Distributed Building ​

{ ... }: {

  ####################################################################
  #
  #  NixOS's Configuration for Remote Building / Distributed Building
  #
  ####################################################################

  # Set local's max-jobs to 0 to force remote building (disable local building).
  # nix.settings.max-jobs = 0;
  nix.distributedBuilds = true;
  nix.buildMachines =
    let
      sshUser = "ryan";
      # Path to the SSH key on the local machine.
      sshKey = "/home/ryan/.ssh/ai-idols";
      systems = [
        # Native architecture.
        "x86_64-linux"

        # Emulated architecture using binfmt_misc and qemu-user.
        "aarch64-linux"
        "riscv64-linux"
      ];
      # All available system features are poorly documented here:
      # https://github.com/NixOS/nix/blob/e503ead/src/libstore/globals.hh#L673-L687
      supportedFeatures = [
        "benchmark"
        "big-parallel"
        "kvm"
      ];
    in
      [
        # Nix seems to always prioritize remote building.
        # To make use of the local machine's high-performance CPU, do not set the remote builder's maxJobs too high.
        {
          # Some of my remote builders are running NixOS
          # and have the same sshUser, sshKey, systems, etc.
          inherit sshUser sshKey systems supportedFeatures;

          # The hostName should be:
          #   1. A hostname that can be resolved by DNS.
          #   2. The IP address of the remote builder.
          #   3. A host alias defined globally in /etc/ssh/ssh_config.
          hostName = "aquamarine";
          # Remote builder's max-jobs.
          maxJobs = 3;
          # SpeedFactor is a signed integer,
          # but it seems that it's not used by Nix and has no effect.
          speedFactor = 1;
        }
        {
          inherit sshUser sshKey systems supportedFeatures;
          hostName = "ruby";
          maxJobs = 2;
          speedFactor = 1;
        }
        {
          inherit sshUser sshKey systems supportedFeatures;
          hostName = "kana";
          maxJobs = 2;
          speedFactor = 1;
        }
      ];
  # Optional: Useful when the builder has a faster internet connection than yours.
	nix.extraOptions = ''
		builders-use-substitutes = true
	'';

  # Define the host aliases for remote builders.
  # This configuration will be written to /etc/ssh/ssh_config.
  programs.ssh.extraConfig = ''
    Host ai
      HostName 192.168.5.100
      Port 22

    Host aquamarine
      HostName 192.168.5.101
      Port 22

    Host ruby
      HostName 192.168.5.102
      Port 22

    Host kana
      HostName 192.168.5.103
      Port 22
  '';

  # Define the host keys for remote builders so that Nix can verify all the remote builders.
  # This configuration will be written to /etc/ssh/ssh_known_hosts.
  programs.ssh.knownHosts = {
    # 星野 愛久愛海, Hoshino Aquamarine
    aquamarine = {
      hostNames = [ "aquamarine" "192.168.5.101" ];
      publicKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIDnCQXlllHoLX5EvU+t6yP/npsmuxKt0skHVeJashizE";
    };

    # 星野 瑠美衣, Hoshino Rubii
    ruby = {
      hostNames = [ "ruby" "192.168.5.102" ];
      publicKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIE7n11XxB8B3HjdyAsL3PuLVDZxWCzEOUTJAY8+goQmW";
    };

    # 有馬 かな, Arima Kana
    kana = {
      hostNames = [ "kana" "192.168.5.103" ];
      publicKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIJ3dDLOZERP1nZfRz3zIeVDm1q2Trer+fWFVvVXrgXM1";
    };
  };
}