Terraform Module Troubleshooting Guide

Source: Dev.to

Symptoms

• Error: An argument named "variable_name" is not expected here
• Error: This object does not have an attribute named "output_name"
• These errors occur even when variables/outputs appear to be correctly defined.

Root Causes & Solutions

1. Stale Terraform Module Cache (Most Common)

When you modify module variables or outputs, Terraform may use cached information.

When to use: After any changes to:

• Module variables (variables.tf)
• Module outputs (outputs.tf)
• Module source paths

Solution: Clear the cache and re‑initialize.

2. Empty or Corrupted Files (Critical Issue)

Files may exist but contain 0 bytes of data.

How to check: Look for files with Length: 0 or 0 bytes.

Solution: Re‑create the file using the command line (or your editor) to ensure it saves correctly.

3. Missing Variable Definitions

Terraform reports a variable that doesn’t exist in the module’s variables.tf.

Solution: Add the missing variable to modules//variables.tf.

After adding, clear the cache and re‑initialize (see Solution #1).

4. Missing Output Definitions

The module doesn’t expose the output being referenced.

Solution: Add the output to modules//outputs.tf:

output "output_name" {
  description = "Description"
  value       = azurerm_resource.resource_name.id
}

Then clear the cache and re‑initialize.

Diagnostic Workflow

Step 1: Verify File Structure

Check for:

• All required files exist
• No files with 0 bytes
• Correct directory hierarchy

Step 2: Verify File Contents

Step 3: Clear Cache and Re‑initialize

Step 4: Check Terraform Version

Example variables.tf:

variable "rg_name" {
  description = "Resource group name"
  type        = string
}

variable "location" {
  description = "Azure region"
  type        = string
}

Example outputs.tf

output "resource_id" {
  description = "ID of the created resource"
  value       = azurerm_resource.name.id
}

Example module call in root/main.tf

module "example" {
  source   = "./modules/module_name"
  rg_name  = "my-rg"
  location = "eastus"
}

# Reference module output
resource "other_resource" "example" {
  dependency_id = module.example.resource_id
}

Common Mistakes to Avoid

❌ Don’t

• Edit files without saving (Ctrl + S in VSCode)
• Forget to run terraform init after module changes
• Use module outputs that aren’t defined in outputs.tf
• Pass variables that aren’t defined in variables.tf

✅ Do

• Always verify files are saved correctly (check file size)
• Clear the .terraform cache after module modifications
• Run terraform fmt to format code
• Run terraform validate to catch errors early
• Keep the module interface (variables / outputs) stable

Quick Reference Commands

When All Else Fails

1. Start fresh

2. Validate each module individually