Lesson 3: Terragrunt & DRY Infrastructure

🧠 The Concept (Explain Like I'm 5)

If you build 10 Lego houses (10 environments), you don't want to rewrite the entire instruction manual 10 times. Terragrunt is a wrapper for Terraform. It lets you write the instruction manual once (Terraform Module) and just pass in a tiny sticky note (Terragrunt configuration) that says "This one is Blue (Dev)" or "This one is Red (Prod)".

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🏢 The Enterprise Context

• DRY (Don't Repeat Yourself): In pure Terraform, you must copy/paste the remote backend configuration (S3/DynamoDB) into every single folder. Terragrunt allows you to define the backend once at the root level and all child folders inherit it.
• Dependency Management: Terragrunt can explicitly orchestrate deployments, ensuring the VPC module deploys before the EKS cluster module.

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🗺️ Visual Architecture: Terragrunt Inheritance

flowchart TD
    subgraph GitRepo ["GitOps Infrastructure Repo"]
        Root["terragrunt.hcl<br/>(Root: Defines S3 Backend & AWS Provider)"]

        subgraph Dev ["/dev"]
            DevTG["terragrunt.hcl<br/>(Includes root, inputs: env=dev)"]
        end

        subgraph Prod ["/prod"]
            ProdTG["terragrunt.hcl<br/>(Includes root, inputs: env=prod)"]
        end
    end

    Root -.->|Inherited by| DevTG
    Root -.->|Inherited by| ProdTG

    DevTG -->|Calls| Module["Terraform EKS Module<br/>(Git Tag v1.2.0)"]
    ProdTG -->|Calls| Module