PROBLEM

Terragrunt has a feature that allows one module to pass outputs to another module.

For example, if “project-prod” module wants to consume “subfolders” output from “folder” module, it can be done like this in “project-prod” module’s terragrunt.hcl:-

include {
    path = find_in_parent_folders()
}

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

inputs = {
    env_folders = dependency.folder.outputs.subfolders
}

The challenge is when running commands such as plan-all, it will fail with the following error:-

Cannot process module Module [...] because one of its 
dependencies, [...], finished with an error: /my/path/folder/terragrunt.hcl 
is a dependency of /my/path/project-prod/terragrunt.hcl 
but detected no outputs. Either the target module has not 
been applied yet, or the module has no outputs. If this 
is expected, set the skip_outputs flag to true on the 
dependency block.

SOLUTION

This error occurs because the generated plan for “folder” module has not been applied yet (ie: the infrastructure does not exist), hence there are no outputs to pass to “project-prod” module to satisfy plan-all.

To fix this, mock outputs can be supplied:-

include {
    path = find_in_parent_folders()
}

dependency "folder" {
    config_path = "../folder"

    mock_outputs = {
        subfolders = {
            "dev" = {
                "id" = "temp-folder-id"
            }
            "prod" = {
                "id" = "temp-folder-id"
            }
            "uat" = {
                "id" = "temp-folder-id"
            }
        }
    }
}

inputs = {
    env_folders = dependency.folder.outputs.subfolders
}

Finally, when running apply-all, it will use the runtime outputs instead of provided mock outputs to build the rest of the infrastructure.