Even DRYer configuration

note

This step outlines optional optimizations.

This guide takes a conservative approach to refactoring the Terragrunt configurations. However, there are additional Terragrunt features that can be leveraged to further DRY your configuration. Note that these features requires a bit more planning and thinking than a step by step tutorial can offer, and thus we cannot provide a step by step tutorial as we offered above. Instead, we will describe the feature that Terragrunt offers with an example, and you can use that as a guide to identify further blocks and attributes that can be moved to the common component configuration.

Deep merge
Expose

Deep merge

Terragrunt supports deep merging included configuration files. By default Terragrunt shallow merges the included configuration, which means that keys that overlap are replaced instead of recursively merged. An included configuration can be deep merged into the current configuration when the merge_strategy attribute is set to "deep". During a deep merge, the following happens:

For simple types (e.g., string and number), the child overrides the parent.
For lists, the two attribute lists are combined together in concatenation.
For maps, the two maps are combined together recursively. That is, if the map keys overlap, then a deep merge is performed on the map value.
For blocks, if the label is the same, the two blocks are combined together recursively. Otherwise, the blocks are appended like a list. This is similar to maps, with block labels treated as keys.

This allows you to define common settings for a complex input variable in the common component configuration, and have the child only inject or override a subset of the attributes.

For example, the k8s-service Service Catalog Terraform module takes in the container image in the format:

type = object({
  # Repository of the docker image (e.g. gruntwork/frontend-service)
  repository = string
  # The tag of the docker image to deploy.
  tag = string
  # The image pull policy. Can be one of IfNotPresent, Always, or Never.
  pull_policy = string
})

It is typical for only the tag attribute to be different across environments. In the guide, you would have to repeat this configuration across the environments, but with deep merge, you can promote the repository and pull_policy attributes to the common component configuration:

common component configuration
inputs = {
  container_image = {
    repository  = "gruntwork/aws-sample-app"
    pull_policy = "IfNotPresent"
  }
}

dev configuration
include "parent" {
  path           = "/path/to/parent/configuration"
  merge_strategy = "deep"
}

inputs = {
  container_image = {
    tag = "v0.0.4"
  }
}

stage configuration
include "parent" {
  path           = "/path/to/parent/configuration"
  merge_strategy = "deep"
}

inputs = {
  container_image = {
    tag = "v0.0.3"
  }
}

In this way, you can leverage deep merge to refactor complex nested inputs in your Terragrunt configuration to further DRY up the config.

Expose `include`

Terragrunt offers the ability to reference values defined in parent configurations through exposed includes. This feature is enabled when the expose = true attribute is set on the include block. Exposed include blocks allow the child configuration to reference values that are defined in the parent configuration. These values are available with the reference include.LABEL.

This is most useful to reference locals that are defined in the common component configuration to reuse them in the child.

For example, if you wanted to test a new version of the module only in dev:

common
locals {
  source_base_url = "git::git@github.com:gruntwork-io/terraform-aws-service-catalog.git//modules/networking/vpc"
}

terraform {
  source = "${local.source_base_url}?ref=v0.65.0"
}

dev
include "parent" {
  path   = "/path/to/parent/configuration"
  expose = true
}

terraform {
  source = "${include.parent.locals.source_base_url}?ref=v0.66.0"
}

Note that the availability of values is subject to the configuration parsing order of Terragrunt. This means that you won’t be able to reference later stage values in early stage blocks, like accessing parent inputs in a child configuration’s locals block, or referencing a parent dependency output in a child configuration’s locals.

You can work around some of this limitation by packing values in the parent configuration’s inputs. Terragrunt passes inputs to Terraform in a way that Terraform ignores input values that do not correspond to an existing variable in the module.

For example, let’s say you want to expose a reference variable in the parent configuration that uses dependency blocks so that you can access it in the child configuration. To do this, you can create a private input value in the parent configuration that references the dependency, e.g.:

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

inputs = {
  # This input variable is for convenience only and is not defined in the underlying Terraform module.
  # We use _ to mark it as "private" and decrease the likelihood of accidentally using a defined variable here.
  _vpc_id = dependency.vpc.outputs.vpc_id
}

You can then access the _vpc_id input variable in the child configuration by using an exposed include block (under the reference, include.PARENT.inputs._vpc_id), like so:

dev
include "parent" {
  path   = "/path/to/parent/configuration"
  expose = true
}

inputs = {
  network_configuration = {
    vpc_id = include.parent.inputs._vpc_id
  }
}

Note that since the dependency reference is in the inputs attribute, you can only reference it in the child configuration’s inputs attribute. You cannot configure a locals block in the child that accesses the parent inputs attribute.

However, in this way, you can bind references in the parent configuration that are then exposed in the child for use in the inputs attribute even if it references dependency blocks.

Even DRYer configuration

Deep merge​

Expose include​

Deep merge

Expose `include`