Certainly! Below is a design document outline that incorporates the suggestions for project division, template file management, environment setup, and module pinning in your Terraform project.

1. Introduction
2. Project Division
3. Template File Management
4. Environment Setup
5. Module Pinning
6. Version Control
7. Conclusion

This document outlines the design and structure for a Terraform project managing multiple downstream clusters with authentication, authorization, and ArgoCD integration. The goal is to divide the project into smaller, manageable modules and templates to streamline development and deployment processes.

The project is divided into several key modules to encapsulate different functionalities and resources:

Contains shared components like VPCs, subnets, and security groups.

Handles IAM roles, policies, and identity provider configurations.

Encapsulates resources for each type of downstream cluster.

Manages ArgoCD installation and configuration on clusters.

For common resources or configurations used by multiple modules.

A template.tfvars file is provided to set default values for common variables across clusters. Users can copy and customize this template for each new cluster.

.
├── templates
│   └── template.tfvars

# template.tfvars
region = "us-west-2"
instance_type = "t2.medium"
instance_count = 3
# ... other default variables

Each environment (dev, staging, production) has its own directory with specific backend.tf and terraform.tfvars.

.
└── environments
    ├── dev
    │   ├── backend.tf
    │   └── terraform.tfvars
    ├── staging
    │   ├── backend.tf
    │   └── terraform.tfvars
    └── production
        ├── backend.tf
        └── terraform.tfvars

Modules are versioned using Git tags or branches. Each environment's terraform.tfvars includes the path to the specific version of the module.

A version control system like Git is used to manage different versions of the tfvars files. Changes are documented in a CHANGELOG.md file.

The proposed structure ensures a clean separation of concerns, making the codebase more maintainable and scalable. It also simplifies the process of creating and managing multiple downstream clusters.

This design document serves as a high-level guide. Detailed implementation instructions should be provided in the actual project repository, along with comments and documentation for each module and template.