How to use Terragrunt dependencies for multi-module deploys?

Question

Accepted Answer

**Terragrunt dependencies** — automatic apply order based on dependencies.

**dependency block:**
```hcl
# infrastructure/production/eks/terragrunt.hcl
dependency "vpc" {
  config_path = "../vpc"
}

inputs = {
  vpc_id     = dependency.vpc.outputs.vpc_id
  subnet_ids = dependency.vpc.outputs.private_subnet_ids
}
```

**dependency outputs:**
```hcl
# vpc/terragrunt.hcl outputs
output "vpc_id" {
  value = aws_vpc.main.id
}
output "private_subnet_ids" {
  value = aws_subnet.private[*].id
}
```

**run-all (sequential apply):**
```bash
terragrunt run-all apply
# 1. vpc (no dependencies)
# 2. eks (depends on vpc)
# 3. apps (depends on eks)
```

**mock outputs (for plan without apply):**
```hcl
dependency "vpc" {
  config_path = "../vpc"
  mock_outputs = {
    vpc_id             = "vpc-mock123"
    private_subnet_ids = ["subnet-mock1", "subnet-mock2"]
  }
  mock_outputs_allowed_terraform_commands = ["plan"]
}
```

**Deploy order:** `vpc → eks → ingress → apps → monitoring`.

DevOps: How to use Terragrunt dependencies for multi-module deploys?