KodeKloud Notes

The Terragrunt dependency block lets you declare dependencies between modules, enabling one module to consume another’s outputs. This promotes modular infrastructure, improves code reuse, and streamlines inter-module communication.

Note

Using dependency blocks helps enforce a clear execution order and guarantees that required resources are available before they’re referenced.

Key Attributes

Attribute	Type / Default	Description
name	string	Identifier for the dependency block. Useful when you have multiple dependencies.
config_path	string	Path to the Terragrunt module or folder with the `terragrunt.hcl`.
enabled	boolean / `true`	Controls whether Terragrunt fetches this dependency. Set to `false` to skip resolution.
skip_outputs	boolean / `false`	If `true`, skips executing `terragrunt output`, reducing runtime when outputs aren’t needed.
mock_outputs	map	Key-value pairs to return when the target module has no real outputs.
mock_outputs_allowed	list	Terraform commands (e.g., `plan`, `apply`) for which mock outputs can be used.
mock_outputs_merge_strategy_with_state	string	Strategy for merging mock outputs with existing remote state (e.g., `merge`, `overwrite`).

Benefits of Using Dependency Blocks

Modularization & Reusability
Break your infrastructure into discrete modules that depend on each other’s outputs.
Output References
Access values via dependency.<name>.outputs.<key> expressions.
Selective Execution
Temporarily disable dependencies using the enabled flag.
Mock Outputs for Testing
Simulate module outputs during validate or plan without applying the dependent module.
Performance Optimization
Skip unnecessary state downloads with skip_outputs and let Terragrunt fetch only the required outputs.
Team Collaboration
Independent development and testing of modules with clearly defined input/output contracts.

Optimization Strategies

Terragrunt fetches dependencies in parallel at each root level but parses nested dependencies serially. When using remote state and optimization flags, it retrieves only the lowest-level outputs, significantly speeding up large infrastructures.

Warning

Overusing mock outputs can mask real configuration issues. Always validate with real outputs before production deployments.

Best Practices

Organize dependencies in a dedicated block at the top of your terragrunt.hcl.
Keep config_path references relative and consistent.
Use mock_outputs only for CI/CD testing and validation.
Ensure remote state backends and optimization flags (skip_outputs) are configured for faster runs.
Name your dependencies meaningfully to convey the resource relationship.

Example Usage

1. EC2 Module Without Dependencies

terraform {
  source = "tfr://terraform-aws-modules/ec2-instance/aws//?version=5.6.1"
}

include "root" {
  path   = find_in_parent_folders()
  expose = true
}

include "common" {
  path   = find_in_parent_folders("common.hcl")
  expose = true
}

inputs = {
  name = include.common.locals.project
}

2. Adding a VPC Dependency

By adding a dependency block, you ensure the EC2 instance uses the VPC created by another module:

terraform {
  source = "tfr://terraform-aws-modules/ec2-instance/aws//?version=5.6.1"
}

include "root" {
  path   = find_in_parent_folders()
  expose = true
}

include "common" {
  path   = find_in_parent_folders("common.hcl")
  expose = true
}

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

inputs = {
  name      = include.common.locals.project
  subnet_id = dependency.vpc.outputs.public_subnets[0]
}

This configuration guarantees that the VPC is provisioned before the EC2 instance and that the instance launches in the correct public subnet.

Links and References

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