Run docker-compose without images with Nix

Wait, what?

With Arion you can fire up containers without creating images for each service. It can use a mostly empty image, and makes the host's Nix store available in the container, allowing the container to run programs without having to re-package them into a docker image.

Arion is configured using Nix with modules, like those in NixOS. Similar to docker-compose it can therefore combine configuration from multiple files. For managing the network and containers it delegates to the docker-compose command.

Project Status

This project was born out of a process supervision need for local development environments while working on Hercules CI. (It was also born out of ancient Greek deities disguised as horses. More on that later.)

If you do want to use Arion for production environments, you'll probably want to either build normal container images or manage garbage collection roots if you control the deployment host. Neither scenario is made easier by arion at this time.

Support for other Linux than NixOS is untested.

Install

Have Nix and Docker installed. Ubuntu users: make sure you've installed via apt-get install docker.io. The Docker snap is broken and apparently unsupported by Canonical.

git clone git@github.com:hercules-ci/arion.git
cd arion
nix-env -iA arion -f .

Example `arion-compose.nix`

This Nix expression serves the Nix manual at host port 8000 when launched with arion up. It is a function from a package set (pkgs) to a configuration.

{ pkgs, ... }:
{
  config.docker-compose.services = {

    webserver = {
      service.useHostStore = true;
      # service.depends_on = [ "backend" ];
      service.command = [ "sh" "-c" ''
                  cd "$$WEB_ROOT"
                  ${pkgs.python3}/bin/python -m http.server
                '' ];
      service.ports = [
        "8000:8000" # host:container
      ];
      service.environment.WEB_ROOT = "${pkgs.nix.doc}/share/doc/nix/manual";
    };

    # backend = { ... }
  };
}

The pkgs argument comes from a file called arion-pkgs.nix. It can be as simple as import <nixpkgs> {} to use the Nixpkgs from your $NIX_PATH, or you can use it to pin a specific Nixpkgs version.

A full featured example

To see how Arion can be used in a project, have a look at todomvc-nix.

git clone git@github.com:nix-community/todomvc-nix.git
cd todomvc-nix/deploy/arion
arion up

How it works

Arion is essentially a thin wrapper around Nix and docker-compose. When it runs, it does the following:

Evaluate the configuration using Nix, producing a docker-compose.yaml and a garbage collection root
Invoke docker-compose
Clean up the garbage collection root

Most of the interesting stuff happens in Arion's Nix expressions, where it runs the module system (known from NixOS) and provides the configuration that makes the Docker Compose file do the things it needs to do.

One of the more interesting built-in modules is the host-store.nix module which performs the bind mounts to make the host Nix store available in the container.

FAQ

Do I need to use Hercules CI?

Nope, it's just Nix and Docker Compose under the hood.

Does Arion support Docker images?

Yes, you can also specify a normal Docker image. For example:

postgres = {
  service.image = "postgres:10";
  service.volumes = [ "${toString ./.}/postgres-data:/var/lib/postgresql/data" ];
  service.environment.POSTGRES_PASSWORD = "mydefaultpass";
};

What about garbage collection?

Arion removes the need for garbage collecting docker images, delegating this task to Nix.

Arion creates a garbage collection root and cleans it up after completing the command. This means that arion up without -d is safe with respect to garbage collection. A deployment that is more serious than local development must leave a GC root on the deployment host. This use case is not supported as of now.

What is messing with my environment variables?

Docker Compose performs its own environment variable substitution. This can be a little annoying in services.command for example. Either reference a script from pkgs.writeScript or escape the dollar sign as $$.

Why "Arion"?

Arion comes from Greek mythology. Poseidon, the god of ~Docker~ the seas had his eye on Demeter. Demeter tried to trick him by disguising as a horse, but Poseidon saw through the deception and they had Arion.

So Arion is a super fast divine horse; the result of some weird mixing. Also it talks.

(And we feel morally obliged to name our stuff after Greek mythology)

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