Terraform: Undeclared Managed Resource Error - How to Fix

• Introduction
• Step-by-Step Guide
• Code Example
• Solutions:
• Additional Notes
• Summary
• Conclusion
• References

Encountering the error message "A managed resource ... has not been declared in the root module" in Terraform can be frustrating, but it usually points to a simple referencing issue. This error means Terraform can't locate the resource you're trying to use. Let's explore common troubleshooting steps to resolve this:

The error message "A managed resource ... has not been declared in the root module" means Terraform can't find the resource you're referencing. Here's how to troubleshoot:

1. Verify Resource Name: Ensure the resource name in your reference matches the actual resource declaration.

   resource "aws_instance" "example" { ... } 
   # Later...
   output "instance_ip" {
     value = aws_instance.example.private_ip 
   }

2. Check for Typos: Double-check for any spelling errors in both the resource declaration and reference.

3. Resource in the Same Scope: If using modules, the resource needs to be declared within the same module or a parent module where it's being referenced.

4. Module Outputs: If referencing a resource from a different module, ensure the module outputs the resource's attributes you need.

   # In the module
   output "subnet_id" {
     value = aws_subnet.example.id
   }
   
   # In the root module
   module "network" { ... }
   resource "aws_instance" "example" {
     subnet_id = module.network.subnet_id 
   }

5. Data Sources for External Resources: If referencing a resource created outside your Terraform code, use a data source to fetch its information.

   data "aws_subnet" "example" {
     id = "subnet-1234567890abcdef0" 
   }

6. Refresh State: Sometimes Terraform's state can become outdated. Try running terraform refresh to update it.

7. Avoid config.yml for Resources: While you can use YAML for variables, define your infrastructure resources directly in .tf files for better Terraform integration.

This code snippet illustrates a common Terraform error: "A managed resource ... has not been declared in the root module." It occurs when a module attempts to reference a resource defined in another module without proper linkage. The example shows a 'compute' module trying to use a subnet created in a 'network' module, causing the error. Two solutions are provided: 1) Using module outputs to expose the subnet ID from the 'network' module for the 'compute' module to use. 2) Utilizing a data source if the subnet is external to the current Terraform project. These methods ensure correct resource referencing and resolve the error.

Let's say you have a Terraform project with two modules: network and compute. The network module creates a VPC and subnet, while the compute module aims to launch an instance within that subnet.

network/main.tf:

resource "aws_vpc" "example" {
  cidr_block = "10.0.0.0/16"
}

resource "aws_subnet" "example" {
  vpc_id     = aws_vpc.example.id
  cidr_block = "10.0.1.0/24"
}

compute/main.tf:

resource "aws_instance" "example" {
  ami           = "ami-0c55b159c9e67027c"
  instance_type = "t2.micro"
  subnet_id     = aws_subnet.example.id  # Incorrect reference!
}

Running terraform apply in the compute module will result in the error:

Error: A managed resource "aws_subnet.example" has not been declared in the root module.

This is because the aws_subnet.example resource is defined in the network module, not within the scope of the compute module.

1. Module Outputs:

Modify the network module to output the subnet ID:

network/main.tf:

# ... (existing VPC and subnet resources)

output "subnet_id" {
  value = aws_subnet.example.id
}

Then, reference this output in the compute module:

compute/main.tf:

resource "aws_instance" "example" {
  ami           = "ami-0c55b159c9e67027c"
  instance_type = "t2.micro"
  subnet_id     = module.network.subnet_id 
}

2. Data Source (if subnet is external):

If the subnet was created outside of your Terraform code, use a data source:

compute/main.tf:

data "aws_subnet" "example" {
  id = "subnet-1234567890abcdef0" # Replace with actual subnet ID
}

resource "aws_instance" "example" {
  ami           = "ami-0c55b159c9e67027c"
  instance_type = "t2.micro"
  subnet_id     = data.aws_subnet.example.id
}

Remember to replace placeholder values with your actual resource names and IDs. By following these steps, you can resolve the "A managed resource ... has not been declared in the root module" error and ensure your Terraform code correctly references the resources it needs.

