terraform-nixos-ng: Modern terraform support for NixOS

Recently I’ve been working on writing a “NixOS in Production” book and one of the chapters I’m writing is on deploying NixOS using terraform. However, one of the issues I ran across was the poor NixOS support for terraform. I’ve already gone through the nix.dev post explaining how to use the terraform-nixos project but I ran into several issues trying to follow those instructions (which I’ll explain below). That plus the fact that terraform-nixos seems to be unmaintained pushed me over the edge to rewrite the project to simplify and improve upon it.

So this post is announcing my terraform-nixos-ng project:

• GitHub - Gabriella439/terraform-nixos-ng

… which is a rewrite of terraform-nixos and I’ll use this post to compare and contrast the two projects. If you’re only interested in trying out the terraform-nixos-ng project then go straight to the README

Using nixos-rebuild

One of the first things I noticed when kicking the tires on terraform-nixos was that it was essentially reinventing what the nixos-rebuild tool already does. In fact, I was so surprised by this that I wrote a standalone post explaining how to use nixos-rebuild as a deployment tool:

• Haskell for all - Announcing nixos-rebuild: a “new” deployment tool for NixOS

Simplifying that code using nixos-rebuild fixed lots of tiny papercuts I had with terraform-nixos, like:

• The deploy failing if you don’t have a new enough version of bash installed

• The inability to turn off the use of the --use-substitutes flag

  That flag causes issues if you want to deploy to a machine that disables outbound connections.

• The dearth of useful options (compared to nixos-rebuild)

  … including the inability to fully customize ssh options

• The poor interop with flakes

  For example, terraform-nixos doesn’t respect the standard nixosConfigurations flake output hierarchy.

  Also, terraform-nixos doesn’t use flakes natively (it uses flake-compat), which breaks handling of the config.nix.binary{Caches,CachePublicKeys} flakes settings. The Nix UX for flakes is supposed to ask the user to consent to those settings (because they are potentially insecure to auto-enable for a flake), but their workaround breaks that UX by automatically enabling those settings without the user’s consent.

I wanted to upstream this rewrite to use nixos-rebuild into terraform-nixos, but I gave up on that idea when I saw that no pull request since 2021 had been merged, including conservative pull requests like this one to just use the script included within the repository to update the list of available AMIs.

That brings me to the next improvement, which is:

Auto-generating available AMIs

The terraform-nixos repository requires the AMI list to be manually updated. The way you do this is to periodically run a script to fetch the available AMIs from Nixpkgs and then create a PR to vendor those changes. However, this shouldn’t be necessary because we could easily program terraform to generate the list of AMIs on the fly.

This is what the terraform-nixos-ng project does, where the ami module creates a data source that runs an equivalent script to fetch the AMIs at provisioning time.

In the course of rewriting the AMI module, I made another small improvement, which was:

Support for aarch64 AMIs

Another gripe I had with terraform-nixos-ng is that its AMI module doesn’t support aarch64-linux NixOS AMIs even though these AMIs exist and Nixpkgs supports them. That was a small and easy fix, too.

Functionality regressions

terraform-nixos-ng is not a strict improvement over terraform-nixos, though. Specifically, the most notable feature omissions are:

• Support for non-flake workflows

  terraform-nixos-ng requires the use of flakes and doesn’t provide support for non-flake-based workflows. I’m very much on team “Nix flakes are good and shouldn’t be treated as experimental any longer” so I made an opinionated choice to require users to use flakes rather than support their absence.

  This choice also isn’t completely aesthetic, the use of flakes improves interop with nixos-rebuild, where flakes are the most ergonomic way for nixos-rebuild to select from one of many deployments.

• Support for secrets management

  I felt that this should be handled by something like sops-nix rather than rolling yet another secrets management system that was idiosyncratic to this deploy tool. In general, I wanted these terraform modules to be as lightweight as possible by making more idiomatic use of the modern NixOS ecosystem.

• Support for Google Compute Engine images

  terraform-nixos supports GCE images and the only reason I didn’t add the same support is because I’ve never used Google Compute Engine so I didn’t have enough context to do a good rewrite, nor did I have the inclination to set up a GCE account just to test the rewrite. However, I’d accept a pull request adding this support from someone interested in this feature.

Conclusion

There’s one last improvement over the terraform-nixos project, which is that I don’t leave projects in an abandoned state. Anybody who has contributed to my open source projects knows that I’m generous about handing out the commit bit and I’m also good about relinquishing control if I don’t have time to maintain the project myself.

However, I don’t expect this to be a difficult project to maintain anyway because I designed terraform-nixos-ng to outsource the work to existing tools as much as possible instead of reinventing the wheel. This is why the implementation of terraform-nixos-ng is significantly smaller than terraform-nixos.