var.environment]: Accesses the map value for the specified environment.

Step 4: Validate Keys

Use validation blocks to ensure only valid keys are used.

Example

variable "environment" {
  description = "Deployment environment."
  type        = string

  validation {
    condition     = contains(keys(var.instance_types), var.environment)
    error_message = "Invalid environment. Must be one of: dev, prod."
  }
}

Explanation

• validation: Ensures the environment variable matches a valid key in the map.
• keys(var.instance_types): Retrieves all keys from the map.
• contains: Checks if the key exists in the map.

Best Practices

• Use Descriptive Keys: Clearly indicate the purpose of each key.
• Validate Inputs: Use validation blocks to enforce constraints on keys.
• Document Maps: Provide clear documentation for map variables and their keys.

By following these steps, you can effectively use variable keys in Terraform maps to create dynamic and flexible configurations.