• Understanding the Error: The "A managed resource ... has not been declared in the root module" error is fundamentally about scope. Terraform needs to know where a resource is defined to use it. The "root module" refers to the main entry point of your Terraform project.
• Importance of Modules: Modules are crucial for organizing and reusing Terraform code. They create logical boundaries, but also introduce the concept of scope that needs to be managed correctly.
• Outputs as Interfaces: Think of module outputs as the public interface of that module. They expose specific values that other parts of your code can use without needing to know the internal implementation details of the module.
• Data Sources vs. Resources:
  • Resources: Manage infrastructure components (create, update, delete).
  • Data Sources: Fetch information about existing infrastructure, whether managed by Terraform or not.
• State File Awareness: Terraform's state file tracks the resources it manages. If you manually create or modify resources outside of Terraform, the state can become inconsistent, leading to errors. terraform refresh helps synchronize the state.
• YAML for Variables, Not Infrastructure: While YAML can store variable values, defining your actual infrastructure resources (like aws_instance, aws_subnet) directly in .tf files provides better clarity and integration with Terraform's planning and execution.

By understanding these points, you'll be better equipped to diagnose and fix "A managed resource ..." errors in your Terraform projects.

This error means Terraform can't locate the resource you're trying to use. Here's a breakdown of common causes and solutions:

| Issue | Solution

By addressing these common causes and implementing the appropriate solutions, you can effectively troubleshoot and resolve the "A managed resource ... has not been declared in the root module" error in your Terraform projects. Understanding the concepts of scope, module outputs, and data sources is essential for writing clean and maintainable Terraform code that accurately references and manages your infrastructure resources. Remember to keep your Terraform state file consistent and leverage the power of modules and data sources to create robust and reusable infrastructure deployments.

• Error: A managed resource "aws_sqs_queue ... | Posted by u/Epicnikos13 - No votes and 3 comments
• Cant solve a " not been declared in the root module " ERROR ... | i have a single folder in which i have 2 files config.yml main.tf in my config.yml the data is - ami: ami-12345678 instance_type: t2.micro name: my-instance in my main.tf i have - provider “aws” { region = “us-east-2” access_key= “AKIASDBYCJ62########” secret_key= “B48Vgh4nRaTMuQDpki8ih2/TYR1M####@@@@#@#@” } locals { my_yaml_data = yamldecode(file(“./config.yml”)) } resource “aws_instance” “my_instance” { source = “./config.yml” ami = config.yml.ami instance_type = config.yml.ins...
• A managed resource "azurerm_subnet" "this" has not been declared ... | Terraform Configuration Files data-sources.tf data "azurerm_subnet" "this" { name = var.subnet_name resource_group_name = var.virtual_network_resource_group_name virtual_network_name = var.virtual_...
• A managed resource "..." "..." has not been declared in the root module | Upgraded to: Terraform v0.12.29 (+) provider.aws v3.1.0 Keep getting error messages on managed resources. Wondering if anyone has run into this. Error: Reference to undeclared resource on route.tf line 15, in resource “aws_route_table_association” “public-subnet-us-east-2-association”: 15: subnet_id = aws_subnet.public-subnet-us-east-2.id A managed resource “aws_subnet” “public-subnet-us-east-2” has not been declared in the root module.
• Solved: Terraform on google cloud - Google Cloud Community | I am trying to create resources and services on google cloud using terraform. following up this guide https://cloud.google.com/docs/terraform/get-started-with-terraform   In this guide we create a vpc network and then a subnet and then the vm. Then we can attach the vm to the subnet we created. But ...
• A managed resource "aws_secretsmanager_secret_version ... | Attempting to use AWS secrets manager and jsondecode throws the following errors: on rds.tf line 27, in resource "aws_db_instance" "dev-test": 27: username = jsondecode(aws_secretsmanager_secret_ve...
• Terraforming Resources into Modules | by Nick Miller | Medium | I dove this week into Modules, an important layer of abstraction for turning Infrastructure into Code.
• amazon web services - Terraform - Unable to get subnet id - Server ... | Feb 4, 2022 ... but everytime I'm getting error: "A managed resource "aws_subnet" "test_subnet" has not been declared in the root module." In the root ...
• SweetOps #terragrunt for June, 2023 | Jun 22, 2023 ... module "ec2-instance" { source = "terraform-aws-modules ... │ A managed resource "locals" "common_tags" has not been declared in the root │ module ...