devenv 1.2: Tasks for convergent configuration with Nix

For devenv, our mission is to make Nix the ultimate tool for managing developer environments. Nix excels at congruent configuration, where the system state is fully described by declarative code.

However, the real world often throws curveballs. Side-effects like database migrations, one-off tasks such as data imports, or external API calls don't always fit neatly into this paradigm. In these cases, we often resort to convergent configuration, where we define the desired end-state and let the system figure out how to get there.

To bridge this gap and make Nix more versatile, we're introducing tasks. These allow you to handle those pesky real-world scenarios while still leveraging Nix's powerful ecosystem.

Usage

For example if you'd like to execute python code after virtualenv has been created:

devenv.nix

{ pkgs, lib, config, ... }: {
  languages.python.enable = true;
  languages.python.venv.enable = true;

  tasks = {
    "python:setup" = {
      exec = "python ${pkgs.writeText "setup.py" ''
          print("hello world")
      ''}";
      after = [ "devenv:python:virtualenv" ];
    };
    "devenv:enterShell".after = [ "python:setup" ];
  };
}

python:setup task executes before devenv:enterShell but after python:virtualenv task:

For all supported use cases see tasks documentation.

Task Server Protocol for SDKs

We've talked to many teams that dropped Nix after a while and they usually fit into two categories:

• 1) Maintaining Nix was too complex and the team didn't fully onboard, creating friction inside the teams.
• 2) Went all-in Nix and it took a big toll on the team productivity.

While devenv already addresses (1), bridging the gap between Nix provided developer environments and existing devops tooling written in your favorite language is still an unsolved problem until now.

We've designed Task Server Protocol so that you can write tasks using your existing automation by providing an executable that exposes the tasks to devenv:

devenv.nix

{ pkgs, ... }:
let
  myexecutable = pkgs.rustPlatform.buildRustPackage rec {
    pname = "foo-bar";
    version = "0.1";
    cargoLock.lockFile = ./myexecutable/Cargo.lock;
    src = pkgs.lib.cleanSource ./myexecutable;
  }
in {
  task.serverProtocol = [ "${myexecutable}/bin/myexecutable" ];
}

In a few weeks we're planning to provide Rust TSP SDK with a full test suite so you can implement your own abstraction in your language of choice.

You can now use your preferred language for automation, running tasks with a simple devenv tasks run <names> command. This flexibility allows for more intuitive and maintainable scripts, tailored to your team's familiarity.

For devenv itself, we'll slowly transition from bash to Rust for internal glue code, enhancing performance and reliability. This change will make devenv more robust and easier to extend, ultimately providing you with a smoother development experience.

Upgrading

If you run devenv update on your existing repository you should already be using tasks, without needing to upgrade to devenv 1.2.

Domen