How to Access a Module Declared on the Parent Level in Terraform

Learn how to effectively access a parent-level module in Terraform! This blog provides a structured approach and clear examples for managing your AWS infrastructure efficiently.
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This video is based on the question https://stackoverflow.com/q/77628751/ asked by the user 'Dean Christian Armada' ( https://stackoverflow.com/u/5568405/ ) and on the answer https://stackoverflow.com/a/77629275/ provided by the user 'Mark B' ( https://stackoverflow.com/u/13070/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Accessing Modules in Terraform: The Challenge of Referencing Outputs

In the world of Terraform, particularly when dealing with multiple AWS accounts, structuring your code correctly is essential. One common challenge developers face is how to access modules declared at the parent level from within sub-modules. In this guide, we will delve into this issue, providing a clear solution to ensure you can manage your AWS infrastructure effectively.

The Scenario: Structuring Your Terraform Code

Imagine you are managing your AWS infrastructure through Terraform, utilizing a modular approach for optimal organization. You have a network account where you create your VPCs, subnets, and security groups. Additionally, you have a sandbox account where you will be deploying your EC2 instances and RDS.

Here’s an overview of your folder structure, which embodies this modular approach:

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The content of your root main.tf file additionally reflects your configuration for both the network and sandbox environments.

Root Main.tf Content

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The Problem

Your sandbox/main.tf attempts to reference an output from the network module like so:

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However, when you execute terraform apply, you receive an error indicating there is no module call named network declared in module.sandbox. This is where many developers hit a roadblock.

The Solution: Properly Passing Variables Between Modules

To resolve this issue, you need to adjust your approach to variable passing. Specifically, you cannot directly reference a module defined in the parent from within a submodule. Instead, you must pass the necessary output values as input variables.

Step-by-step Solution

Update Your Sandbox Module

Modify the sandbox module to accept an input variable for the VPC ID. This way, when you create the module, you can provide the VPC ID as an input.

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Ensure Your Sandbox Module Receives the Variable

Next, within the sandbox module, you must reference this variable when passing it to the sg module.

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Conclusion

By implementing these adjustments, you can successfully access outputs from your parent modules in your sandbox modules without encountering errors. This structured approach not only enhances the functionality of your code but also maintains the modularity necessary for managing complex AWS infrastructures efficiently.

Now you can streamline your Terraform scripts and confidently deploy your resources across multiple AWS accounts!

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