Arion is a tool for building and running applications that consist of multiple docker containers using NixOS modules. It has special support for docker images that are built with Nix, for a smooth development experience and improved performance.
It is built on top of Docker Compose, which implements the container orchestration functionality.
Instead of configuring the compositions in YAML files like
docker-compose.yaml
, Arion uses the Nix
language to declare the compositions. Because of this, Arion gives you
the ability to declare your deployments, configuration and packaging
in the same language. By replacing multiple tools with a single
language, you decrease your mental load and you can more easily
refactor and maintain your configurations.
Although Arion can be used as a Docker Compose with an improved configuration front end, there is more to be gained from integrating with Nix. In particular, the more structured approach of Nix compared to Dockerfiles allows the following:
-
Build components of your image in parallel, automatically
-
Share packages between images, regardless of the order they were added
-
Improve performance by skipping container image creation
-
Work with structured data instead of strings, templates and a multitude of expression languages
-
Refactor across deployments, configuration and packaging
Arion allows to compose containers with different granularity:
Arion is configured declaratively with two files:
Arion needs arion-pkgs.nix
to import nixpkgs, it’s contents can be as simple as:
import <nixpkgs> {}
or more sophisticated (recommended) setup with Niv.
Describe containers using NixOS-style modules. There are a few options:
examples/minimal/arion-compose.nix
:
{ pkgs, ... }:
{
config.docker-compose.services = {
webserver = {
service.useHostStore = true;
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";
};
};
}
examples/nixos-unit/arion-compose.nix
:
{
docker-compose.services.webserver = { config, pkgs, ... }: {
nixos.configuration = {config, pkgs, ...}: {
boot.isContainer = true;
services.nginx.enable = true;
services.nginx.virtualHosts.localhost.root = "${pkgs.nix.doc}/share/doc/nix/manual";
system.build.run-nginx = pkgs.writeScript "run-nginx" ''
#!${pkgs.bash}/bin/bash
PATH='${config.systemd.services.nginx.environment.PATH}'
echo nginx:x:${toString config.users.users.nginx.uid}:${toString config.users.groups.nginx.gid}:nginx web server user:/var/empty:/bin/sh >>/etc/passwd
echo nginx:x:${toString config.users.groups.nginx.gid}:nginx >>/etc/group
${config.systemd.services.nginx.runner}
'';
};
service.command = [ config.nixos.build.run-nginx ];
service.useHostStore = true;
service.ports = [
"8000:80" # host:container
];
};
}
examples/full-nixos/arion-compose.nix
:
{
docker-compose.services.webserver = { pkgs, ... }: {
nixos.useSystemd = true;
nixos.configuration.boot.tmpOnTmpfs = true;
nixos.configuration.services.nginx.enable = true;
nixos.configuration.services.nginx.virtualHosts.localhost.root = "${pkgs.nix.doc}/share/doc/nix/manual";
service.useHostStore = true;
service.ports = [
"8000:80" # host:container
];
};
}
To see how Arion can be used in a project, have a look at todomvc-nix.
$ git clone https://github.com/nix-community/todomvc-nix
$ cd todomvc-nix/deploy/arion
$ arion up
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.
Arion has run successfully on Linux distributions other than NixOS, but we only perform CI for Arion on NixOS.
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.
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.
Restart it with arion restart <name>
or if you’ve changed the image rebuild
them using arion up -d --always-recreate-deps <name>
.
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
$$
.
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)