Switching to Nix Flakes

Posted on January 9, 2021

# Motivation

I switched to nix flakes over christmas vacation. It could do with some better mechanics, but in general it’s pretty great and I feel much more comfortable with the stability and flexibility of my system than I did before.

Nix the package manager has rolling release on all it’s packages. It uses a channel system and a release system. This means that for each release, there’s a stable and unstable channel. The stable channel is probably not broken, but has older packages. This generally works quite well if you live on stable. You can also mix and match stable and unstable packages. However, this is poor for long term stability and reproducibility. Given that this is the entire point of using Nix as a package manager, this seem bad.

Flakes is the solution to this problem. In a similar fashion to npm, flakes will generate a lock file. Building your system with the lock file will guarantee the same system down to the commit hash. This is essentially the main motivating factor for me.

# Mechanics

## Getting nix flakes

Switching flakes was nearly trivial, even if you’re using home-manager. The first step is to install flakes on your system. This is done by including the following in your configuration.nix:

  nix = {
      package = pkgs.nixUnstable;
      extraOptions = ''
        experimental-features = nix-command flakes

This moves the nix package manager to unstable. Then, add nixFlakes to the list of packages to install flakes.

## Enabling flakes

This part is simple enough; it’s outlined on the wiki. The input list will probably include the stable, unstable, and home-manager channels, and the output list can contain only your configuration.nix and home.nix (for home-manager) nixos modules, as outlined on the home-manager readme.

Before building your system, flakes requires that the directory you’re building from be a git repo, and that all files being used in the build be staged.

Then you can build your system by running (in the repo root directory):

sudo nixos-rebuild switch --flake $HOST_NAME

Note that since we’re using flakes, flakes will create a flake.lock file that will allow us to build the repo on any machine down to the commit used.

Note what we’re already getting with very little expended effort:

  • a monolithic repo for both our user and system configuration. I was already doing this pre-flakes, but I had to run two separate build commands. The fact that I can manage them both without having to deal explicitly with mutliple channels is neat.
  • identical builds proccesses. Of course, compiler non-determinism cannot be avoided and service management is sometimes sketchy, but in general everything up to that is the same. Specifically, the commit hash of the repos used and the way the repos are built will not change. This is a long winded way of saying if it built and ran on one machine targeting amd64, it will also build on all other machines targeting amd64.

## nixflk

The initial flakes are pretty cool, but there are some nifty abstractions that one can add on top. I ended up using nixflks and my friend’s fork of nixflks as my starting ground. I’ll talk more to the fork, since those are the abstractions I’ve been using. My dotfiles are here.

The basic structure I settled with looks like this:

├── flake.nix
├── hosts
│   ├── default.nix
│   ├── desktop.nix
│   ├── laptop.nix
│   ├── hw
│   │   ├── desktop.nix
│   │   ├── laptop.nix
│   │   └── shared.nix
│   └── shared
│       ├── apps.nix
│       ├── default.nix
│       ├── dotfiles
│       ├── home.nix
│       ├── misc.nix
│       └── services.nix
└── overlays
    ├── deepfry
    └── imagemagick

The idea is that I have two computers: a desktop and a laptop. They have:

  • different hardware
  • same dotfiles (window manager, text editor, services)
  • same applications installed
  • same overlays

As a result, the hosts/default.nix chooses a host based on hostname (e.g. either laptop or desktop). This includes $HOSTNAME.nix file, which gets the relevant hardware and includes the entirety of the shared directory.

This means that I’m able to keep different hw and other small differences separate, while avoiding code duplication with the shared directory. The shared directory includes my dotfiles, common services (like zerotier), my dev tools, etc. This generally feels super clean and gives me a flexible way to manage both my machines.

## gotchas

One of the slight gotchas for doing this is the inclusion of a hardware-configuration.nix file. I copied over most of it to the hw directory. However, you’ll need to go onto nixpkgs, grab the not-detected.nix, and cut and paste that into the common hardware file.

The whole having to stage the files you use is really annoying. Whenever I added a new file, I’d build, scratch my head, then realize that I had forgot to stage it.

Error messages could definitely be a bit more forgiving…

Sometimes the cache gets into an inconsistent state and fails immediately when trying to build. The remedy is to make a trivial change (perhaps add a newline) to the flake.nix and rebuild. This was also pretty annoying.

## cachix

The caching for unstable doesn’t work very well. As a result, I’m sometimes stuck compiling on my laptop. This is not ideal since my laptop is old and not powerful. To solve this, I can compile programs on my desktop, push the results very easily to cachix, and then pull them down on my laptop (using the same lock file). This is pretty nice.

## other perks

Another perk of using flakes is that you can effortlessly import and use others flakes by listing them as inputs.