Using One Module's Local Variable in Another Module in Terraform

When working with Terraform, managing resources across multiple modules is a common practice. However, one of the challenges is efficiently sharing information between these modules. This can include using a variable from one module within another. In Terraform, direct local variable sharing across modules is not inherently supported. Instead, you follow best practices using outputs, inputs, state files, and possibly the data block.

Understanding Terraform Modules

To appreciate the complexities and methods for using one module's local variable in another module, you must understand how modules work in Terraform.

What is a Module?

A module is a container for multiple resources that are used together. Every Terraform configuration has at least one module (the root module), and it can then call zero or more child modules.

1. Structure: A module typically contains resource definitions, input variables, output values, and any local variables needed to support these.
2. Reusability: Modules promote reusability, allowing you to manage configurations efficiently and reduce repetition.

Sharing Data Between Modules

Because local variables are scoped to a module, they can't be directly accessed by other modules. However, you can effectively share data by understanding and using variables and outputs effectively.

The Role of Outputs

Outputs help in passing data from one module to other modules or to external systems. By defining output values in a module, you make certain information available outside the module's boundary.

Example: Using Outputs to Share Data

Let's consider two modules: `module_a` and `module_b`. Suppose we want to use a local variable from `module_a` in `module_b`.

`module_a` Definition

• Modular Independence: Ensure each module can be used independently when possible. Avoid excessive inter-dependence that complicates configurations.
• Modular Outputs: Carefully design outputs to provide necessary data without exposing too much details.
• Inputs and Variables: Use input variables judiciously to parameterize module configurations effectively.
• Remote State: Alternatively, if tight coupling is needed and state boundaries are consistent, consider using `terraform_remote_state` to access shared data.