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Terraform

Terraform: Pass Resources as Module Variables?

By Filip on 10/07/2024

Learn how to effectively manage your infrastructure by passing Terraform resources as variables into modules for improved code organization and reusability.

Terraform: Pass Resources as Module Variables?

Table of Contents

Introduction

You can't directly pass a Terraform resource as a variable into a module. This is because Terraform evaluates resources and variables in a specific order, and modules need to receive their input values before they can create resources. However, there are a few ways to achieve a similar outcome: Pass Resource Attributes as Variables, Use Data Sources, Employ the depends_on Meta-Argument, and Output and Input Chaining. While passing entire resources as variables isn't directly supported, these methods provide flexibility and maintain the desired separation between modules and resource creation. Remember to prioritize clear and concise code organization for better maintainability.

Step-by-Step Guide

You can't directly pass a Terraform resource as a variable into a module. This is because Terraform evaluates resources and variables in a specific order, and modules need to receive their input values before they can create resources.

However, there are a few ways to achieve a similar outcome:

  1. Pass Resource Attributes as Variables: Instead of passing the entire resource, pass its attributes (like ID, name, etc.) as input variables to the module. You can then use these attributes within the module to reference the existing resource.

  2. Use Data Sources: Utilize data sources within your module to fetch information about existing resources. You can pass the necessary arguments to the data source to locate the specific resource you need.

  3. Employ the depends_on Meta-Argument: If you need to ensure a resource is created before a module is provisioned (even without passing its attributes), you can use the depends_on meta-argument within the module block. This establishes an explicit dependency, guaranteeing the resource is created beforehand.

  4. Output and Input Chaining: When dealing with multiple modules, you can output the necessary resource attributes from one module and pass them as input variables to another. This allows modules to share information and reference resources created by other modules.

While passing entire resources as variables isn't directly supported, these methods provide flexibility and maintain the desired separation between modules and resource creation. Remember to prioritize clear and concise code organization for better maintainability.

Code Example

This code demonstrates how to use Terraform modules to manage resources. It shows how to pass resource attributes as variables, use data sources, employ the depends_on meta-argument, and chain outputs and inputs. These techniques help manage resource dependencies and information flow between modules.

This example demonstrates the different ways to interact with resources when using modules in Terraform.

1. Passing Resource Attributes as Variables:

main.tf:

resource "aws_instance" "example" {
  ami           = "ami-0c55b159c9e67025c"
  instance_type = "t2.micro"
}

module "my_module" {
  source = "./modules/my_module"
  instance_id = aws_instance.example.id
}

modules/my_module/main.tf:

resource "aws_volume_attachment" "ebs_attach" {
  device     = "/dev/sdh"
  volume_id   = var.volume_id
  instance_id = var.instance_id
}

variable "instance_id" {}
variable "volume_id" {}

2. Using Data Sources:

main.tf:

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

module "my_module" {
  source = "./modules/my_module"
  subnet_name = aws_subnet.example.name
}

modules/my_module/main.tf:

data "aws_subnet" "selected" {
  filter {
    name   = "tag:Name"
    values = [var.subnet_name]
  }
}

resource "aws_instance" "example" {
  # ... instance configuration ...
  subnet_id = data.aws_subnet.selected.id
}

variable "subnet_name" {}

3. Employing the depends_on Meta-Argument:

main.tf:

resource "aws_security_group" "example" {
  # ... security group configuration ...
}

module "my_module" {
  source = "./modules/my_module"
  depends_on = [aws_security_group.example]
}

4. Output and Input Chaining:

main.tf:

module "vpc_module" {
  source = "./modules/vpc"
}

module "subnet_module" {
  source = "./modules/subnet"
  vpc_id = module.vpc_module.vpc_id
}

modules/vpc/main.tf:

resource "aws_vpc" "example" {
  # ... VPC configuration ...
}

output "vpc_id" {
  value = aws_vpc.example.id
}

modules/subnet/main.tf:

resource "aws_subnet" "example" {
  # ... subnet configuration ...
  vpc_id = var.vpc_id
}

variable "vpc_id" {}

These examples illustrate different approaches to manage resource dependencies and information flow between modules. Choose the method that best suits your specific use case and promotes code clarity and maintainability.

Additional Notes

  • Why this restriction? Terraform's execution model requires it to know the values of all variables before it can determine the resources to create. Passing a resource directly would create a circular dependency.
  • Data Sources vs. depends_on: Data sources are for fetching information about a resource. depends_on is for ensuring a resource is created before a module runs, but doesn't provide its data.
  • Module Composition: Output and input chaining is crucial for building complex systems from smaller, reusable modules.
  • Best Practices:
    • Favor passing minimal information (like IDs) to modules for loose coupling.
    • Document module inputs and outputs clearly.
    • Consider using Terraform's null value and conditional logic within modules to handle optional resources.
  • Alternatives: Tools like Terragrunt can offer more advanced resource referencing capabilities, but come with their own learning curve.
  • Terraform Version Awareness: Features and behaviors can change between Terraform versions. Always consult the documentation for the version you're using.

Summary

Method Description Use Case
Pass Resource Attributes Pass specific attributes (e.g., ID, name) of the resource as input variables to the module. When the module needs to reference or utilize specific properties of an existing resource.
Use Data Sources Employ data sources within the module to fetch information about existing resources. When the module needs to dynamically access information about a resource based on specific criteria.
Employ depends_on Establish an explicit dependency between a resource and a module using the depends_on meta-argument. When the module's provisioning relies on the prior creation of a specific resource, but doesn't require direct access to its attributes.
Output and Input Chaining Output resource attributes from one module and pass them as input variables to another. When multiple modules need to interact and share information about resources created by each other.

Key Takeaway: While directly passing entire resources as variables to Terraform modules is not possible, these alternative methods provide flexibility and maintain modularity in your infrastructure code.

Conclusion

In conclusion, while Terraform's structure doesn't allow passing entire resources as module variables, alternative methods provide the necessary flexibility for managing resources across modules. Passing resource attributes, utilizing data sources, employing the depends_on meta-argument, and chaining outputs and inputs offer robust ways to handle resource dependencies and information flow. Choosing the appropriate method depends on the specific use case and desired level of coupling between modules. As you build more complex infrastructures, mastering these techniques, along with clear documentation and adherence to best practices, will lead to more maintainable and scalable Terraform code.

References

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