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Terraform

Terraform for_each Error: Cannot Determine Resource Attributes

By Ondřej Dolanský on 12/30/2024

Learn how to resolve Terraform errors when your "for_each" value depends on resource attributes that can't be determined until runtime.

Terraform for_each Error: Cannot Determine Resource Attributes

Table of Contents

Introduction

The Terraform error "The "for_each" value depends on resource attributes that cannot be determined until apply" can be frustrating, but it's ultimately rooted in how Terraform operates. This error pops up when you're using the for_each construct with a resource, but the values Terraform needs to populate for_each aren't available until the very moment it applies your changes.

Step-by-Step Guide

The error "The "for_each" value depends on resource attributes that cannot be determined until apply" occurs in Terraform when you're using for_each with a resource, but the values you need for for_each aren't known until Terraform actually applies your changes.

Here's a simplified example:

resource "aws_instance" "example" {
  # ... other configurations ...

  for_each = { for subnet in aws_subnet.example : subnet.id => subnet }
}

resource "aws_subnet" "example" {
  # ... subnet configurations ...
}

In this case, you're trying to create instances based on the available subnets. However, Terraform can't know how many subnets aws_subnet.example will create until it actually starts creating them.

This is a common issue because Terraform plans your infrastructure before applying it. To fix this, you can use the -target argument with terraform apply to first create the resources your for_each depends on:

terraform apply -target=aws_subnet.example

Once the subnets are created, Terraform will know their IDs, and you can run terraform apply again to create the instances.

Code Example

This code demonstrates a Terraform dependency error scenario. The code defines an AWS instance resource that depends on AWS subnets. However, it attempts to use the subnet IDs within a for_each loop before the subnets are actually created, leading to an error. The solution involves using the -target argument with terraform apply to explicitly create the subnets first. Once the subnets are created and their IDs are available, Terraform can successfully create the instances. This highlights the importance of understanding resource dependencies and using appropriate Terraform commands to manage them effectively.

This example showcases the "The "for_each" value depends on resource attributes that cannot be determined until apply" error and how to resolve it.

File: main.tf

terraform {
  required_providers {
    aws = {
      source = "hashicorp/aws"
      version = "~> 4.0"
    }
  }
}

provider "aws" {
  region = "us-east-1" # Replace with your desired region
}

# This resource will cause the error
resource "aws_instance" "example" {
  ami           = "ami-0c55b17e9e212caf0" # Replace with a valid AMI ID for your region
  instance_type = "t2.micro"

  # Trying to use subnet IDs before they are created
  for_each = { for subnet in aws_subnet.example : subnet.id => subnet }

  subnet_id = each.value.id 
}

resource "aws_subnet" "example" {
  count = 2 # Create two subnets
  vpc_id     = aws_vpc.main.id
  cidr_block = cidrsubnet("10.0.0.0/16", 8, count.index)
  availability_zone = "us-east-1a" # Replace with a valid availability zone
}

resource "aws_vpc" "main" {
  cidr_block = "10.0.0.0/16"

  tags = {
    Name = "main"
  }
}

Running the code:

  1. Execute terraform init: This will initialize your Terraform project.
  2. Run terraform apply: You will encounter the error mentioned in the article.
  3. Apply the fix: Execute terraform apply -target=aws_subnet.example to create the subnets first.
  4. Run terraform apply again: This time, Terraform will successfully create the instances as the subnet IDs are now known.

Explanation:

  • The aws_instance resource depends on the aws_subnet.example resource for the subnet_id.
  • Terraform cannot determine the subnet IDs until the aws_subnet.example resource is created.
  • Using -target allows us to create the dependent resource (aws_subnet.example) first.
  • Once the subnets are created, Terraform can access their IDs and successfully create the instances.

This example demonstrates a common scenario where resource dependencies can lead to errors during the Terraform apply phase. By understanding the error message and utilizing the -target argument, you can effectively resolve such issues and ensure a smooth infrastructure provisioning process.

Additional Notes

  • Root of the issue: This error stems from Terraform's core principle of planning infrastructure before applying changes. It cannot predict resource attributes that are generated during the apply phase, leading to this error when those attributes are needed for for_each.
  • Impact of the error: The error prevents Terraform from determining the number of instances to create, hindering the planning process and halting the apply phase.
  • Alternative solutions:
    • Data sources: If possible, use data sources to fetch information about existing resources instead of relying on attributes of resources created within the same Terraform configuration.
    • Resource dependencies: Explicitly define dependencies between resources using the depends_on argument to ensure resources are created in the correct order.
  • Best practices:
    • Modularize your code: Break down your infrastructure into smaller, reusable modules to minimize dependencies and make your code easier to manage.
    • Use descriptive variable names: Clear and concise variable names improve code readability and make it easier to understand resource dependencies.
  • Beyond for_each: This error can also occur with other constructs that rely on resource attributes not known until apply time, such as count and dynamic blocks. The principles and solutions discussed here apply to those situations as well.
  • Terraform versions: While the error message and underlying behavior are consistent across Terraform versions, newer versions might offer improved error messages or alternative solutions. Always refer to the official Terraform documentation for the version you are using.

Summary

This article explains the Terraform error "The "for_each" value depends on resource attributes that cannot be determined until apply" and provides a solution.

Problem: This error occurs when using for_each with a resource that depends on attributes of another resource not yet created. Terraform needs to know these attributes during the planning phase, but they are only available after applying the dependent resource.

Example: Trying to create multiple AWS instances (aws_instance) using for_each based on the IDs of AWS subnets (aws_subnet) before the subnets are actually created.

Solution: Use the -target argument with terraform apply to first create the dependency:

  1. Target the dependent resource: terraform apply -target=aws_subnet.example
  2. Apply remaining resources: After the subnets are created, run terraform apply again to create the instances.

This two-step process ensures Terraform knows the required attributes (subnet IDs in the example) during the second apply command, avoiding the error.

Conclusion

The Terraform error "The "for_each" value depends on resource attributes that cannot be determined until apply" arises from the dependency between a resource using for_each and the attributes of another resource that are only available after its creation. This situation highlights the distinction between Terraform's planning and application stages. To resolve this, you can utilize the -target argument with terraform apply to first create the resource on which the for_each depends. Once the dependent resource is established and its attributes are known, a subsequent terraform apply will proceed without encountering this error. Understanding resource dependencies and leveraging Terraform's capabilities, such as the -target argument, are crucial for effectively managing infrastructure provisioning and avoiding common pitfalls during the Terraform workflow.

References

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