Feature Request: Support `from-module` parameter

[osterman]

First of all, I love the direction of this utility. It's a generalized approach to orchestrated complex, multi-phased applies for terraform. It's a nice alternative to terragrunt that's less opinionated. We use a similar tool designed for helm that's called helmfile.

what

• Support an init.from-module directive for each module.

why

• Terraform init supports -from-module=SOURCE when initializing a module
• Enables ultra DRY innovation of modules (what terragrunt uses) and would make it easier to drop terragrunt and use astro instead
• Rapidly define a complex architecture by referring to root module invocations

example

This example would copy the /aws/app module to core/app and then invoke the commands accordingly

modules:
  - name: app
    init:
      from-module: https://github.com/cloudposse/terraform-root-modules//aws/app
    path: core/app
    deps:
      - module: users
      - module: vpc
    remote:
      backend_config:
        bucket: acme-terraform-states
        key: "{{.aws_region}}/app-{{.environment}}.tfstate"
        region: us-east-1
    variables:
      - name: region
      - name: environment
        values: [dev, prod]

[dansimau]

I am all for this.

I'm even thinking of a generalised approach, i.e.: within each terraform block, have a way to specify arbitrary arguments to each Terraform command:

E.g.:

modules:
  - name: app
    terraform:
      flags:
        init:
          - from-module=https://github.com/cloudposse/terraform-root-modules//aws/app
    path: core/app
    deps:
      - module: users
      - module: vpc
    remote:
      backend_config:
        bucket: acme-terraform-states
        key: "{{.aws_region}}/app-{{.environment}}.tfstate"
        region: us-east-1
    variables:
      - name: region
      - name: environment
        values: [dev, prod]

Then you get access to all the underlying Terraform flags. For example, one could increase parallelism for all apply commands by doing:

terraform:
  flags:
    apply:
      - parallelism=20

modules:
  - name: app
    terraform:
      flags:
        init:
          - from-module=https://github.com/cloudposse/terraform-root-modules//aws/app
    path: core/app
    deps:
      - module: users
      - module: vpc
    remote:
      backend_config:
        bucket: acme-terraform-states
        key: "{{.aws_region}}/app-{{.environment}}.tfstate"
        region: us-east-1
    variables:
      - name: region
      - name: environment
        values: [dev, prod]

Only consideration there is that users could potentially cause an issue by changing something astro doesn't expect under the hood. I can't see any catastrophic cases of that right now but it could become a problem.

[gerardaus]

This is an important feature for greater levels of composition and versioning of infrastructure. Would love to see this implemented or help in the implementation.

If you could init from a module, would it negate the need for the "path" attribute?

Initially when I looked at the project, I was under the impression that the "path" attribute would act like the -from-import functionality of terraform.