Terraform Conditionals: Handling Non-Existent Variables

TLDR

To handle non-existent variables in Terraform, use the lookup function or the ternary conditional operator. These techniques allow you to provide default values or handle undefined variables gracefully.

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Terraform configurations often rely on variables to define dynamic values. However, there are cases where a variable might not be defined. This guide will show you how to handle such scenarios using Terraform conditionals.

Why Handle Non-Existent Variables?

• Flexibility: Allow configurations to work even when some variables are not defined.
• Default Values: Provide sensible defaults for optional variables.
• Error Prevention: Avoid runtime errors caused by missing variables.

Using the lookup Function

The lookup function retrieves a value from a map and allows you to specify a default value if the key does not exist.

Example: Using lookup

variable "config" {
  type = map(string)
}

output "example" {
  value = lookup(var.config, "key", "default-value")
}

In this example, if var.config does not contain the key key, the output will use default-value.

Using the Ternary Conditional Operator

The ternary operator allows you to check if a variable is defined and provide a fallback value.

Example: Using the Ternary Operator

variable "optional_var" {
  default = null
}

output "example" {
  value = var.optional_var != null ? var.optional_var : "default-value"
}

In this example, if var.optional_var is not defined, the output will use default-value.

Best Practices

• Use Defaults: Define default values for variables whenever possible.
• Validate Inputs: Use validation blocks to enforce constraints on variable values.
• Document Variables: Clearly document which variables are optional and their default values.

By using these techniques, you can handle non-existent variables in Terraform gracefully, making your configurations more robust and flexible.