Terragrunt dependency and mock_outputs

dependency block

Reads outputs from another Terragrunt unit - cleaner than terraform_remote_state and aware of the apply order.

# environments/prod/eks/terragrunt.hcl
include "root" {
  path = find_in_parent_folders("root.hcl")
}

dependency "vpc" {
  config_path = "../vpc"
}

dependency "rds" {
  config_path = "../rds"
}

inputs = {
  vpc_id          = dependency.vpc.outputs.vpc_id
  private_subnets = dependency.vpc.outputs.private_subnet_ids
  db_endpoint     = dependency.rds.outputs.endpoint
}

mock_outputs

Without mock outputs, terragrunt plan fails if a dependency hasn't been applied yet - it can't fetch real outputs. Mock outputs provide placeholder values for plan only.

dependency "vpc" {
  config_path = "../vpc"

  mock_outputs = {
    vpc_id             = "vpc-00000000"
    private_subnet_ids = ["subnet-00000000", "subnet-11111111"]
  }

  mock_outputs_allowed_terraform_commands = ["plan", "validate"]
}

mock_outputs_allowed_terraform_commands

Controls which commands use mocks - apply always uses real outputs.

mock_outputs_allowed_terraform_commands = ["plan", "validate", "destroy"]

dependency graph

Terragrunt resolves the graph automatically with run-all - units apply in the right order.

vpc  ──►  eks
     ──►  rds  ──►  app

# applies vpc first, then rds and eks in parallel, then app
terragrunt run-all apply

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Originally published at https://bard.sh/posts/terragrunt_dependency/