Terraform: Use Module Output as Input Variable

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

To use the output of one Terraform module as input for another, you'll need to define an output variable in the provider module, call the provider module from the consumer module, and access the output variable in the consumer module. This approach ensures data flow between modules, requires type matching between output and input variables, and promotes well-structured, modular code.

To use the output of one Terraform module as input for another, follow these steps:

1. Define an output variable in the provider module: Within the module that creates the resource you want to reference, declare an output block. Give it a descriptive name and set its value to the attribute of the resource you want to expose. For example, if you have a module that creates a VPC and you want to output the VPC ID, the output block might look like this:

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

2. Call the provider module from the consumer module: In the module that needs to use the output, use a module block to call the provider module. Ensure both modules are in accessible locations within your project structure.

3. Access the output variable in the consumer module: Reference the output variable from the provider module using the syntax module.[module_name].[output_name]. For instance, to use the VPC ID from the previous example in a module named "subnet", you would write:

module "vpc" {
  source = "./vpc"
  # ... other input variables for the VPC module ...
}

resource "aws_subnet" "main" {
  vpc_id = module.vpc.vpc_id
  # ... other subnet configurations ...
}

Key Points:

• Data flow: Terraform manages the execution order, ensuring the provider module is executed before the consumer module that relies on its output.
• Type matching: The type of the output variable in the provider module must match the type of the input variable it's assigned to in the consumer module.
• Module structure: Organize your modules logically to represent different parts of your infrastructure. This makes your code more readable and maintainable.

By following these steps, you can effectively pass information between Terraform modules, enabling you to create reusable and modular infrastructure code.

The code demonstrates Terraform module communication by creating a VPC network and a subnet within that VPC. The VPC module defines the VPC resource and outputs its ID. The subnet module calls the VPC module and receives the VPC ID as input, using it to create a subnet associated with the correct VPC. This showcases modularity and reusability in infrastructure code.

This example demonstrates how to output the VPC ID from a VPC module and use it as input for a subnet module.

Project Structure:

terraform-modules/
├── vpc/
│   └── main.tf
└── subnet/
    └── main.tf

vpc/main.tf:

# Configure the AWS Provider
terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 4.0"
    }
  }
}

# Configure the AWS Region
provider "aws" {
  region = "us-west-2"
}

# Create a VPC
resource "aws_vpc" "main" {
  cidr_block = "10.0.0.0/16"

  tags = {
    Name = "main-vpc"
  }
}

# Output the VPC ID
output "vpc_id" {
  value = aws_vpc.main.id
}

subnet/main.tf:

# Configure the AWS Provider
terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 4.0"
    }
  }
}

# Configure the AWS Region
provider "aws" {
  region = "us-west-2"
}

# Call the VPC module
module "vpc" {
  source = "../vpc"
}

# Create a subnet using the VPC ID from the VPC module
resource "aws_subnet" "main" {
  vpc_id     = module.vpc.vpc_id
  cidr_block = "10.0.1.0/24"
  availability_zone = "us-west-2a"

  tags = {
    Name = "main-subnet"
  }
}

Explanation:

1. vpc/main.tf: This module defines a VPC resource and outputs its ID using the output block.
2. subnet/main.tf: This module calls the vpc module using the module block. It then accesses the vpc_id output variable from the vpc module using module.vpc.vpc_id and uses it as input for the vpc_id argument of the aws_subnet resource.

This example demonstrates how to pass information between Terraform modules, allowing you to create reusable and modular infrastructure code. You can extend this approach to pass other types of data, such as security group IDs, subnet IDs, and more.

Best Practices:

• Descriptive Naming: Use clear and concise names for both output and input variables to enhance code readability.
• Modularity: Break down your infrastructure into logical modules. For example, separate modules for networking, compute, and storage.
• Versioning: Use Git or another version control system to track changes to your modules and ensure consistency across environments.
• Documentation: Document your modules clearly, including input variables, output values, and any dependencies.

Advanced Concepts:

• Output Data Structures: You can output complex data structures like lists, maps, and objects from modules.
• Conditional Outputs: Use conditional expressions within the output block to dynamically define output values based on input or resource attributes.
• Terraform Registry: Leverage pre-built modules from the Terraform Registry to accelerate your infrastructure provisioning.

Troubleshooting:

• Type Errors: Double-check that the output variable type matches the input variable type.
• Circular Dependencies: Avoid creating circular dependencies between modules, where module A depends on module B, and module B depends on module A.
• Terraform State: Understand how Terraform state is managed and how it affects module outputs.

Security:

• Sensitive Data: Avoid outputting sensitive information like passwords or API keys from modules. Use alternative mechanisms like HashiCorp Vault for secrets management.

By understanding these concepts and following best practices, you can effectively use Terraform modules to create reusable, maintainable, and scalable infrastructure code.

This table summarizes how to use the output of one Terraform module as input for another:

Key Considerations:

• Data flow: Terraform automatically manages execution order, ensuring the provider module runs before the consumer module.
• Type matching: The output variable type in the provider module must match the input variable type in the consumer module.
• Module structure: Organize modules logically to represent different infrastructure components for better readability and maintainability.

By following these steps and considering the key points and best practices, you can leverage Terraform modules to create efficient, reusable, and maintainable infrastructure as code. This approach not only streamlines your workflow but also enhances the organization and scalability of your projects. Understanding how to pass data between modules is crucial for harnessing the full potential of Terraform and building complex, interconnected cloud infrastructure.

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