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Terraform Configurations With Nickel

This repository contains tooling for generating Nickel contracts out of Terraform provider schemas.

It enables configurations to be checked against provider specific contracts before calling Terraform to perform the deployment. Nickel can natively generate outputs as JSON, YAML or TOML. Since Terraform can accept its deployment configuration as JSON, you can straightforwardly export a Nickel configuration, adhering to the right format, to Terraform. Tf-Ncl provides a framework for ensuring a Nickel configuration has this specific format. Specifically, Tf-Ncl is a tool to generate Nickel contracts that describe the configuration schema expected by a set of Terraform providers.

Starting a Tf-Ncl configuration

The easiest way to get started is to use the hello-tf flake template:

nix flake init -t github:tweag/tf-ncl#hello-tf

This will leave you with a flake.nix file containing some glue code for getting a Nickel contract out of tf-ncl, evaluating a Nickel configuration and calling Terraform. It's as easy as

nix develop -c run-terraform init
nix develop -c run-terraform apply

Without Nix it's a bit more complicated. You will need to obtain the Nickel contract using the tools in this repository. Take a look at the working principle for an overview of the process. The most involved step will be calling schema-merge with extracted Terraform provider schemas, see the nix code for inspiration.

Once you have a project set up, you can start writing Nickel configuration code. To quote from the hello-tf example:

let Tf = import "./schema.ncl" in
{
  config.resource.null_resource.hello-world = {
    provisioner.local-exec = [{
      command = m%"
        echo 'Hello, world!'
      "%
    }],
  },
} | Tf.Config

Anything goes! You just need to ensure that your Terraform configuration ends up in the toplevel attribute config and that your entire configuration evaluates to a record satisfying the Tf.Config contract.

To actually turn the Nickel code into a JSON configuration file understood by Terraform, you need to call nickel to export the renderable_config toplevel attribute introduced by the Tf.Config contract:

nickel export <<<'(import "./<your-toplevel-file>.ncl").renderable_config'

This can be useful for inspecting the result of your code. But usually it will be easier to use the wrapper script for Terraform provided in the hello-tf flake template.

For inspiration on what's possible with Nickel, take a look at the examples. Happy hacking!

How?

Unfortunately, Terraform doesn't expose an interface for extracting a machine readable specification for the provider independent configuration it supports. Because of that this repository contains two tools and some glue written in Nix. Maybe this flowchart helps:

flowchart LR
    subgraph Nix
        direction TB
        providerSpec(Required Providers);
        providerSchemasNix(Terraform provider schemas);
        providerSpec -- generateJsonSchema --> providerSchemasNix;
    end
    
    subgraph schema-merge
        direction TB
        providerSchemasGo(Terraform provider schemas);
        merged-json-go(Merged JSON w/ Terraform builtins);
        providerSchemasGo --> merged-json-go;
    end

    subgraph tf-ncl
        direction TB
        merged-json-rust(Merged JSON w/ Terraform builtins);
        nickel-contracts(Monolithic Nickel contract)
        merged-json-rust --> nickel-contracts;
    end

    Nix --> schema-merge
    schema-merge --> tf-ncl
Loading

The entire process is packaged up in a Nix function generateSchema which is exposed as a flake output. Also, to generate a Nickel contract for a single provider, there is a flake output schemas:

nix build github:tweag/tf-ncl#schemas.aws

All providers available in nixpkgs are supported. The generateSchema function can also be called manually. For example, to get a monolithic Nickel schema for the aws, github and external Terraform providers, you could use

nix build --impure --expr \
  '(builtins.getFlake "github:tweag/tf-ncl).generateSchema.${builtins.currentSystem} (p: { inherit (p) aws github external; })'

Status

This project is in active development and breaking changes should be expected.

  • Automatic contracts for Terraform provider schemas
  • Contracts for Terraform state backends #14, #15
  • More documentation #13
  • Natural handling of field references #12