Fixing Terraform Plan Error: Missing Resource Instance Key

Encountering errors during terraform plan can be frustrating, especially after upgrading versions or during a fresh installation. This article addresses a specific error encountered in version 0.2.0 of the terraform-aws-vantage-integration module: "Error: Missing resource instance key". We'll break down the cause of this error and provide a solution to get your Terraform plans running smoothly again.

Understanding the Error

The error message Error: Missing resource instance key arises because the aws_cur_report_definition.vantage_cost_and_usage_reports and aws_s3_bucket.vantage_cost_and_usage_reports resources have their count attribute set. When a resource has a count attribute, Terraform requires you to specify which instance of the resource you're trying to access. Without specifying an index, Terraform doesn't know which instance's attribute to retrieve, hence the error.

Specifically, the error occurs in the outputs.tf file of the vantage-integration module when trying to access attributes like arn, report_name, id without specifying the instance index. The relevant lines in outputs.tf look like this:

output "vantage_cost_and_usage_report_arn" {
  value       = try(aws_cur_report_definition.vantage_cost_and_usage_reports.arn, null)
}

output "vantage_cost_and_usage_report_name" {
  value       = try(aws_cur_report_definition.vantage_cost_and_usage_reports.report_name, null)
}

output "vantage_cost_and_usage_reports_bucket_arn" {
  value       = try(aws_s3_bucket.vantage_cost_and_usage_reports.arn, null)
}

output "vantage_cost_and_usage_reports_bucket_id" {
  value       = try(aws_s3_bucket.vantage_cost_and_usage_reports.id, null)
}

Let's dive deeper into the reasons behind this and how to resolve it.

Root Cause Analysis

The core issue stems from how Terraform handles resources created with the count parameter. When count is used, Terraform creates multiple instances of the resource, each identified by an index. To access a specific instance's attribute, you must use the bracket notation [index] to specify which instance you're referring to. The error message clearly indicates this:

For example, to correlate with indices of a referring resource, use:
    aws_cur_report_definition.vantage_cost_and_usage_reports[count.index]

The try() function is used here to gracefully handle cases where the resource might not exist (e.g., when count is set to 0). However, even with try(), Terraform still needs to resolve the attribute access, which fails when the index is missing.

The problem likely arose in version 0.2.0 due to changes in how the module manages the creation of these resources, potentially introducing or modifying the count parameter without updating the output references accordingly. Even on a fresh installation, this issue would surface because the module's code, specifically the outputs.tf file, contains the incorrect resource attribute access.

Solution: Specifying the Resource Instance Key

The solution is to modify the outputs.tf file to correctly reference the resource instances using the [0] index (assuming you only have one instance, which is often the case). Here's how you can update the output definitions:

output "vantage_cost_and_usage_report_arn" {
  value       = try(aws_cur_report_definition.vantage_cost_and_usage_reports[0].arn, null)
}

output "vantage_cost_and_usage_report_name" {
  value       = try(aws_cur_report_definition.vantage_cost_and_usage_reports[0].report_name, null)
}

output "vantage_cost_and_usage_reports_bucket_arn" {
  value       = try(aws_s3_bucket.vantage_cost_and_usage_reports[0].arn, null)
}

output "vantage_cost_and_usage_reports_bucket_id" {
  value       = try(aws_s3_bucket.vantage_cost_and_usage_reports[0].id, null)
}

By adding [0] to the resource references, you're explicitly telling Terraform to access the attributes of the first instance of the resource. If you have multiple instances, you'll need to adjust the index accordingly, potentially using a for loop or other logic to iterate through the instances.

Step-by-Step Resolution Guide

1. Locate the outputs.tf file: This file is located within the vantage-integration module directory, typically under .terraform/modules/vantage-integration/outputs.tf in your project.
2. Edit the outputs.tf file: Open the file in a text editor and modify the output definitions as shown above, adding the [0] index to the resource references.
3. Save the changes: Save the modified outputs.tf file.
4. Run terraform plan again: Execute terraform plan in your terminal to verify that the error is resolved. If the plan runs without errors, you've successfully fixed the issue.
5. Apply the changes (if the plan is successful): If the plan looks good, run terraform apply to apply the changes to your infrastructure.

Additional Considerations and Best Practices

• Module Versions: Always pin your module versions in your Terraform configuration to avoid unexpected issues caused by updates. Use `version =