A Quick Fix for “Backend initialization required” from Terragrunt

Today I ran terragrunt apply against a IaC directory, and got this response:

╷
│ Error: Backend initialization required: please run "terraform init"
│ 
│ Reason: Backend configuration block has changed
│ 
│ The "backend" is the interface that Terraform uses to store state,
│ perform operations, etc. If this message is showing up, it means that the
│ Terraform configuration you're using is using a custom configuration for
│ the Terraform backend.
│ 
│ Changes to backend configurations require reinitialization. This allows
│ Terraform to set up the new configuration, copy existing state, etc. Please
│ run
│ "terraform init" with either the "-reconfigure" or "-migrate-state" flags
│ to
│ use the current configuration.
│ 
│ If the change reason above is incorrect, please verify your configuration
│ hasn't changed and try again. At this point, no changes to your existing
│ configuration or state have been made.
╵
ERRO[0000] Hit multiple errors:
Hit multiple errors:
exit status 1 

But wait, I hear you say, Terragrunt runs terraform init for you… so what gives?

Well, in this case, the terragrunt.hcl has a dependency block, and one of those dependencies has not run properly, so… let’s fix it

Read the content of your terragrunt.hcl

terraform {
  source = "git@github.com:example/example-terraform-modules.git//module"
}

include {
  path = find_in_parent_folders()
}

dependency "dependency_1" {
  config_path = "${get_terragrunt_dir()}/../dependency"

  mock_outputs_allowed_terraform_commands = ["destroy", "force-unlock"]
  mock_outputs = {
    output_1 = []
    output_2 = ""
  }
}

inputs = {
  name      = "some_module"
  some_key  = dependency.dependency_1.outputs.output_1
  other_key = dependency.dependency_1.outputs.output_2
}

Right, so for some reason the dependency won’t run. Change into that directory, and run terragrunt apply --terragrunt-source-update. Hopefully, you’ll get something like this:

Initializing the backend...

Successfully configured the backend "example"! Terraform will automatically
use this backend unless the backend configuration changes.

Initializing provider plugins...
- Reusing previous version of example/example from the dependency lock file
- Installing example/example v1.0.0...
- Installed example/example v1.0.0 (signed by Example)

Terraform has been successfully initialized!

You may now begin working with Terraform. Try running "terraform plan" to see
any changes that are required for your infrastructure. All Terraform commands
should now work.

If you ever set or change modules or backend configuration for Terraform,
rerun this command to reinitialize your working directory. If you forget, other
commands will detect it and remind you to do so if necessary.
example_module.this: Refreshing state... [id=an-example]

No changes. Your infrastructure matches the configuration.

Terraform has compared your real infrastructure against your configuration
and found no differences, so no changes are needed.

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Outputs:
output_1 = {"some_key": "some_value"}
output_2 = "some_string"

You may find yourself having to traverse several different dependencies until you get to the one which is missing… and then it should work :)

Featured image is “Jenga” by “Mara Tr.” on Flickr and is released under a CC-BY license.