What is Terragrunt and why is it needed?

Question

Accepted Answer

**Terragrunt** — wrapper over Terraform for DRY configuration, dependency management and multi-environment.

**Terraform limitation:** lots of code duplication for dev/staging/prod.

**Structure with Terragrunt:**
```
infrastructure/
  _envcommon/          # Shared modules
    vpc.hcl
    eks.hcl
  dev/
    env.hcl            # dev-specific variables
    vpc/
      terragrunt.hcl   # Includes _envcommon/vpc.hcl
    eks/
      terragrunt.hcl
  staging/
    env.hcl
    vpc/
      terragrunt.hcl
  production/
    env.hcl
    vpc/
      terragrunt.hcl
```

**env.hcl:**
```hcl
locals {
  aws_region = "us-east-1"
  environment = "dev"
}
```

**terragrunt.hcl (in vpc/ folder):**
```hcl
include "root" {
  path = find_in_parent_folders()
}

include "envcommon" {
  path   = "${get_repo_root()}/_envcommon/vpc.hcl"
  merge_strategy = "deep"
}
```

**_envcommon/vpc.hcl:**
```hcl
terraform {
  source = "${get_repo_root()}/modules/vpc"
}

inputs = {
  vpc_cidr = local.vpc_cidr
  environment = local.environment
}
```

**Key commands:** `terragrunt plan`, `terragrunt apply-all`, `terragrunt run-all apply`.
**Dependencies:** `dependency` block for automatic deployment order (VPC → EKS → Apps).

DevOps: What is Terragrunt and why is it needed?